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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Awesome sea-going crocodyliforms of the Mesozoic

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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).

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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.

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

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