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Awesome sea-going crocodyliforms of the Mesozoic

The views expressed are those of the author and are not necessarily those of Scientific American.


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Suddenly... THALATTOSUCHIANS!!

The Mesozoic was not a ‘dinosaurs-only theme park’. Numerous other tetrapod lineages were around as well, and there was enough ‘ecospace’ for members of at least some of these groups to evolve giant size and macropredatory lifestyles, and even to dominate certain sections of the Mesozoic world.

It’s well known in particular that this was true of the Mesozoic seas. They weren’t ‘ruled’ by dinosaurs, but by members of some wholly different reptile groups. In this article I want to briefly – yes, very, very briefly – rattle through the mostly marine, mostly long-snouted crocodyliforms that shared the Mesozoic seas with such other non-dinosaurs as the ichthyosaurs and plesiosaurs.

PS – I wrote those introductory paragraphs before finishing the rest of the article. I totally, totally failed on the “very, very briefly” bit. And my original intention was to write about all longirostrine crocodyliform lineages. I totally failed on that, too.

One of the world's most important metriorhynchid specimens: the Rhacheosaurus gracilis specimen NHMUK R3948, from the Upper Jurassic of Daiting, Germany. The specimen is unique in preserving the soft-tissue impression of the upper tail lobe. The oval forelimbs are so small in Rhacheosaurus that they are scarcely visible in this old photograph. Rhacheosaurus and Cricosaurus are unusual in having such reduced, paddle-shaped forelimbs.

Several groups concern us here. The most specialised and most remarkable marine Mesozoic crocodyliforms are the metriorhynchids, a Jurassic and Early Cretaceous group best known for the many excellent fossils described from England, France and Germany during the early 1800s. Later discoveries made in Russia, Cuba, Mexico, Chile and Argentina have added substantially to our knowledge.

Soft-tissue outline and tail-tip skeleton of Rhacheosaurus gracilis, drawn by Andrews (1913) after the NHMUK R3948 specimen show above. The tail fins of some other metriorhynchid taxa must have been much larger than this.

Especially well-preserved, complete or near-complete fossils show us that metriorhynchids had paddle-shaped limbs and a vertically oriented tail fluke (the vertebrae at the tail-tip bent towards to support the lower lobe; the upper lobe was unsupported by bone). They also wholly lack the bony osteoderms that normally cover the dorsal surface of the crocodyliform body, and their skulls are highly streamlined and lightweight.

Several cranial specialisations present in the group are associated with pelagic marine life. For all their prowess as amphibious and aquatic predators, crocodyliforms have almost entirely stuck with terminal external nostrils. Metriorhynchids are the exception, since members of Cricosaurus retracted their nostrils such that they’re present a short distance posterior to the premaxillary bones at the tip of the snout. We also know (due to exceptional fossils) that metriorhynchids had large salt-excreting glands housed in massive, projecting prefrontal bones (Fernández & Gasparini 2000).

I wrote about metriorhynchids on Tet Zoo ver 1 (during 2006), but that was a totally different time: metriorhynchid diversity mostly went unappreciated back then, and – while there was a vague idea that metriorhynchids came in slender-snouted and robust-snouted forms – a massively over-lumped taxonomy, a poor understanding of the group’s phylogenetic structure, and a lack of functional studies contributed to the idea that metriorhynchids were all much alike, and certainly less interesting and less important than the diverse, pelagically adapted ichthyosaurs and plesiosaurs that they lived alongside.

Here's the latest version of my crocodylomorph montage, now with new metriorhynchids. Image by Darren Naish.

The metriorhynchid radiation, 2012

Recent phylogenetic analyses have shown that species referred to the ‘classic’ metriorhynchid genera Metriorhynchus and Geosaurus are not all close relatives. In other words, these genera (as classically conceived) are not monophyletic. New genera have been named for some of the taxa concerned, but several old names have been resurrected as well (Young & Andrade 2009, Andrade et al. 2010, Cau & Fanti 2010, Young et al. 2010, 2012a, b).

Metriorhynchidae is currently understood to consist of two major clades. One includes Metriorhynchus geoffroyi and its close relatives as well as Gracilineustes, Rhacheosaurus and the many species of Cricosaurus. Since the type species of MetriorhynchusM. geoffroyi von Meyer, 1830 – belongs here, this clade has to be called Metriorhynchinae. The other clade includes Geosaurus giganteus, Suchodus, Neptunidraco, Purranisaurus, Plesiosuchus, Torvoneustes and Dakosaurus. Since the type species of GeosaurusG. giganteus (von Sömmerring, 1816) – goes here, this is Geosaurinae (Young & Andrade 2009, Young et al. 2010). A couple of taxa – Teleidosaurus and Eoneustes – appear to be close to Metriorhynchidae, but just outside the group (Young et al. 2010).

Late Jurassic geosaurine metriorhynchids from Europe, illustrated to scale from Young et al. (2012b). Life reconstructions by Dmitry Bogdanov.

We now know that some geosaurine species were robust-snouted, and in fact some were deep-snouted and (for a crocodylomorph) short-snouted to boot, with skulls superficially resembling those of tyrannosaurs and other macropredatory theropod dinosaurs. The serrated, laterally compressed teeth of these geosaurines were suited for slicing through the tissues of prey animals (Young & Andrade 2009) and some taxa evolved tight tooth-to-tooth occlusion with extensive wear on the teeth resulting from both tooth-food contact as well as tooth-tooth contact (Young et al. 2012a, b). These wear patterns are thought to have increased the shearing capability of the teeth and therefore improved the ability of these animals to cut prey items into chunks.

Skulls of (top to bottom) the geosaurines Plesiosuchus manselii, Dakosaurus maximus and D. andiniensis, from Young et al. (2012b).

It has most recently been suggested that some of these deep-snouted geosaurines were capable of suction-feeding, and in fact that they were ecological analogues of killer whales and other macropredatory odontocetes (Young et al. 2012a, b). All in all, a strong body of exciting work has shown that sea-going crocodyliforms became archosaurian mimics of both gracile, slender-snouted and robust, short-snouted predatory toothed cetaceans.

Incidentally, this succession of studies – virtually all of them led by Mark T. Young of the University of Edinburgh – provide an excellent example of what sort of difference resolving taxonomy and phylogeny can have on our appreciation of evolution, ecology, functional morphology and behaviour.

A brief diversion: dude, where’s my viviparous archosaur?

An enormous amount remains unknown about metriorhynchid biology and behaviour. If these animals were so strongly adapted for aquatic life – and given that some of them got very large (the geosaurine Plesiosuchus reached 6.9 m) – were they able to haul up on beaches to bask, or to dig nests and lay eggs? It really looks unlikely*, so does this mean that they were viviparous? Viviparity has indeed been implied for these animals: Young et al. (2010, p. 802) noted that their pelvic girdles are proportionally wide compared to those of other crocodylomorphs, and their substantially reduced pectoral and pelvic girdles, hydrofoil-shaped forelimbs and hypocercal tails make it extremely unlikely that they were capable of any terrestrial movement. If metriorhynchids really were viviparous, they would (so far as we know) be unique within Archosauria.

* Young et al. (2010) argued that the loss of the external mandibular fenestra and shortening of the retroarticular process in metriorhynchids indicate reduction of the jaw muscles normally involved in thermoregulatory jaw-gaping. It therefore seems that metriorhynchids didn’t (and/or couldn’t) gape to thermoregulate, and thus didn’t (and/or couldn’t) indulge in terrestrial thermoregulatory behaviour.

Hypothetical sauropod viviparity, as depicted by Doug Henderson. No, it didn't happen.

Of course, this brings up the whole issue of why viviparity has (otherwise) not evolved within Archosauria, so far as we know. Despite occasional speculative suggestions of viviparity within sauropod (see this Tet Zoo article), pachycephalosaurian (Maryańska & Osmolska 1974) and hesperornithine (Currie 1991) dinosaurs, the total absence of this reproductive mode in archosaurs has led some authors to propose the existence of various constraining factors that might prevent its evolution (Blackburn & Evans 1986, Andrews & Mathies 2000).

If metriorhynchids did evolve viviparity, then maybe those proposed constraints are not so constraining after all… or, is it that metriorhynchids somehow evolved ‘around’ them… and, if they did ‘evolve around’ them, why didn’t other groups? Questions questions questions.

There are teleosaurids too

Beautiful skeleton of Steneosaurus bollensis from the Early Jurassic of Holzmaden; image by Heinrich Mallison, used with permission.

Moving on… add Metriorhynchidae to those near-relatives Teleidosaurus and Eoneustes and you get Metriorhynchoidea (Young et al. 2010). Metriorhynchoids are closely related to another group of Mesozoic marine* crocodyliforms, the mostly Early Jurassic teleosaurids. Metriorhynchoids and teleosaurids share a suite of characters not seen in other crocodyliforms (the most obvious of which include loss of premaxillary-nasal contact and especially enlarged supratemporal fenestra) and are grouped together as the thalattosuchians.

* Mostly marine. A Chinese crocodyliform usually (but not universally) regarded as a teleosaurid – Peipehsuchus teleorhinus – is from non-marine deposits, and a few other non-marine teleosaurids have been reported as well. It hasn’t been determined whether these forms are non-marine because they’re early members of the radiation, or because they’ve switched from an ancestral marine to a specialised, freshwater lifestyle.

Teleosaurids weren’t as specialised for pelagic life as metriorhynchids were and consequently are typically considered less interesting. Their limbs and tails aren’t specialised like those of metriorhynchids, and they possess extensive dorsal and ventral coverings of osteoderms. Superficially, they mostly look gharial-like, with long, tubular snouts that look suited for rapid lateral snapping and teeth probably best for grabbing small fish and other such prey.

Machimosaurus jaw fragments. The mandibular symphysis (scale bar = 20 mm) and tooth (scale bar = 5 mm) shown at the top (from Bardet & Hua 1996) are from the Upper Jurassic of Ethiopia and were first misidentified as those of a pliosaur. The partial dentary at the bottom (from Buffetaut 1982a) is from France.

However, Machimosaurus (apparently the only teleosaurid to cross the Jurassic-Cretaceous boundary) has blunt-tipped, robust teeth that look suited for the grabbing and crushing of large or hard-bodied prey.

Some Machimosaurus remains indicate a skull length of about 1.5 m and, apparently, an incredible total length of 9.5 m (Steel 1973) [UPDATE: see comment below from Mark Young]. Lower jaw fragments from Ethiopia, originally thought to represent a new pliosaur named Simolestes nowackianus, were later shown to belong to Machimosaurus (Bardet & Hua 1996). If you don’t know what Simolestes is and need a reminder, here’s a brief article at Tet Zoo ver 2.

Some studies find the best known teleosaurid genus – Steneosaurus – to be non-monophyletic, with some species (S. pictaviensis and S. megarhinus) grouping together with Teleosaurus, and not with the other Steneosaurus species (Mueller-Töwe 2005). At least some species included within the ‘classic’ teleosaurid genera Teleosaurus and Steneosaurus look much alike, making it seem likely that Teleosauridae is a clade. However, Jouve (2009) found teleosaurid relationships to vary according to the choice of outgroup taxa, and they formed a paraphyletic series of lineages successively closer to Metriorhynchidae in some data runs. Teleosaurid species-level taxonomy is in a bit of a mess; major revision is required.

Diagram showing (top to bottom) skulls of Metriorhynchus superciliosus, Teleidosaurus bathonicus and Pelagosaurus typus, all in dorsal view (from Buffetaut 1980b). The prefrontal bones are marked in black: those of metriorhynchids are, obviously, especially large.

A small, delicately built thalattosuchian from the Lower Jurassic of Germany, France and England – Pelagosaurus typus – has dorsal and ventral osteoderms and overall looks teleosaurid-like. ‘Traditionally’, this animal is indeed a teleosaurid. However, it resembles metriorhynchoids in the proportions of some of its skull bones, its possession of a prominent sagittal crest and the shape of its scapula (Pierce & Benton 2006): for these reasons, some authors (e.g., Buffetaut 1980a, b) have regarded it as especially close to metriorhynchoid ancestry. Yet others have regarded it as the sister-taxon to the rest of Thalattosuchia (e.g., Clark 1994, Wu et al. 2001, Brochu et al. 2002). Most recently, it has been recovered as a basal teleosaurid (Mueller-Töwe 2005, Pol & Gasparini 2009). It was these conflicting ideas that led Pierce & Benton (2006) to basically say that detailed study is needed before we can better understand the position (and significance) of Pelagosaurus. To anyone who ends up doing a PhD on teleosaurids… be sure to sort Pelagosaurus out while you’re at it.

Where within the croc radiation?

Where thalattosuchians fit within the crocodylomorph radiation is controversial and there are two main competing hypotheses. In his pioneering work on crocodylomorph phylogeny, Clark (1994) found thalattosuchians to group together with the other so-called longirostrine crocodyliforms (the dyrosaurids and pholidosaurids)*; this whole assemblage was found to be closely related to the goniopholidids (a group you’ll recall from last time), Bernissartia and the eusuchians (crown-crocs and some closely related lineages).

* The name Tethysuchia – originally coined by Buffetaut (1982b) – can be applied to the dyrosaurid + pholidosaurid clade (Andrade et al. 2012). In some cladograms (as in the one shown below), Thalattosuchia is close to Pholidosauridae, meaning that thalattosuchians are also part of Tethysuchia in these topologies.

Substantially simplified version of one of Clark’s (2004) favoured most parsimonious cladograms (see his Fig. 5.2). Here, thalattosuchians are part of a ‘longirostrine clade’ (the name Tethysuchia can be applied to it) that’s only a couple of nodes away from Crocodylia, and hence deeply nested within Neosuchia. Notosuchians were recovered as a series of outgroups to Neosuchia.

According to this hypothesis, thalattosuchians are thus part of Neosuchia, the crocodyliform clade that includes crown-crocs and all crocodyliforms closer to them than to notosuchians (a major clade I intend to write about soon). A string of cladistic studies essentially support this position: see Pol & Gasparini (2009) for a full discussion (and all the citations), since a major part of their paper was devoted to this subject. In short, Clark’s hypothesis posits thalattosuchians as fairly ‘modern’ crocodyliforms, deeply nested within the radiation.

The competing position is regarded as more ‘traditional’ by some authors; the idea here is that Thalattosuchia is outside much of Crocodyliformes, and only convergently similar to the other longirostrine lineages (Langston 1973, Buffetaut 1982). This position has also been supported by several cladistic analyses (again, see Pol & Gasparini 2009); it would essentially mean that thalattosuchians originated early in crocodylomorph history from forms of ‘protosuchian’-type grade, that they are not at all close to eusuchians or eusuchian-like crocodyliforms, and that they are outside of Neosuchia. An even more extreme version of this hypothesis is apparently due to be presented soon (that is, it posits a position for Thalattosuchia even further away from the crown).

Substantially simplified cladogram - this time based on that of Sereno & Larsson (2009) - where thalattosuchians are well away from Crocodylia, and indeed wholly separate from other longirostrine taxa.

The idea that thalattosuchians might group with other longirostrine taxa due to convergent similarity is appealing, but you can’t just pretend that the characters linked with the longirostrine condition don’t exist: sure, they might exist due to convergence, but, alternatively, they might well exist due to a close evolutionary relationship. In fact, a critical look at the characters pulling the longirostrine taxa together (Pol & Gasparini 2009) showed that the majority of them occur independently of longirostry: that is, they are not the obvious convergent side-effects of the evolution of a long, slender rostrum.

One criticism levelled at analyses that put thalattosuchians far away from the crown is that these studies don’t include a representative sampling of longirostrine taxa. However, Sereno & Larsson (2009), Young & Andrade (2009), Young et al. (2012b) and other works have included what looks like a good selection of taxa, and they still found thalattosuchians to be outside of Neosuchia and far away from the more crown-ward dyrosaurids and pholidosaurids.

Rhacheosaurus, as reconstructed by Samuel Williston in 1914. This illustration was almost certainly based on the complete German specimen shown at the top of this article - look how tiny the forelimbs are. I assume that this view of metriorhynchids is unfamiliar to most people who've heard of them.

For now, disagreement remains on the phylogenetic position of thalattosuchians, and different studies are still producing contradictory results. Both proposed positions are ‘interesting’. The ‘more crown-ward’ position means that a major radiation of mostly marine, mostly longirostrine crocodyliforms, maybe not all that different from extant crocodylians in behaviour and biology, played a major role in numerous Mesozoic ecosystems and continued to do so in the Cenozoic; the ‘less crown-ward’ position means that a radically modified, pelagic radiation of sleek, cetacean-like crocodyliforms evolved from terrestrial or semi-terrestrial, relatively short-snouted crocodyliforms very different in biology and behaviour from crown crocs.

How can this controversy be resolved? I’m not sure, but I’m holding out hope for the discovery of a very early thalattosuchian, dating to the earliest Jurassic or even the Late Triassic, that provides the key anatomical information we need.

For previous Tet Zoo articles on crocodyliforms ancient and modern, see…

Refs – -

Andrade, M. B., Edmonds, R., Benton, M. J. & Schouten, R. 2012. A new Berriasian species of Goniopholis (Mesoeucrocodylia, Neosuchia) from England, and a review of the genus. Zoological Journal of the Linnean Society 163, S66–S108.

- ., Young, M. T., Desojo, J. B. & Brusatte, S. L. 2010. The evolution of extreme hypercarnivory in Metriorhynchidae (Mesoeucrocodylia: Thalattosuchia): evidence from microscopic denticle morphology and a new tri-faceted Kimmeridgian tooth from Germany. Journal of Vertebrate Paleontology 30, 1451-1465.

Andrews, C. W. 1913. A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay, Part II. British Museum (Natural History).

Andrews, R. M. & Mathies, T. 2000. Natural history of reptilian development: constraints on the evolution of viviparity. BioScience 50, 227-238.

Bardet, N. & Hua, S. 1996. Simolestes nowackianus Huene, 1938 from the Upper Jurassic of Ethiopia is a teleosaurid crocodile, not a pliosaur. Neues Jahrbuch fur Geologie und Paläontologie, Monatshefte 1996, 65-71.

Blackburn, D. G. & Evans, H. E. 1986. Why are there no viviparous birds? The American Naturalist 128, 165-190.

Brochu, C. A., Bouaré, M. L., Sissoko, F., Roberts, E. M. & O’Leary, M. A. 2002. A dyrosaurid crocodyliform braincase from Mali. Journal of Paleontology 76, 1060-1071.

Buffetaut, E. 1980a. Position systematique et phylogenetique du genre Pelagosaurus Bronn, 1841 (Crocodylia, Mesosuchia), du Toarcien d’Europe. Géobios 13, 783-786.

- . 1980b. Teleosauridae et Metriorhynchidae: l’évolution de deux familles de crocodiliens mésosuchiens marins du Mésozoïque. 105 Congres national des Societes savantes, Caen, 1980, sciences 3, 11-22.

- . 1982a. Le crocodilien Machimosaurus VON MEYER (Mesosuchia, Teleosauridae) dans le Kimmeridgien de l’Ain. Bulletin trimestrielle Societe de la géologique Normandie et Amis du Museum, Havre 69, 17-27.

- . 1982b. Radiation évolutive, paléoécologie et biogéographie des crocodiliens mésosuchiens. Mémoires de la Société Géologique de France 60,1-85.

Cau, A. & Fanti, F. 2011. The oldest known metriorhynchid crocodylian from the Middle Jurassic of North-eastern Italy: Neptunidraco ammoniticus gen. et sp. nov. Gondwana Research 19, 550-565.

Clark, J. M. 1994. Patterns of evolution in Mesozoic Crocodyliformes. In Fraser, N. C. & Sues, H.-D. (eds) In the Shadow of the Dinosaurs – Early Mesozoic Tetrapods. Cambridge University Press (Cambridge, NY, Melbourne), pp. 84-97.

Currie, P. J. 1991. The Flying Dinosaurs. Red Deer College Press, Red Deer, Alberta.

Fernández, M. S. & Gasparini, Z. 2000. Salt glands in a Tithonian metriorhynchid crocodyliform and their physiological significance. Lethaia 33, 269-276.

Jouve, S. 2009. The skull of Teleosaurus cadomensis (Crocodylomorpha: Thalattosuchia), and phylogenetic analysis of Thalattosuchia. Journal of Vertebrate Palaeontology 29, 88-102.

Langston, W. 1973. The crocodilian skull in historical perspective. In Gans, C. & Parsons, T. S. (eds) Biology of the Reptilia, Vol. 4, Morphology D. Academic Press (London/NY), pp. 263-284

Maryańska, T. & Osmólska, H. 1974. Pachycephalosauria, a new suborder of ornithischian dinosaurs. Palaeontologica Polonica 30, 45-102.

Mueller-Töwe, I. J. 2005. Phylogenetic relationships of the Thalattosuchia. Zitteliana A45: 211-213.

Pierce, S. E. & Benton, M. J. 2006. Pelagosaurus typus Bronn, 1841 (Mesoeucrocodylia: Thalattosuchia) from the Upper Lias (Toarcian, Lower Jurassic) of Somerset, England. Journal of Vertebrate Paleontology 26, 621-635.

Pol, D. & Gasparini, Z. 2009. Skull anatomy of Dakosaurus andinensis (Thalattosuchia: Crocodylomorpha) and the phylogenetic position of Thalattosuchia. Journal of Systematic Palaeontology 7, 163-197.

Sereno, P. C. & Larsson, H. C. E. 2009. Cretaceous Crocodyliforms from the Sahara. ZooKeys 28, 1-143. doi:10.3897/zookeys.28.325

Steel, R. 1973. Handbuch der Paläoherpetologie. Part 16. Crocodylia. Gustav Fischer Verlag (Stuttgart).

Wu, X.-C., Russell, A. P. & Cumbaa, S. L. 2001. Terminonaris (Archosauria: Crocodyliformes): new material from Saskatchewan, Canada, and comments on its phylogenetic relationships. Journal of Vertebrate Paleontology 21, 492-514.

Young, M. T. & Brandalise de Andrade, M. 2009. What is Geosaurus? Redescription of Geosaurus giganteus (Thalattosuchia: Metriorhynchidae) from the Upper Jurassic of Bayern, Germany. Zoological Journal of the Linnean Society 157, 551-585.

- ., Brusatte, S. L., de Andrade, M. B., Desojo, J. B., Beatty, B. L., Steel, L., Fernández, M. S., Sakamoto, M., Ruiz-Omeñaca, J. I. & Schoch, R. R. 2012b. The cranial osteology and feeding ecology of the metriorhynchid crocodylomorph genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLoS ONE 7(9): e44985. doi:10.1371/journal.pone.0044985

- ., Brusatte, S. L., Beatty, B. L., Andrade, M. B., Desojo, J. B. 2012a. Tooth-on tooth interlocking occlusion suggests macrophagy in the Mesozoic marine crocodylomorph Dakosaurus. Anatomical Record 295, 1147-1158.

- ., Brusatte, S. L., Ruta, M. & Brandalise de Andrade, M. 2010. The evolution of Metriorhynchoidea (mesoeucrocodylia, thalattosuchia): an integrated approach using geometric morphometrics, analysis of disparity, and biomechanics. Zoological Journal of the Linnean Society 158, 801-859.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jwmorenob 1:11 am 10/10/2012

    Just to enjoy being the first one to make a comment. Great post series!

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  2. 2. Dartian 3:16 am 10/10/2012

    Darren:
    If these animals were so strongly adapted for aquatic life – and given that some of them got very large (the geosaurine Plesiosuchus reached 6.9 m) – were they able to haul up on beaches to bask, or to dig nests and lay eggs? It really looks unlikely

    Perhaps the combination of very large size and aquatically-modified limbs did indeed prevent large metriorhynchids from leaving the water. But, for the record, neither large body size nor highly modified limbs are traits that by themselves prevent certain extant aquatic tetrapods from leaving the water on a regular basis. Male elephant seals are perfectly able to haul up themselves on beaches and also to fight, mate, and to move around there (rather unelegantly, perhaps, but with surprising speed); notably, they do not use their limbs when moving on land*. And the very largest extant crocodylian species, which, like the metriorhynchids, may reach total lengths of 6+ m, are also able to leave the water in order to bask and to dig nests for their eggs.

    * Are there any body mass estimates available for those largest metriorhynchids? Would they have been comparable in weight (if not in body length) to male elephant seals?

    Lower jaw fragments from Ethiopia

    I was so surprised by a reference to Mesozoic marine deposits in Ethiopia that I had to look it up. I was under the impression that in the Jurassic and the Cretaceous, Ethiopia would have been well inland as part of Gondwana, and bordering India (or perhaps even Madagascar). Apparently, my impression was wrong: Ethiopia did indeed have a marine coastline both in the Jurassic and in the Cretaceous. (India and Madagascar were still attached to Africa in the Late Jurassic but much further south than I had imagined. And present-day Somalia seems to have been submerged.)

    reconstructed by Samuel Williston in 1914

    Is that Archaeopteryx that I see boldly flying over the sea? Well that settles the dispute about its flight capabilities then! ;)

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  3. 3. Mark Young 5:06 am 10/10/2012

    Great to see you going over Thalattosuchia!

    The 9.5 m length of the Portuguese Machimosaurus hugii is a considerable overestimate. Probably by just over 2 metres.

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  4. 4. naishd 9:57 am 10/10/2012

    Dartian (comment 2) – you’re right: as I say in the article, it is indeed the combination of features (large size, proportionally tiny forelimbs, fluked tail) that suggest an absence of terrestrial behaviour in these animals. Elephant seals and modern crocodylians all have features that allow proficient terrestrial locomotion. Metriorhynchids do not.

    I’m not aware of any mass estimates for metriorhynchids, nor (so far as I’ve heard) has anyone ever tried to work them out.

    Darren

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  5. 5. Hai~Ren 10:26 am 10/10/2012

    I wonder why both groups of thalattosuchians went extinct in the Early Cretaceous, way before the pliosaurs and ichthyosaurs disappeared.

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  6. 6. Tayo Bethel 10:39 am 10/10/2012

    I’m glad to see that this brief article was nowhere near brief.

    Perhaps live birth never evolved in other archosaur groups because there was no strong selective pressure for iit it.

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  7. 7. Hastley 11:09 am 10/10/2012

    I’m still not sure, even with their suite of features, you can rule out terrestrial locomotion entirely, especially with their evidently powerful axial propulsion system. I could easily envision them doing something similar to the snake “slide-push” (used in environments that lack suitable contact points, especially if the snake is heavy-bodied and large, like an anaconda), where they use the mass of their posterior body to anchor themselves while they push the anterior portion forward, then pull the posterior forward while the anterior anchors. It’s slow and cumbersome, but ditto for sea turtles. Plus, I could actually see the hypocercal tail helping in that case.

    Ironically, I could see larger metriorhynchids doing better at this than small ones. If you’re massive, you make a bigger imprint in the sand and thus have better traction, and if you’re longer, you can traverse a greater absolute distance per cycle (all other kinematics being equal). After all, the nest doesn’t need to be much farther away from the high tide line in a big animal than a small one, but just far away enough to not be inundated.

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  8. 8. souhjiro 11:17 am 10/10/2012

    Hello!
    The metriorhinchid hind legs look very well developed, even on the giant forms, and coupled with an awkward, nocturnal beaching,with serpentine movements, on remote islands and such, could be compatible with terrestrial egglaying,and the hind limbs useful on dig a nest in the sand.

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  9. 9. OkieWhaler 1:48 pm 10/10/2012

    Darren – I loved this. Thanks for pulling it together! I have a couple thoughts/questions:

    First, what is up with the pelicaniform in the crocodyliform montage? Is that real or did you throw that in there to see if anyone was looking? If real, do we think it was a lunge-feeder?

    Second, the similarity with Cetacea is remarkable, particularly the gross skull morphology and general process/diversity (not surprisingly the taxonomy is also similar – there are only a few ways to say ‘narrow’ in Greek and “long-nosed” in Latin). You mentioned the loss of premaxillary-nasal contact, but is there any sign of posterior shift in the airways in the more pelagic crocodyliforms?

    Lastly, a comment on the evolutionary hypotheses – the radiation of cetaceans looks a lot more like your second tree than the first (e.g., Price et al 2005). That doesn’t mean anything at all about which is correct, but in many ways we are surprised to see the diversity of cetaceans given the ability for long distance movements and the lack of geographic barriers. All that to say, it has been shown that large vertebrates can radiate in a pelagic environment on a relatively rapid timescale. Certainly one can imagine niche-exploitation opportunity.

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  10. 10. kuartus 2:44 pm 10/10/2012

    Metriorhynchids even got a little bit of attention from hollywood. I believe a metriorhynchid was featured in the popular animated movie, Ice Age: The meltdown, as a villain.

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  11. 11. Hai~Ren 2:52 pm 10/10/2012

    OkieWhaler: The pelican-like croc is Stomatosuchus, from the Late Cretaceous of Egypt. Unfortunately, the only known specimen (a skull) was obliterated during a 1944 Allied bombing raid that destroyed the Munich Museum. There’s a relative, Laganosuchus, from deposits of similar age in Morocco and Niger.

    Mourasuchus, a giant caiman from the Miocene of South America, is supposedly similar in form, although it’s not closely related to the stomatosuchids.

    It’s been suggested that these may have been filter-feeders, although I don’t know if anyone has followed up on that idea.

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  12. 12. Halbred 4:09 pm 10/10/2012

    What’s odd to me is that viviparity evolves to readily in lizards and snakes but not archosaurs. Turtles, I can understand, but I don’t see what constrains would be put on archosaurs that aren’t put on lepidosaurs.

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  13. 13. OkieWhaler 4:23 pm 10/10/2012

    Thanks Hai~Ren! Damn those Allies… oh wait! I didn’t mean that. A very important unintended casualty of war, very sad. It would be interesting and fun to follow-up on feeding mechanisms given there is some groundwork from whales/pelicans. Thanks again!

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  14. 14. naishd 4:38 pm 10/10/2012

    Thanks for all the great comments. Those of you interested in comparing thalattosuchian evolution with that of cetaceans might be interested to know that cetaceans have been covered quite a bit on Tet Zoo. See…

    All the whales of the world, ever (part I)

    All the whales of the world, ever (part II)

    Darren

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  15. 15. LeeB 1 10:10 pm 10/10/2012

    Cetaceans, Metriorhynchids ,Ichthyosaurs and Mosasaurs all seem to have a similar evolutionary pattern of adapting to marine life.
    They start off smallish, rapidly diversify, develop a tail fin and begin to develop much larger species.

    Metriorhynchids didn’t get as large as the others but they were the only ones evolving in the presence of larger competitors, with Pliosaurs already occupying the macropredator niche.
    It would be interesting to know just how large Aggiosaurus was though.

    Even though they retain hindlimbs that look as though they could possibly dig a nest, for the larger species there would be problems moving away from any completed nest with their bulk and dragging tail tending to compress sand to the point that any eggs would be crushed.

    So they really do look like they had found a way to be fully aquatic with live birth (or possibly ovoviviparity) as part of the lifestyle.

    Incidentally turtles, which have to come ashore to breed never seem to approach the size of the other marine reptiles, they also do not have the problem of a large dragging tail compressing the finished nest as they leave.

    LeeB.

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  16. 16. Dartian 3:46 am 10/11/2012

    Darren:
    Elephant seals [...] have features that allow proficient terrestrial locomotion.

    But are they skeletal features? If elephant seals were only known as fossils, how could we tell that they weren’t exclusively aquatic?

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  17. 17. naishd 5:42 am 10/11/2012

    Dartian – yes, I’ve had this argument about elephant seals before (smiley). They possess forelimb, hindlimb and pelvic features that – if the animal was wholly extinct and only known from bones – would still be indicative of a terrestrial ability (in my opinion, anyway). In fact, Mirounga has more ‘terrestrial correlates’ in its limbs than many other phocids. This stuff is only hinted at in the literature, by the way, and hasn’t been the focus of any specific technical study. I’m aiming to write it up as a short paper (you might know that I’ve published on pinnipeds in the past; have also spent a lot of time with phocid skeletal material).

    Darren

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  18. 18. Dartian 6:53 am 10/11/2012

    Darren:
    They possess forelimb, hindlimb and pelvic features that – if the animal was wholly extinct and only known from bones – would still be indicative of a terrestrial ability

    Fair enough – but I still can’t entirely shake off the feeling that there is something rather post hoc about this line of reasoning (note that these are just general musings; they’re not directed against you or your forthcoming paper!). I mean, we know for a fact that seals are able to move on land – because we can observe them doing so. If we didn’t know that (in the hypothetical case of all phocids – not just the elephant seals – being extinct and only known as fossils), would we even know what to look for in terms of signs of terrestrial abilities?

    (Conversely, if the sea otter Enhydra lutris was extinct and only known as a fossil, would palaeontologists be able to conlude that it did, in fact, not come ashore regularly?)

    /Devil’s advocacy mode off.

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  19. 19. naishd 7:04 am 10/11/2012

    I wholly agree that it’s difficult to evaluate a hypothesis like this without the (conscious or unconscious) benefit of hindsight… That aside, I am of the opinion that forelimb, hindlimb and pelvic characters seen in pinnipeds – and in sea otters – indicate proficient terrestrial abilities that we could discern even if the animals were wholly extinct, whereas the absence of such features in some Mesozoic marine reptile groups indicate lack of terrestrial abilities entirely. I’ve decided not to elaborate on the details, otherwise I’ll end up writing the better part of the paper concerned (smiley).

    Darren

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  20. 20. josimo70 7:52 am 10/11/2012

    I know there’s not necessarily a “right” answer, but… why long necks in sauropterygians/plesiosaurs (evolving twice, at least) and not in sea crocs or mosasaurs? Any anatomical clue?

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  21. 21. Heteromeles 10:10 am 10/11/2012

    Here’s a question that might actually start to get at whether sea crocs laid eggs.

    The question was whether there were atolls in the Mesozoic. The answer (from a hasty reading, cf http://coral.aims.gov.au/speciesPages/html/key/reefs-mesozoic.html) seems to be “yes in the Jurassic, but perhaps not so much in the later Cretaceous.” In the later Cretaceous, there’s a messy confluence of rapid sea level variation with ocean acidification that favored rudist clams over corals, and it’s not clear that rudist-dominated reefs were as tough as coral-dominated reefs (again, from my hasty reading).

    Still, if we’re postulating that sea crocs laid eggs on oceanic island beaches or atolls, it’s reasonable to ask whether those atolls even existed. It looks like they did, but more in the Jurassic than the Cretaceous. Then again, it looks like sea crocs existed more in the Jurassic than the Cretaceous. While this is likely coincidence, it could suggest that they laid their eggs on atolls, and that this might have limited their survival in the Cretaceous.

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  22. 22. Jerzy v. 3.0. 1:30 pm 10/11/2012

    Marine crocs might slither like eels. Or slither and roll slightly to the sides, pulling themselves on short flippers, like Clarias fish which moves on land on their gill covers.

    Unless they had underdeveloped lower ribcage which would be crushed on land, for example.

    At less than 7 m they are within weight range of the largest marine turtles.

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  23. 23. Jerzy v. 3.0. 1:30 pm 10/11/2012

    PS. Speaking of crocodiles.

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  24. 24. eldri 8:50 pm 10/11/2012

    In comment 18. Dartian asked:
    (Conversely, if the sea otter Enhydra lutris was extinct and only known as a fossil, would palaeontologists be able to conlude that it did, in fact, not come ashore regularly?)
    ****They *Used to* come ashore, according to old reports. They were hunted to near extinction, and only those populations which Never (or almost never) came ashore surivived. ‘Culture’ is a pretty strong factor in animals as intelligent and social as sea otters.
    I expect that, as numbers grow, we may begin to see some individuals exploring onshore–

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  25. 25. LeeB 1 9:59 pm 10/11/2012

    Deep diving air breathing vertebrates have collapsible ribcages to deal with the water pressure at depth, this would seem to be a problem on land yet elephant seals manage to breathe while ashore none the less.
    Perhaps their ability to lift their chests up off the ground helps with this.
    I wonder if anything is known about whether Metriorhynchid rib cages could collapse, which might tell us whether they were deep divers or not; and also if their back vertebrae would allow the dorsal flexion required to lift their thoraxes off the ground.

    LeeB.

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  26. 26. Tayo Bethel 11:25 pm 10/11/2012

    The speculative comments are getting more and more interesting …
    If metriorhynchids did indeed lay eggs on land rather than giving birth to live young, it would imply something about their parental behavior. Being ill equipped to move around on land, nest guarding would seem unlikelys. Furthermore, the newly hatched metriorhynchids would have to struggle into the oceanwithout assistance or protection of any kind.

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  27. 27. Tayo Bethel 11:35 pm 10/11/2012

    LOL excuse the typos

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  28. 28. Dartian 3:07 am 10/12/2012

    LeeB 1:
    Incidentally turtles, which have to come ashore to breed never seem to approach the size of the other marine reptiles

    Didn’t Archelon have an estimated body weight of well over 2,000 kg?

    Darren:
    I’ve decided not to elaborate on the details, otherwise I’ll end up writing the better part of the paper concerned

    You naughty tease, you! ;)

    Heteromeles:
    it could suggest that they laid their eggs on atolls, and that this might have limited their survival in the Cretaceous

    An interesting idea; however, how can it be reconciled with the fact that sea turtles (all of which presumably had to come ashore somewhere to lay their eggs) started do diversify in the Cretaceous?

    Eldri:
    only those populations which Never (or almost never) came ashore surivived

    But we can’t really tell from their skeletal anatomy alone which populations regularly came ashore and which did not, can we? (That was my point with bringing up sea otters in this discussion.)

    Tayo:
    the newly hatched metriorhynchids would have to struggle into the oceanwithout assistance or protection of any kind

    It would be a costly reproductive strategy, for sure. But sea turtles have adopted it anyway. Sea kraits too*.

    * At least I presume so; I don’t know how well documented the reproductive biology of Laticauda (or other sea snakes, for that matter) actually is. There is an abundance of such footage for sea turtles, but has the hatching and ‘race to the sea’ by young sea kraits ever been filmed in the wild?

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  29. 29. Dartian 3:49 am 10/12/2012

    An addendum to my previous comment, regarding the weight of Archelon. It might be of interest to put the size of this extinct turtle in the context of extant large water-living tetrapods that are demonstrably capable of coming ashore.

    According to Wikipedia, the estimated body mass of the largest Archelon specimens is about 2,200 kg. For comparison, and still according to Wikipedia (yes, I was being lazy!):

    -A male southern elephant seal typically weighs 2,200-4,000 kg;
    -A male northern elephant seal typically weighs 1,500-2,300 kg;
    -A male walrus typically weighs 800-1,700 kg;
    -A male hippopotamus typically weighs 1,500-1,800 kg;
    -The heaviest reliably measured saltwater crocodiles (with total lengths well exceeding 6 m) have weighed surprisingly little – just slightly over 1,000 kg.

    (Note that those body masses for the elephant seals, walrus, and hippo represent typical weights; the maximum weights for all these species are, of course, significantly greater.)

    Thus, assuming that Archelon‘s weight hasn’t been grossly overestimated, it would seem that we’re talking here of a chelonian the size of a hippo (and much heavier than even the largest extant crocodile). Just some food for thought. ;)

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  30. 30. LeeB 1 4:43 am 10/12/2012

    The largest turtles like Archelon, Cratochelone and Stupendemys do seem to be heavier than the largest known Metriorhynchids, but their carapace would give then support while on land, whereas the limbs of metriorhynchids and the attachment of the femur to the pelvis do not seem capable of supporting their weight while ashore; also the turtles do not have long heavy tails which would drag over their nests.
    Other crocodilians can stand up high enough on their hind limbs to keep the tail clear of the nest.
    It is interesting that Metriorhynchids did not reach lengths in excess of 10 metres, unlike plesiosaurs, ichthyosaurs and mosasaurs; but perhaps competition with pliosaurs was the reason.

    However the teeth of Aggiosaurus are reportedly bigger than those of Plesiosuchus so it should have been longer than 7 metres.

    LeeB.

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  31. 31. naishd 4:50 am 10/12/2012

    One brief thing to say on some of the comments so far… we can’t yet offer hard data, and my apologies for being a bit arm-wavy here, but eels, walking catfish (comment 22) and sea-turtles all possess features consistent with their occasional terrestrial excursions. Eels are hugely muscular (the whole body is massively muscled and highly flexible) and the only species that travel across land are less than 1 m or so long. Walking catfish, again, have highly muscular, flexible bodies and pectoral fins suited for terrestrial locomotion. Turtles have powerful, proportionally large flippers and the plastron supports their weight ventrally.

    Now, compare all of this with what’s present in metriorhynchids. These are not fish, they don’t have a massively muscled, highly flexible body. The tail is going to be very powerful, for sure, but I find it very doubtful that an animal built like this is able to wiggle, squirm, flip or twist around on land to any useful degree. The limbs are often pretty small, proportionally, and their pectoral and pelvic girdles are also small and in fact rather dainty.

    Empirical work is needed, but I really get the impression here that these animals were not built to come on to land. The argument is sounding very similar to the one that went on for years with regard to plesiosurs. In the end, we discovered a pregnant specimen that demonstrates viviparity.

    Darren

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  32. 32. Dartian 5:19 am 10/12/2012

    Darren:
    I find it very doubtful that an animal built like this is able to wiggle, squirm, flip or twist around on land to any useful degree

    So you don’t think they could behave like the killer whales in Patagonia, which intentionally beach themselves in order to catch pinnipeds? (Sorry, I know that now I’m just being mean. But I couldn’t resist.)

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  33. 33. naishd 5:50 am 10/12/2012

    Dartian (comment 32): sure, maybe they could flop around at the edge of the water (a bit of behaviour that’s also been speculatively attributed to pliosaurs). That’s not the same as climbing up above the high-water line to dig a nest though, is it?

    Darren

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  34. 34. Dartian 6:04 am 10/12/2012

    Darren:
    That’s not the same as climbing up above the high-water line to dig a nest though, is it?

    No, of course not. But it’s behaviour for which it would be very hard or perhaps even impossible to find any osteological/fossil correlates. That’s my point.

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  35. 35. naishd 6:13 am 10/12/2012

    Well, I think you’re throwing things off track here (smiley).

    Issue 1. Could a big, wholly extinct aquatic tetrapod – a predatory cetacean, a metriorhynchid, a pliosaur, a mosasaur – have grabbed prey animals from the water’s edge, and then flopped, twisted and thrashed until it got itself back to safety? Sure, YES, several species of extant cetacean show that this behaviour is plausible, and you can’t say anything definitive about its presence or absence based on fossils.

    Issue 2. Could any one of those animals – a metriorhynchid, say – have climbed/shuffled/dragged itself up a beach, away from the water’s edge, and far enough away from the high-water mark to dig a nest where the eggs wouldn’t be drowned? I’m saying… probably not.

    On eggs being drowned, we do actually know that some shelled eggs (those of certain pleurodires in particular) can survive submergence for a short while without the embryos drowning, but I think these are a very special case.

    Of relevance to this discussion, incidentally, are the very weird trace-fossils, made by big, limb-dragging quadrupeds of some sort, discovered in certain Mesozoic beach sediments. No reason to think that they were made by metriorhynchids, however.

    Darren

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  36. 36. Jerzy v. 3.0. 8:52 am 10/12/2012

    Hi Darren,

    Sorry me, I meant not Clarias but Anabas testudineus fish (climbing gourami) which uses not fins but gill coverts. It moves ungainly but rather effective, turning to its sides most of the time. See BBC Wild India documentary, episode deserts.

    I am not sure there is no misunderstanding. Your argument seems to center around the weak/strong limb and pectoral girdle skeleton. However many vertebrates move on land without using limbs at all. Seagoing crocodile would be very ungainly on land, but so are sea turles.

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  37. 37. Jerzy v. 3.0. 8:57 am 10/12/2012

    @29
    It stuck me that some extinct crocodiliomrphs would have smaller legs and bigger size than C. porosus, but are assumed to move on land the normals way.

    BTW, maybe tail fin with a skeleton in its lower part in some reptile groups is evolutionary result of semi-aquatic ancestors, where a fin with fleshy lower part would be squashed when out of water?

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  38. 38. David Marjanović 10:43 am 10/12/2012

    * Young et al. (2010) argued that the loss of the external mandibular fenestra and shortening of the retroarticular process in metriorhynchids indicate reduction of the jaw muscles normally involved in thermoregulatory jaw-gaping. It therefore seems that metriorhynchids didn’t (and/or couldn’t) gape to thermoregulate, and thus didn’t (and/or couldn’t) indulge in terrestrial thermoregulatory behaviour.

    See comment 8: try again at night.

    (Conversely, if the sea otter Enhydra lutris was extinct and only known as a fossil, would palaeontologists be able to conlude that it did, in fact, not come ashore regularly?)

    Geological context would of course help. One reason we’re so sure Tulerpeton didn’t come ashore was that the nearest shore was 200 km away.

    Also, Enhydra is able to come ashore, and the skeleton shows that just fine.

    What’s odd to me is that viviparity evolves to readily in lizards and snakes but not archosaurs. Turtles, I can understand, but I don’t see what constrains would be put on archosaurs that aren’t put on lepidosaurs.

    Archosaur embryos get their calcium from the eggshell, not from the yolk. So, there has to be a hard eggshell.

    Though… I don’t actually know if that’s really true for crocodiles.

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  39. 39. naishd 11:00 am 10/12/2012

    The ‘calcium constraint’ just alluded to by David has indeed been mentioned as one reason ‘preventing’ archosaurs from evolving viviparity. Same has also been suggested for gekkotans – this works so long as soft-shelled and viviparous lineages are outside the clade that includes the hard-shelled clade.

    Darren

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  40. 40. Yodelling Cyclist 11:23 am 10/12/2012

    @Heteromeles

    I’m not sure I follow the requirements for atolls: certainly sea turtles and seals (covering both terrestrial reproduction scenarios) are capable of breeding on continental beaches. Biological reefs (of any sort) are not required, certainly, for green sea turtles breeding on Pacific Mexican beaches.

    The huge fresh water snapping turtles also cope without biological reefs, yet have similar issues with leaving the water.

    My money’s on Darren being right, btw, for two reasons:

    1.) Those are really weedy limbs compared with the power in a turtle or a seal forelimbs.
    2.) It’s Darren.

    Have hesperornithean nests been found, btw? There’s another archosaur that will have really sucked on land.

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  41. 41. David Marjanović 1:22 pm 10/12/2012

    this works so long as soft-shelled and viviparous lineages are outside the clade that includes the hard-shelled clade

    Is that an allusion to the pt word?

    Have hesperornithean nests been found, btw? There’s another archosaur that will have really sucked on land.

    A bonebed interpreted as a nesting ground has been found, IIRC, but not any eggshells or nests. That’s an old memory, though.

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  42. 42. Tayo Bethel 2:01 pm 10/12/2012

    If memory serves, the most specialized aquatic archosaurs alive today–aquatic birds–all have trouble moving around on land. Even in the most extreme cases though, either the forelimbs or the hind limbs are strong enough to assist in locomotion. Could metriorhynchids have progressed–very clumsily, of course–by usingjust their hind limbs to push themselves along?

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  43. 43. vdinets 3:56 pm 10/12/2012

    eldri: sea otters still regularly come ashore in many places in the northern part of their range, i. e. on Commander Islands and Kamchatka (where predation by brown bears is an issue). I think they don’t do it in California because of the risk of overheating. I don’t think this behavior could have been reduced by hunting, because industrial-scale sea otter hunting was always done from boats.

    Dartian: I’ve seen one site off Sabah where sea kraits come to breed. It’s a beach completely covered with flotsam, broken coral, and seashells. The kraits lay eggs under larger logs, and I think the hatchlings can get to the water by moving from one piece of cover to another, never exposing themselves.

    Tayo: the most aquatic extant birds are arguably penguins, and they can march across the Antarctic for unbelievable distances. Divers/loons can’t stand on their legs, but they can crawl on land for at least 20 m, possibly more.

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  44. 44. souhjiro 9:09 pm 10/12/2012

    @37

    In all the videos of crocodiles and marine iguanas swimming actively, the tail more distal part tends to droop slightly downwards even when the motion is very vigorous.Perhaps was also the same on the similarly shaped ancestors of metriorhynchids, mosasaurs and ichthyosaurs and this is the reason of the three groups ending with the same curious tailfin design with the vertebrae on the lower lobe

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  45. 45. LeeB 1 9:48 pm 10/12/2012

    Apologies for being off topic but people interested in marine reptiles may like to know that the latest issue of the Norwegian Journal of Geology is out, with lots of papers on new Plesiosaurs, Pliosaurs and Ichthyosaurs from Svalbard.
    One of the papers sorts out (for now) the taxonomy of Pliosaurus.

    And it is all open access.

    Interestingly there seem to be no Metriorhynchids in the Svalbard fauna, I wonder why?
    Water temperature or some other reason.

    LeeB.

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  46. 46. Heteromeles 11:32 am 10/14/2012

    The idea about atolls was that Metriorhynchids (or all sea crocs) were so clumsy on land that nesting on the mainland, where they could get chomped by a theropod, would be a non-starter. Oceanic islands such as atolls are, in modern times, refuges for species with vulnerable reproductive cycles, from turtles to albatrosses, and there’s no reason to think the same wouldn’t have been true in the Mesozoic. If atolls existed during Mesozoic, which was the unquestioned assumption I wanted to bring out.

    Reef composition has changed drastically and repeatedly over geologic time, and it seems (again from hasty reading) that corals makes better reefs and atolls than rudist clams do. If ocean chemistry favors the clams, there are going to be fewer (or no) atolls, and anything that depends on them as refuges is going to be out of luck. We’re going into a similar regime now, where sea level rise and dropping pH selecting against coral reef growth will combine to cost us a lot of really nice islands in the coming centuries.

    The counter-argument for this idea (pointed out above) is that sea turtles seem to manage to nest on the mainland as well as islands, and it seems that they have been able to do this since at least the Tertiary (wasn’t Andrewsarchus found with turtle remnants?) and probably it was true in the Mesozoic as well. Assuming this is the case, being marginally competent in moving on land appears to be no barrier to nesting on mainland beaches.

    I think the one useful thing to come from playing with this idea is that fluctuating sea levels and low sea pH (as in the later Cretaceous) seem to select against atolls, and they may also select against species that have to haul out to give birth, especially if they are limited to a few beaches that can be inundated. It might be fun and useful to see whether these swings correlate with megafaunal shifts in oceanic predators.

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  47. 47. naishd 12:22 pm 10/14/2012

    Cretaceous atolls: good idea. Firstly, note that thalattosuchians were a Jurassic, more than a Cretaceous, group. Secondly, there is plenty of evidence for atolls in the Cretaceous: here’s a paper on Lower Cretaceous atolls; here’s another; here’s a paper on Jurassic atolls.

    Darren

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  48. 48. Heteromeles 2:10 pm 10/14/2012

    @Darren: Agreed. The sea-level weirdness started later in the Cretaceous, after the sea-crocs were (apparently) gone. There seemed to have been plenty of atolls during the Jurassic, and I’d pitch the notion that these might have been breeding grounds for sea-crocs.

    There was an earlier point about sea turtles undergoing some sort of radiation during the Cretaceous. I don’t know anything about that, but our current regime of oceanic acidification and overfishing seems to favor jellyfish over corals. Since many sea turtles eat jellies, it’s at least theoretically possible that a mid-Cretaceous bloom of jellyfish might have allowed diversification of jelly-eating turtles. This is, of course, pure speculation, but again, I don’t have a clue what giant sea turtles might eat, either. Giant Cretaceous jellyfish (Megacyanea hypothetica?)

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  49. 49. David Marjanović 6:54 pm 10/14/2012

    I don’t have a clue what giant sea turtles might eat, either

    Ammonites. Archelon had serious jaws.

    The current increase in jellyfish seems to be due to overfishing: if you take the vertebrates away, the jellyfish are the top predators, as they were in the Cambrian.

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  50. 50. Dartian 11:28 pm 10/14/2012

    David:
    “Enhydra is able to come ashore, and the skeleton shows that just fine.

    Yes, but that was precisely what I was getting at: this animal’s skeletal anatomy and its actual behaviour are not in close agreement. For example, if the sea otter was only known as a fossil, I don’t think that any palaeontologist could tell – or even suspect – that it usually gave birth to its young in water and not on land (unlike both pinnipeds and other otters).

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  51. 51. eldri 3:03 pm 10/15/2012

    Reply to: vdinets (in 43.)
    Thank you– it is good to know they are back to land.

    The accounts of I was thinking of Were of onshore clubbing, (moms and pups)-those went Fast, and *Early*.
    …”low hanging fruit” gets picked first..I supect there may not have been too many to begin with, most of them perhaps on islands(bears, as you say, and natives )
    By the time demand became Huge, they were Gone.

    Kelp bed populations were harder to find, lasted longer.

    California also has few offshore Islands compaired with futher North.

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  52. 52. vdinets 10:05 am 10/16/2012

    eldri: The 18th century population is estimated to have been around 300,000, including 16,000 in California.

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  53. 53. Gigantala 2:31 pm 10/18/2012

    Hi everyone. This article and the comments brings about many interesting implications, so here’s a list (some of them are questions):

    1- If thalattosuchians diverged before Neosuchia, does this mean that these animals could ostensibly be endothermic? Crocodyllians are, after all, often interpreted as having evolved from endothermic ancestors, notosuchians in particular being almost mammal like, so would thalattosuchians have been “warm blooded”?

    2- Similarly, given the whole lip controversy on notosuchians, is the whole suction feeding indicative that metriorhynchids actually had lips?

    3- The atoll dependency also brings interesting questions about pelagic pteroaurs. If ornithocheirids and the like were dependent on atolls, their disappearence might explain their absence in the later Cretaceous.

    4- Vivipary/ovovivipary might not had been necessary. Metriorhynchids could had just developed specialised cloacal pouches to hold the eggs, like a more specialised version of the emperor penguin pouches. Likewise, they could have restricted their broods to a few large eggs, as the young would have less to fear from predators.

    5- Is there any particular convergent traits between thalattosuchians and champsosaurs? Both lineages seem very similar in several anatomical aspects.

    6- What is the third phylogenetic possibility that makes places them further apart?

    Link to this
  54. 54. Michał 11:48 am 10/21/2012

    6- What is the third phylogenetic possibility that makes places them further apart?

    Not sure if we can already talk about this year SVP annual meeting abstracts, but the hypothesis I suspect is hinted at here was in fact presented for the first time back in 2010: Wilberg (2010) recovered thalattosuchians as non-crocodyliform crocodylomorphs.

    Ref:
    Eric Wilberg (2010) “The phylogenetic position of Thalattosuchia (Crocodylomorpha) and the importance of outgroup choice” Journal of Vertebrate Paleontology 30 (Suppl.), p.187A

    Link to this
  55. 55. David Marjanović 8:50 pm 10/23/2012

    I wonder if anything is known about whether Metriorhynchid rib cages could collapse

    Bone histology should tell whether they regularly did: if not, we’d expect pachyosteosclerosis (very small marrow cavity, bones looking “inflated” but being very massive to act as ballast that counteracts the lungs); if yes, we’d expect osteoporosis (spongy bones, so that the whole animal with collapsed lungs has about the density of water).

    However, even human ribcages can collapse safely. One or two extreme deep-diving athletes do that regularly.

    Wilberg (2010) recovered thalattosuchians as non-crocodyliform crocodylomorphs.

    Yes, and an SVP talk this year says this always happens if you put enough outgroups into your analysis.

    Link to this
  56. 56. David Marjanović 10:09 pm 10/23/2012

    (…as opposed to either using a protosuchian as the outgroup, making it impossible for the analysis to find the thalattosuchians as non-crocodyliforms, or using Gracilisuchus as the only outgroup even though it’s probably pretty far away from Crocodylomorpha. Adding Postosuchus to either of the big published matrices changes things.)

    Link to this
  57. 57. Michał 7:55 am 10/24/2012

    Nonetheless, I do wonder whether this is similar to the results of Gauthier et al. squamate analysis, where dolichosaurs, aigialosaurs and mosasaurids were recovered as non-scleroglossan scincogekkonomorphs, far away from both varanids and snakes (a result that suspisciously wasn’t replicated when mosasaurids were excluded, or when amphisbaenians and dibamids were excluded). But then, all I have now are Wilberg’s SVP abstracts, so I guess I will wait for the paper.

    Link to this
  58. 58. David Marjanović 10:33 am 10/24/2012

    In that case, we’re looking at convergence between elongate, limb-reduced animals that share states of lots of correlated characters. I don’t know how such an effect could move Thalattosuchia away from the other sea & freshwater crocodiles to within just a few nodes from Postosuchus.

    Link to this
  59. 59. Michał 11:30 am 10/24/2012

    In that case, we’re looking at convergence between elongate, limb-reduced animals that share states of lots of correlated characters.

    That’s one of the problems, yeah, but when all “krypteians” were included in the analysis, and mosasaurids were excluded, dolichosaurs and Aigialosaurus at least moved into Scleroglossa, close to Varanoidea and “Krypteia”. Conversely, without dolichosaurs and Aigialosaurus mosasaurids stayed outside Scleroglossa. Doesn’t that mean that there’s something in mosasaurid anatomy that’s also responsible for such an unorthodox result?

    Link to this
  60. 60. David Marjanović 1:45 pm 10/26/2012

    Hm. Interesting.

    Link to this
  61. 61. Yodelling Cyclist 2:35 pm 10/27/2012

    Uh, possibly a dumb question, but anyway here goes: if these maritime crocs are endothermic as has been mooted above, and these animals are performing live birth, how is sex selection being achieved without egg temperature variations (as happens in modern crocodiles). My understanding was that all modern crocs lacked sex chromosomes.

    Also, re Gigantala’s 4th suggestion: the pouch would need to be very highly modified to somehow prevent the egg and embryo from being drowned.
    Something just doesn’t seem to add up in this picture, unless some marine crocs developed sex chromosomes, or they’ve been lost in modern crocs.

    Link to this
  62. 62. David Marjanović 2:05 pm 10/28/2012

    unless some marine crocs developed sex chromosomes, or they’ve been lost in modern crocs

    …both of which are entirely possible.

    Link to this
  63. 63. Yodelling Cyclist 10:32 pm 10/29/2012

    I’m amazed, are sex chromosomes really gained and lost so easily in tetrapods?

    Link to this
  64. 64. Finback 4:41 am 11/6/2012

    “Of relevance to this discussion, incidentally, are the very weird trace-fossils, made by big, limb-dragging quadrupeds of some sort, discovered in certain Mesozoic beach sediments. No reason to think that they were made by metriorhynchids, however.”

    Marine gorgonopsians.

    Link to this

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