Paleontology Newsflash: Paraxenisaurus normalensis and Stellasaurus ancellae


There's a starsaur waiting in the ground/ He'd like to come and meet us/ But he thinks he'd blow our minds.

The great 2020 saurischian streak has broken! Finally we get a new bird-hipped dinosaur, and it's a ceratopsian. I'm including this new horned dinosaur, Stellasaurus, along with another recently discovered and also fragmentary new taxon from Mexico: Paraxenisaurus. Without further fuss and ado . . . .

Paraxenisaurus

  • We know this animal only from partial remains of the hands, feet and ankles, and a few fragments of the tail, recovered from a few different individuals in the Cerro del Pueblo formation of Coahuila, Mexico. It's enough to build a decent case for deducing its place in the dinosaur family tree, but not much more than that so far. Phylogenetic analysis places it in the deinocheirid group of ornithomimosaurs, making it the first representative known from North America. This makes it a surprising find, though it doesn't completely topple all hypotheses of this group and associated North American fauna. During the mid- to later Cretaceous, many animal groups crossed back and forth between North America and Asia, so the prospect of a North American deinocheirid was always plausible, if unexpected.

  • What can we say about Paraxenisaurus itself? Well, it was big. We can't say how big, but it has some adaptations for carrying a bigger body. Some of its toe bones almost look like a tyrannosaur's, not unlike Deinocheirus itself. Those two groups developed similar foot designs for speed, and so when they got big, they developed some of the same features convergently for dealing with the problems of bigness.

  • Generally speaking, ornithomimosaurs split into two groups. The best known, ornithomimidae, focused on speed, stayed relatively small, and had compacted metatarsals that could deal with acceleration. Gallimimus, as popularized by Jurassic Park, and Ornithomimus, as seen at the Dinosaur Park, belong to this group. Deinocheirids adopted gigantism for defense, and what features for speed they couldn't co-opt for other uses, they lost. We haven't been able to study them nearly as well, so any evidence for this group helps clarify our knowledge concerning them.

  • As for determining what it looked like, well, I've already seen professed "reconstructions" based on related animals. That's never a good idea, and I can't consider such art reconstructive in any way. Deinocheirus itself offers up a lot of good reasons against drawing anything but fingers and toes for this taxon at this point. I'll let the authors that described the recent discovery of more Deinocheirus skeletal material at the type quarry explain: "The discovery of the original specimen almost half a century ago suggested that this was an unusual dinosaur, but did not prepare us for how distinctive Deinocheirus is—a true cautionary tale in predicting body forms from partial skeletons, even for animals in which the relationships are known." If you see a picture labeled as Paraxenisaurus and it shows more than fingers and toes, take it only as speculation, not reconstruction.

  • Paraxenisaurus joins a slough of new taxa named over the past few years from the Cerro del Pueblo formation, including the ceratopsians Coahuilaceratops and Yehuecauhceratops, the hadrosaurs Velafrons and Latirhinus, and the nodosaur Acantholipan. So far we only have fragmentary skeletons for most of these taxa, so there's still a lot to learn about this area of Cretaceous Mexico.

  • The name Paraxenisaurus means "strange reptile." Which is funny, because even if future discoveries make it out to be a real weirdo, it still has to compete with Deinocheirus, which is a bonafide master of the bizarre—an ornithomimosaur with a spinosaur sail and a hadrosaur ducky bill. The name of Utah's state fossil, Allosaurus, also means "strange reptile," it just uses different Greek words for the same meaning. Perhaps that's why the irony of its species name tickles my sense of poetry; "normalensis" refers to a geographic region, and it's pronounced with the stress on the "mal" as appropriate for Spanish, but it combines with the genus name to translate as "the strange reptile from Normal." And that is awesome.

Stellasaurus

  • Before getting into Stellasaurus itself, it'll help to know some technical background. Dinosaur names do not refer to dinosaurs . . . that is, they don't apply to the original animals that lived and died during the Mesozoic era. These names apply to taxa. Think of a taxon as a model made up of everything that we know about a particular population of similar organisms. As a model, a taxon is also a hypothesis, and scientists work to disprove hypotheses. New discoveries can overturn taxa at any time, even popular ones: remember Brontosaurus? Some taxa prove more stable than others under testing. That doesn't make disproved taxa mistakes; it's just part of the scientific process. For clarity's sake, we try to avoid naming new taxa without as solid a case as possible, but sometimes we can't learn anything without hazarding a hypothesis and seeing how the tests turn out.

  • Scientists first referred bones of Stellasaurus to the now defunct genus Rubeosaurus. Like most taxonomic histories, the story gets technical and convoluted really quickly, but here are the most important points. Most scientists seem to agree that Styracosaurus is at the base of an evolutionary branch that somehow leads to Pachyrhinosaurus. Einiosaurus, Achelousaurus, and now Stellasaurus occur at different levels in the rocks between Styracosaurus and Pachyrhinosaurus. Scientists regard them with different opinions as to their validity, usually driven by two competing hypotheses of what happened in those middle levels. One hypothesis posits a branching evolution, known as cladogenesis. The other, which this study "fails to reject" (in science, we try to refute hypotheses, and when we fail to refute them, they become the strongest point of view) represents anagenesis, a condition where one genus leads directly to another in a direct line of descent. This study proposes Stellasaurus as a "missing link" between Styracosaurus and the later links in this hypothesized evolutionary chain. Since Stellasaurus as a genus has arisen from a larger theoretical framework, challenges to this framework could affect its validity in the future.

  • The holotype—that is, the first specimen referred to the genus that forms its ultimate basis for comparison—consists of parts of the frill and top of the skull. That's not much to go on, but since most of horned dinosaur taxonomy centers on skull features, it may prove adequate. However, only one side of the frill survived. A recent paper describing a pathological specimen of Styracosaurus might represent a different challenge to this taxon. This Styracosaurus skull was not symmetric: one side of the frill sported an extra horn. Since some of the other taxa on this evolutionary line are likewise based on scanty evidence, Stellasaurus may come and go with the fate of the others for a while before we can test it against a better set of data. Sometimes that's just how science has to work. So Stellasaurus might not be with us for long. Or it might. Time and more fossils will tell.

  • The name Stellasaurus refers to the star-like shape of its frill, what with all those spikes jutting off the edges. It also refers—and I swear I'm not making this up—to David Bowie's song, "Starman." The only rules to naming new taxa involve naming taxa after the author(s) of the study. This particular song doesn't have much to do with the dinosaur, but that makes no difference to the science. Scientific names don't have to make sense, leading to all sorts of fun inside jokes. If ever you're up for a little Coffee Talk: Basilosaurus is neither a saurian, nor basil. Discuss.

Serrano-Brañas, C. I., Espinosa-Chávez, B., Maccracken, S. A., Gutiérrez-Blando, C., de León-Dávila, C., & Ventura, J. F. (2020). Paraxenisaurus normalensis, a large deinocheirid ornithomimosaur from the Cerro del Pueblo Formation (Upper Cretaceous), Coahuila, Mexico. Journal of South American Earth Sciences, 102610.

Lee, Y. N., Barsbold, R., Currie, P. J., Kobayashi, Y., Lee, H. J., Godefroit, P., ... & Chinzorig, T. (2014). Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature, 515(7526), 257-260.

Wilson JP, Ryan MJ, Evans DC. 2020 A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the ‘Styracosaurus-line’ dinosaurs. R. Soc. Open Sci. 7: 200284.

Holmes, R. B., Persons, W. S., Rupal, B. S., Qureshi, A. J., & Currie, P. J. (2020). Morphological variation and asymmetrical development in the skull of Styracosaurus albertensis. Cretaceous Research, 107, 104308.

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