This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
If you’re interested in the wildlife of the past – particularly in the many varied reptiles of the Mesozoic Era – then you’re surely already a big fan of the Triassic, the stage of Earth history that extended from 250 to 200 million years ago. True, it wasn’t a time dominated by giant, long-necked dinosaurian behemoths like those of the Jurassic and Cretaceous (the sauropods), nor did bipedal super-predators (like allosauroid and tyrannosaurid theropods) stalk the landscape. There were no feathery theropods, no horned or armour-plated ornithischians, and no mega-huge soaring, striding super-pterosaurs.
But the reptiles that did inhabit the Triassic world were just so much more enigmatic. An incredible panoply of weird and spectacularly weird beasts stalked the Triassic world: some of the more remarkable ones have no close post-Triassic relatives or counterparts. What’s more, giant sauropods and enormous theropods are well known compared to many of the beasts of the Triassic – these creatures were not just strange, but also mysterious to the extreme in terms of biology, behaviour and lifestyle. So much about them has yet to be discovered.
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Research on Triassic animals and the communities they belonged to has always bubbled away in the background of Mesozoic palaeontological research. However, a series of major discoveries made in recent years have revealed a previously unappreciated complexity to Triassic tetrapod diversity and evolution, and have helped make discoveries in the Triassic big news. We now know that the archosaur clade that includes modern crocodilians included a number of bipedal, superficially dinosaur-like Triassic lineages as well as a radiation of bizarre, sail-backed forms. Various Triassic archosaurs previously thought to be early ornithischian dinosaurs have proved not to be dinosaurs at all, a discovery that has fundamentally changed our ideas about dinosaurian diversity and evolution in the Triassic. Indeed, new studies have added further support to the idea that dinosaurs were ‘victors by default’ and that crocodile-group archosaurs exhibited more morphological (and behavioural?) disparity than Triassic members of the dinosaur clade (e.g., Brusatte et al. 2008, 2010).
The Triassic story is most certainly not limited to dinosaurs and other archosaurs – heavens, no – but my point here is that it is these discoveries that have mostly put Triassic studies into the limelight. This research has occurred against a background of exciting work on faunal and floral turnovers, mass extinctions and changing climatic conditions, and debates about faunal and stratigraphic correlations. Should you wish to better keep up with news on the Triassic and its animals, keep an eye on the blogosphere: there’s Chinleana, Triassic Critters, Madygen & Co. and The Forgotten Archosaurs for starters.
Yes, these are exciting times for anyone interested in the Triassic and its wildlife. Some of us even like to speak of an intellectual movement that seeks to appreciate and promote Triassic life. We call it Scientific Triassicism* and there’s even a t-shirt to prove it (hat-tip here to Tet Zoo stalwart Neil Kelley of microecos who started Scientific Triassicism here**).
* Not Global Triassicism as I stupidly said on twitter. I’m at @TetZoo, by the way.
** No no no, I am completely wrong. See comments!
Where might you go if you want a good, substantially comprehensive source of information on the life of the Triassic? The reason we’re here is that Hans-Dieter Sues and Nicholas Fraser recently published Triassic Life on Land: the Great Transition (Sues & Fraser 2010). While not really designed for the lay-reader, this well illustrated volume - written by two highly respected workers, well known for their many technical projects on Triassic tetrapods - takes us on a chronological tour through the terrestrial fossil record of the Triassic (note: no coverage of the extensive marine record). An introductory chapter discusses Triassic stratigraphy and palaeogeography and also brings the reader up to speed on tetrapod phylogeny. You aren’t treated to any sumptuous reconstructed Triassic vistas (bar one obligatory Doug Henderson piece between the Preface and Introduction, and the one on the cover), but the book does feature skeletal reconstructions, photos, drawings and maps throughout.
If I had to summarise this book I’d say that it’s a concise, global review of current knowledge of terrestrial Triassic biotas, written for advanced students and other researchers. The bibliography is huge: 34 pages long. If you’ve read any literature on Triassic biotas, you’re likely familiar with the Chinle Formation of the western US, the Newark Supergroup of eastern North America and the Lossiemouth Sandstone of Scotland. But what about Triassic assemblages from elsewhere in the world? Have they yielded the same sorts of fossils, or is there evidence for impressive provinciality in the Triassic world?
As just one example, how many people outside of the Triassic research community have heard about the fossils from the Argana Valley in Morocco? Metoposaurs, Placerias-like dicynodonts and a rauisuchian (Arganasuchus) are known from here. The beauty of the ‘global coverage’ provided here is that biotas from all those more obscure geological units get written about and are not ignored.
How many weird Triassic reptiles is enough?
I know full well that tetrapods aren’t the only living things that exist, neither today nor in the Triassic, but it’s difficult not to regard reptiles and other tetrapods as the stars of the Triassic stage. And while we always have to remember that members of those sexy Triassic tetrapod groups didn’t all live side-by-side, it’s difficult not to imagine the Triassic world as the most incredible menagerie imaginable. Temnospondyls, weird, early turtles, procolophonoids, tree-climbing drepanosaurids, tubby, beaked rhynchosaurs, protorosaurs, trilophosaurs, quadrupedal, vaguely crocodilian-like proterochampsids and phytosaurs, armour-plated aetosaurs, predatory ornithosuchids, rauisuchians and a variety of leggy early crocodilians, silesaurids. Then there are dinosaurs and their close relatives, the early pterosaurs, all those synapsid groups…
You’ve heard about all of these groups, of course. What about some of the really special unfamiliars of the Triassic? One of my favourites is the incredible Teraterpeton hrynewichorum, named by Sues (2003). This animal – apparently an archosauromorph of some sort (perhaps related to trilophosaurs) – is long-snouted and with both a long edentulous region at the front of the jaws and transversely broad, complex teeth lining its jaws further back. It has deep, blade-like hand claws. What about Mecistotrachelos – a slender-necked reptile perhaps related to protorosaurs like Tanystropheus (Fraser et al. 2007)... but equipped with wings! Then there’s Uatchitodon, known only from its blade-like, groove-bearing teeth. These grooves – open and gutter-like in some specimens but completely closed but for a small aperture near the tooth tip in others – strongly suggest that this reptile (whatever it was) had a venomous bite.
Among the other remarkable reptiles of the Triassic are the gliding, superficially Draco-like kuehneosaurid lepidosauromorphs, known from the Upper Triassic of both the British fissure fill deposits and from the Lockatong Formation of New Jersey. How these kuehneosaurid taxa should be named and classified continues to be the source of disagreement. Stein et al. (2008) recently argued that the British specimens differ enough in wing shape to warrant recognition as the distinct taxa Kuehneosaurus and Kuehneosuchus, though they also noted that the two might be sexual dimorphs. One slightly controversial idea that Sues & Fraser (2010) promote is that the name Icarosaurus should be used for the British specimens (it’s otherwise been used only for the Lockatong Formation taxon I. siefkeri), and they further say that Kuehneosaurus is a nomen nudum since Robinson (1962) neither assigned a holotype nor provided a diagnosis.
The section on Madygen Formation fossils is of special interest (to me, anyway) for its coverage of the incredibly weird Sharovipteryx and Longisquama. Any discussion of Longisquama just has to include a section on its abuse at the hands of those who would wish it to be potential bird ancestor. I wish it didn’t need to be said again, but there is nothing bird-like at all about this animal, it fundamentally lacks any characters that might make it especially bird-like, those ridiculous structures on its back are not feather-like except in the most gross superficial sense (Reisz & Sues 2000), and the idea that it’s more bird-like than any non-avialan theropod, or even a sauropodomorph or ornithischian, is difficult to appreciate. We’re not even sure what Longisquama is – it seems to be a diapsid, but beyond this it’s not yet possible to say anything with confidence about its affinities.
Sues & Fraser (2010) say all of this too, albeit in less vitriolic fashion. Regarding the use of Longisquama as a possible bird ancestor, I like what Luis Chiappe had to say about this and those other Triassic taxa mistakenly favoured as potential bird ancestors by the BANDits*: “In the end, claims for the origin of birds from this stock of small Triassic reptiles seems to adopt a formula more apt for the world of art than for scientific endeavour, as the identity of the object (the ancestor) is defended more by its symbolism than by its own physical attributes” (Chiappe 2007, p. 39).
* Birds Are Not Dinosaurs.
Big picture stuff
Toward the end of their book, Sues and Fraser review the biotic changes and extinction events that occurred before and during the Triassic. Again, a very nice, concise review of these areas.
But I find it notable that they’re somewhat sceptical of claims made by other specialist Triassic researchers. To start with, they argue that the end-Permian event wasn’t really as severe as some authors (Mike Benton in particular) have made it out to be, though Sues and Fraser are hardly the first to argue this of course. They point to evidence showing that certain ‘Triassic’ plant clades are not Triassic novelties, but actually originated in the Permian and survived across the Permo-Triassic boundary. Furthermore, we now know that procolophonoids, temnospondyls and a few other tetrapod groups didn’t fare so badly across the boundary. These discoveries seemingly lessen the impact of the end-Permian extinction.
As for events later in the Triassic, recent studies have mostly supported Benton’s argument that dinosaurs were not ‘destined for success’ when compared to other Triassic archosaurs and tetrapods. Rather, it seems that luck was a big factor and that dinosaurs simply took advantage of ecological opportunities made available when poor fortune befell contemporaneous archosaur clades (Benton 1983, Irmis et al. 2007, Brusatte et al. 2008, 2011a, b). Sues and Fraser, however, say that “the situation is less clear-cut” (p. 180) and point to evidence from growth rates which implies that dinosaurs and their close kin may indeed have had some sort of evolutionary edge against contemporaneous archosaurs. I don’t know. I can agree with Sues and Fraser that “Testing hypotheses concerning competition often poses considerable challenges even in present-day ecosystems” and that “it comes as no surprise that it is difficult, perhaps even impossible, to test such hypotheses convincingly in the fossil record” (p. 180), but I still think that the evidence for historical contingency in the rise of dinosaurs and decline of other archosaurs is pretty good.
Incidentally, Sues and Fraser use the term Crurotarsi for crocodile-group archosaurs. That was fine when they were writing the book and many other authors have done this too. I’ve always preferred Crurotarsi to the massively baggage-laden and etymologically inappropriate Pseudosuchia… unfortunately, the latter name is branch-based (Crurotarsi is node-based) and thus superior in terms of ‘capturing’ taxa (Brochu 1997, Senter 2005). However, a major recent study (Nesbitt 2011) found phytosaurs to be outside crocodile-group archosaurs, and in fact even outside crown-group Archosauria (the crocodile + bird clade) altogether. That’s a problem for nomenclature, as Crurotarsi was specifically defined to include phytosaurs by Sereno (1991). If we follow both Sereno’s (1991) phylogenetic definition and Nesbitt’s (2011) phylogeny, the name Crurotarsi now goes to the phytosaur + crown-archosaur clade, a usage completely inconsistent with tradition. Exactly what we should do about this hasn’t yet been resolved (patience, young ones) but, whatever, we should stop using Crurotarsi for members of the crocodilian clade.
There are insects and plants too?
I liked Sues and Fraser’s coverage of tetrapods, of course, but I’m especially appreciative of what they say about other animals, and also about plants. Take the amazing titanopterans from Australia and Kyrgzstan. These orthopteran-like insects were adapted for fast running, possessed large stridulatory structures on their forewings, large spurs on their limb segments that were probably used in catching prey and, in cases, had wingspans of at least 40 cm. Numerous insects fossils from the Solite Quarry on the state line between Virginia and North Carolina show that early relatives of craneflies, midges, thrips and scorpionflies were all living in the Late Triassic.
Plants. I bet most of you interested in Mesozoic animals think that plants are boring and that the palaeobotanical bits of the book are the ones you’ll be skipping. I’m not necessarily going to disagree, but do note that artists of prehistoric scenes have long struggled to fine good reference material on Mesozoic flora. Indeed, it’s somewhat telling that one of the best resources on the life appearance of Mesozoic plant taxa is a colouring book produced for children (Long & Welles 1990). Sues and Fraser synthesise a large amount of information on Triassic plants that’s otherwise widely scattered in the literature, and include diagrams of fronds and other plant parts needed for scientific reconstruction. It’s a shame that there isn’t more of this sort of stuff in the literature – we often need more information on ‘background biota’ when trying to understand the possible ecologies, lifestyles and behaviours of our favourite animals.
Is Sues & Fraser (2010) the definitive, comprehensive volume we’ve been waiting for on the wildlife and world of the Triassic? To be honest, when these animals are mentioned, it’s often all too briefly and you’re reminded that the book is about reviewing faunal and floral assemblages, not about discussing individual taxa in depth. Nevertheless, there’s enough coverage that you won’t want to miss the book should you find these creatures interesting. Also, some of their discussions include novel, even controversial opinions.
All in all, Triassic Life on Land: the Great Transition is an exemplary review of current knowledge of the world’s terrestrial Triassic fossil record. It’s a great resource (and great distillation of the literature) for specialist researchers, but should also prove of significant interest to anyone else who really likes the Triassic and its fossils.
Sues, H.-D. and Fraser, N. C. 2010. Triassic Life on Land: the Great Transition. Columbia University Press (New York), 280pp. ISBN 978-0-231-13522-1.
For previous Tet Zoo articles on Triassic animals, see...
Tet Zoo picture of the day # 4 (Tanystropheus)
Your Triassic armadillodiles (aetosaurs)
The armadillodile diaries, a story of science ethics (more aetosaurs)
The surprising and hitherto undocumented late survival of non-dinosaurian dinosauromorphs
Tet Zoo picture of the day # 26 (placodonts)
One of so many bizarre Triassic marine reptiles (Helveticosaurus)
The 'Birds Come First' hypothesis of dinosaur evolution (some mention of Longisquama)
An extreme environment invaded by an 'extreme' marine reptile: Henodus part II
Refs - -
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Brusatte, S. L., Benton, M. J., Ruta, M. & Lloyd, G. T. 2008. Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs. Science 321, 1485-1488.
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