May 14, 2013 | 27
Tet Zoo readers with supernatural memories will doubtless recall the January 2012 article ‘Rigid swimmer’ and the Cretaceous Ichthyosaur Revolution (part I) [link below]. I’ll refresh your memory by telling you that the article was all about the PLoS ONE paper on Acamptonectes, a Cretaceous ophthalmosaurid ichthyosaur from the UK and Germany described by Valentin Fischer, Michael Maisch, Jeff Liston, myself and a team of colleagues (Fischer et al. 2012).
Acamptonectes is a close relative of the ophthalmosaurine ophthalmosaurid Ophthalmosaurus, and the persistence of its lineage into the Early Cretaceous is significant in demonstrating that yet another ophthalmosaurid lineage survived across the Jurassic-Cretaceous boundary. The old idea that ichthyosaurs suffered an end-Jurassic extinction event is no longer supported by the data. As we explained in the Acamptonectes paper (Fischer et al. 2012), it now seems that something like nine (NINE) different ophthalmosaurid lineages of the Late Jurassic (in other words, all the ophthalmosaurid lineages known from the time) survived into the Cretaceous (see also Druckenmiller & Maxwell 2010, Fisher et al. 2011a, b, Zammit 2012). Since 2012, the ophthalmosaurine Leninia stellans has been described from the Lower Cretaceous of Russia (Fischer et al. 2013a), adding a tenth lineage to the roster.
I finished the Tet Zoo Acamptonectes article by saying that more on this story is set to appear soon – more, that is, on the matter of ichthyosaur persistence into the Cretaceous. That day has finally arrived, since the latest issue of Biology Letters includes a paper by Valentin, myself, Jeff Liston and our co-authors on another new Cretaceous ichthyosaur.
This newest ichthyosaur is Malawania anachronus and it’s from the Hauterivian-Barremian part of the Lower Cretaceous (Fischer et al. 2013b). And it’s from Iraq, making it only the second ichthyosaur to be reported from the Middle East and the first post-Triassic one (the other one is a mixosaurid-like Triassic form from Saudi Arabia (Vickers-Rich et al. 1999)). We’re excited by the fact that Malawania really does change our understanding of ichthyosaur diversity and disparity in the Cretaceous. How so? Read on.
The long Malawania back-story
Malawania’s back-story is lengthy and convoluted, stretching, in fact, over six decades. Yes, it isn’t really a new discovery at all, but another of those fossils that was discovered a while ago, worked on intermittently by an interested party, but – otherwise – predominantly ignored and left in museum storage. Journalists like to make gentle fun of these sorts of stories, implying that palaeontologists are idiots for not noticing the new (and sometimes potentially really exciting) species that are metaphorically sitting in front of them. The fact remains that scientists are frequently already working themselves to death, are over-committed, are busy churning out projects from over-long to-do lists, and are already confronted with scores of other exciting new species that also need to be worked on.
Malawania’s history starts in 1952 when the single known specimen – a nice, articulated, partial skeleton that consists of much of the animal’s front half (though with only the back part of the skull preserved) – was discovered by a team of British oil geologists (D. M. Morton, F. R. S. Henson, R. J. Wetzel and L. C. F. Damesin) while they were working at Chia Gara, Armadia, in the Kurdistan region of Iraq (Fischer et al. 2013b, ESM*). The slab that contains the fossil had been transported an unknown distance and was being used to dam a small river and as a stepping-stone for a mule track. People and pack animals were thus routinely walking over the fossil, though they didn’t damage it so far as we can tell. As you can see from the photo above, the specimen was preliminarily prepared while still in the field.
* Electronic Supplementary Material.
Morton and colleagues recognised the potential significance of the specimen and transported it back to the UK, donating it in 1959 to the august institution now known as the Natural History Museum, London. At this stage, the specimen became known to the only British scientist really interested in ichthyosaurs during the 1950s and 60s: Robert M. Appleby, then based at University College in Cardiff, Wales.
Appleby worked, on and off, for years on the Iraqi ichthyosaur. He was impressed with the fact that it seemed to be ‘archaic’ and mostly similar to the famous Lower Jurassic west European ichthyosaur Ichthyosaurus (more on this in a minute). What really slowed his progress, however, is that he was never able to satisfactorily resolve the specimen’s age. Seeing as it’s preserved on a loose slab that didn’t match the adjacent strata, its exact stratigraphic origin was never obvious. Aspects of the anatomy led Appleby to wonder if it was Rhaetian (Upper Triassic) in age, but (supposed) experts who had worked in the immediate region strongly advised Appleby that the specimen ‘must’ have come from the Jurassic Sargelu Formation, and the argument became whether the specimen was Middle or Upper Jurassic in age. The first plant microfossils extracted from the slab’s matrix were said to indicate a Lower Cretaceous origin, and this so conflicted with the strong advice that Appleby had received that it seemed more likely that some mistake had been made with the sample (Fischer et al. 2013b, ESM). If the animal really was Ichthyosaurus-like, then a Middle Jurassic origin seemed far more likely than a Cretaceous one, and it was this opinion that Appleby preferred.
Appleby did actually get as far as submitting a manuscript on the specimen to the journal Palaeontology in 1979. Unfortunately, the unresolved nature of stratigraphic provenance was deemed unsatisfactory by one of the paper’s reviewers, and Appleby was essentially sent back to the drawing board. He continued throughout the 1980s and 90s in his efforts to get the issue resolved but never succeeded; other ichthyosaur projects occupied his time right up to his death in 2004.
As I’ve mentioned on a few occasions, I and colleagues have been working for the past several years on the many ichthyosaur-themed manuscripts that Appleby worked on during his lifetime but was never able to complete. Acamptonectes was one of those (Fischer et al. 2012); Malawania is another. Leading this endeavour is Jeff Liston, currently of the National Museums Scotland, Edinburgh. Jeff assembled a team that was able to do what Appleby had not: resolve the age of the specimen as well as analyse it within a modern phylogenetic framework.
Jeff took new samples of matrix from the specimen and had them properly processed. They yielded excellent microfossils including dinoflagellate cysts, pollen and spores (Fischer et al. 2013b, ESM). As you’ll know if you’re at all familiar with microfossils, these fossils are typically highly specific to particular segments of geological history, are abundant and easily preserved, and are fairly uniform in morphology and hence easy to identify to species (if you know what you’re doing). Extracting and studying them properly is therefore essential stuff if you want to date a chunk of sediment.
And the news was good. Among the microfossils preserved in the same slab as Malawania is the dinoflagellate cyst Muderongia staurota, a species unique to the Hauterivian and Barremian parts of the Early Cretaceous. Two pollen taxa are also present in the slab – both of which are typical of the Early Cretaceous – as are three spores, all of which are – again – typical Early Cretaceous taxa (Fischer et al. 2013b, ESM). Based on this microfossil assemblage, we have to conclude with some degree of confidence that the Malawania slab is from the late Hauterivian or the Barremian.
Malawania is an Early Cretaceous ichthyosaur, not a Jurassic one.
An ‘Early Jurassic’ ichthyosaur in the Cretaceous
Here we come back to what I said above about Malawania being similar to the Early Jurassic European ichthyosaur Ichthyosaurus. We now know of quite a few Cretaceous ichthyosaurs, but what’s notable is that all of them belong to Ophthalmosauridae, the major clade of thunnosaurian parvipelvian* ichthyosaurs that includes Acamptonectes mentioned earlier, as well as the more famous, widely distributed taxa Ophthalmosaurus and Platypterygius (though, for the record, note that both of these genera as conventionally understand are almost certainly not monophyletic).
* Parvipelvia is the node-based ichthyosaur clade – named by Motani (1999) – that includes Hudsonelpidia, Macgowania, Ichthyosaurus and all descendants of their most recent common ancestor. All members of clade possess substantially reduced, narrow pubic bones, a widened radius, and a tibia and fibula that are similar in size and shape.
Ichthyosaurs might not have undergone a devastating drop in diversity at the close to the Jurassic as traditionally thought, but it does still seem that ophthalmosaurids were the one and only group that persisted beyond the Middle Jurassic.
Until now. There’s no doubt that Malawania is not an ophthalmosaurid. It lacks all the derived features of this advanced, tuna-shaped group and – compared to the members of Ophthalmosauridae – is archaic in overall anatomy. Unlike ophthalmosaurids, the forefin bones in Malawania form a tightly packed mosaic of hexagonal elements and the forefin itself is tetradactyl and lacks accessory digits, notching is present on the leading digit, and the scapula lacks a prominent acromion (Fischer et al. 2013b).
Yet in these features and others, Malawania is similar to the parvipelvians of the Early Jurassic and even Late Triassic. In our phylogenetic analysis – the most comprehensive yet produced for Parvipelvia – Malawania is recovered as the sister-taxon to Ichthyosaurus, a discovery which requires the existence of a ghost lineage nearly 70 million years long (Fischer et al. 2013b).
At the insistence of our reviewers (there’s an especially tortuous back-story to the treatment this paper received at the hands of certain of its reviewers, but that’s a story I won’t relate here), we didn’t just include Malawania within our own novel analysis; we also coded it for inclusion in the analyses produced by just about all other contemporary ichthyosaur workers. Malawania wasn’t consistently recovered as the Ichthyosaurus sister-taxon in all of the resulting trees (note, however, that character and taxon sampling in these other analyses is poorer than in ours). It was, however, consistently and confidently found to be outside of Ophthalmosauridae, and was frequently surrounded by taxa that belong to lineages with a Late Triassic or Early Jurassic origin.
What does this all mean? It means that ophthalmosaurids weren’t the only ichthyosaurs that persisted into the Cretaceous after all: close relatives of Ichthyosaurus from the Early Jurassic did too (we opted to be conservative and did not use the name Ichthyosauridae for the Ichthyosaurus + Malawania clade, but this decision would seem justifiable based on our current understanding).
Ophthalmosaurids were diverse, strongly adapted for pelagic life, globally distributed, and appear to have given rise to new species at a fairly rapid rate. Meanwhile, members of the Malawania lineage seem to have been extremely rare, extremely low in diversity, restricted in distribution, and without clear pelagic adaptations. Furthermore, the apparent conservatism in Malawania’s lineage indicates a sort of long evolutionary stasis – a real contrast to the general trend seen elsewhere throughout Mesozoic marine reptiles whereby lineages appear to steadily and continually become more streamlined, more efficient, and more specialised for pelagic life throughout their history.
This doesn’t mean that Malawania is boring and without peculiarities. Among its diagnostic characters are a horn-shaped projection on the posterodorsal part of the humerus, an especially short, squat humerus, and trapezoidal neural spines on the cervical and anterior dorsal vertebrae (Fischer et al. 2013b). However, in general form and the majority of its known anatomical details, Malawania was an anachronism: an ‘Early Jurassic-style’ ichthyosaur that had survived well into the Cretaceous.
Perhaps Malawania and its ancestors were rare, lagoon-dwelling specialists that hung on in low-latitude refugia different from the sorts of habitats frequented by ophthalmosaurids and certain other marine reptile groups. In fact, Late Jurassic and Early Cretaceous Iraq appears – from palaeoenvironmental reconstructions – to be well suited to serve as such a ‘safe haven’, since it was a partially enclosed marine basin for much of this time (Jassim & Goff 2006).
A new phylogeny for Parvipelvia
Our phylogenetic analysis confirms many details recovered in previous studies but does somewhat modify our understanding of parvipelvian evolution. Previous analyses have tended to arrange parvipelvians in a pectinate arrangement: that is, where each taxon represents an additional step on a phylogenetic tree that has ophthalmosaurids at its ‘tip’.
We do find a pectinate arrangement at the base of Parvipelvia (where Hudsonelpidia and Macgowania represent successive sister-taxa to Neoichthyosauria), and another near the origin of Thunnosauria (where Suevoleviathan and Hauffiopteryx represent successive sister-taxa to Thunnosauria, and where Stenopterygius and Chacaicosaurus* are successive sister-taxa to Ophthalmosauridae). However, a novel feature of our analysis is the recovery of a clade that includes temnodontosaurs as well as leptonectids (Fischer et al. 2013b) – an intuitively pleasing discovery, given the similar gestalt of these often big, often especially long-snouted, long-flippered ichthyosaurs.
* This taxon is regarded by some authors as a synonym of Stenopterygius, and both are indeed quite similar. However, the fact that the two do not group together when included in the same analysis suggests that they are best retained as distinct genera.
Within Thunnosauria, studies have either found Ichthyosaurus or Stenopterygius to be closest to Ophthalmosauridae, with the hypothesis of a Stenopterygius + Ophthalmosauridae clade being most frequently recovered (Godefroit 1993, Maisch & Matzke 2000, Druckenmiller & Maxwell 2010, 2013, Caine & Benton 2011, Fischer et al. 2011b, 2012, 2013a). We also support this relationship, and indeed Stenopterygius is more ophthalmosaurid-like than other thunnosaurians in the configuration of its skull bones, the large acromion on its scapula and its reduced chevron bones.
We feel it’s appropriate to give this clade a name and we call it Baracromia, a name that means ‘heavy acromion’ (Fischer et al. 2013b). Baracromia is a node-based clade anchored on Stenopterygius quadriscissus and Ophthalmosaurus icenicus, and defined to specifically exclude Ichthyosaurus communis. The idea here is that, should future studies find a topology where Ichthyosaurus is closer to ophthalmosaurids than is Stenopterygius, Baracromia self-destructs. Of course, we’ve named the Stenopterygius + Ophthalmosauridae clade because we’re fairly confident that it’s real and will persist in future studies.
Disparate origins and conservative relicts
So, Malawania changes things. Cretaceous ichthyosaurs were not all members of the same single radiation after all. In fact, members of two distinct, distantly related groups both made it across the Jurassic-Cretaceous boundary, meaning that Cretaceous ichthyosaur taxa have disparate evolutionary origins. Malawania also has implications for the rate of evolutionary change seen in ichthyosaurs, since it seems that members of some lineages could be surprisingly conservative on occasion.
Was this sort of thing more widespread, or is Malawania truly exceptional? You may recall recent work showing that the mostly small, mostly lagoonal and estuarine leptocleidid plesiosaurs are late-evolving novelties (Benson et al. 2012), not conservative relicts as previously argued. It isn’t lost on me that Malawania seems to represent the very opposite sort of situation: it is a conservative relict, not a Cretaceous novelty. But that’s ok. The world is complicated.
Is there more to come on the story of ichthyosaur diversity and disparity across the Jurassic-Cretaceous boundary? Yes, yes there is.
Another article on the Malawania back-story has been penned by my co-author Jeff Liston and is available here on Mr Wood’s Fossils. Do check it out.
For previous articles on ichthyosaurs and some of the other issues discussed here, see…
Refs – -
Benson, R. B. J., Ketchum, H. F., Naish, D. & Turner, L. E. 2012. A new leptocleidid (Sauropterygia, Plesiosauria) from the Vectis Formation (Early Barremian-early Aptian; Early Cretaceous) of the Isle of Wight and the evolution of Leptocleididae, a controversial clade. Journal of Systematic Palaeontology DOI: 10.1080/14772019.2011.634444
Caine, H. & Benton, M. J. 2011. Ichthyosauria from the upper Lias of Strawberry Bank, England. Palaeontology 54, 1069-1093.
Druckenmiller, P. S. & Maxwell, E. E. 2010. A new Lower Cretaceous (lower Albian) ichthyosaur genus from the Clearwater Formation, Alberta, Canada. Canadian Journal of Earth Sciences 47, 1037-1053.
- . & Maxwell, E. E. 2013. A Middle Jurassic (Bajocian) ophthalmosaurid (Reptilia, Ichthyosauria) from the Tuxedni Formation, Alaska and the early diversification of the clade. Geological Magazine doi: 10.1017/S0016756813000125
Fischer, V., Arkhangelsky, M. S., Uspensky, G. N., Stenshin, I. M. & Godefroit. 2013a. A new Lower Cretaceous ichthyosaur from Russia reveals skull shape conservatism within Ophthalmosaurinae. Geological Magazine doi: 10.1017/S0016756812000994
- ., Clément, A., Guiomar, M., & Godefroit, P. 2011a. The first definite record of a Valanginian ichthyosaur and its implication for the evolution of post-Liassic Ichthyosauria. Cretaceous Research 32, 155-163.
- ., Maisch, M. W., Naish, D., Kosma, R., Liston, J., Joger, U., Krüger, F. J., Pérez, J. P., Tainsh, J. & Appleby, R. M. 2012. New ophthalmosaurid ichthyosaurs from the European Lower Cretaceous demonstrate extensive ichthyosaur survival across the Jurassic-Cretaceous boundary. PLoS ONE 7(1): e29234. doi:10.1371/journal.pone.0029234
- ., Masure, E., Arkhangelsky, M. S. & Godefroit, P. 2011b. A new Barremian (Early Cretaceous) ichthyosaur from western Russia. Journal of Vertebrate Paleontology 31, 1010-1025.
- ., Appleby, R. M., Naish, D., Liston, J., Riding, J. B., Brindley, S. & Godefroit, P. 2013b. A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs. Biology Letters 9, 20130021 http://dx.doi.org/10.1098/rsbl.2013.0021
Godefroit, P. 1993. The skull of Stenopterygius longifrons (Owen, 1881). Revue de Paléobiologie de Genève volume spécial 7, 67-84.
Jassim, S. Z. & Goff, J. C. 2006. Geology of Iraq. Dolin, Brno, Czech Republic.
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McGowan, M. 1995. A remarkable small ichthyosaur from the Upper Triassic of British Columbia, representing a new genus and species. Canadian Journal of Earth Sciences 32, 292-303.
Motani, R. 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19, 473-496.
Sander, P. M. 2000. Ichthyosauria: their diversity, distribution, and phylogeny. Paläontologische Zeitschrift 74, 1-35.
Vickers-Rich, P., Rich, T. H., Rieppel, O., Thulborn, R. A. & McClure, H. A. 1999. A Middle Triassic vertebrate fauna from the Jilh Formation, Saudi Arabia. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 213, 201-232.
Zammit, M. 2012. Cretaceous ichthyosaurs: dwindling diversity, or the Empire Strikes Back? Geosciences 2, 11-24.