A paper in Nature, “A new hypothesis of dinosaur relationships and early dinosaur evolution,” presents fresh ideas about dinosaur relationships that reveal the extent to which dinosaur traits are not distributed in a treelike pattern. One news article calls this a “radical shakeup of the dinosaur family tree” because it would overturn a century of evolutionary thinking about dinosaurs:
The analysis, which has already sparked controversy in the academic world, suggests that the two basic groups into which dinosaurs have been classified for more than a century need a fundamental rethink. If proved correct, the revised version of the family tree would overthrow some of the most basic assumptions about this chapter of evolutionary history, including what the common ancestor of all dinosaurs looked like and where it came from.
The basic issue is this: For the past hundred years, dinosaurs were classified into two primary groupings. Dinosaurs within Ornithischia, which have hips like a bird, and dinosuars within Saurischia, with hips like a lizard. Before you read any further, don’t presume that these old divisions foreshadow the now-popular theory that birds evolved from dinosaurs: the theropod dinosaurs, the group from which birds supposedly evolved, belong to the “lizard-hipped” Saurischia and are not “bird-hipped”! This obviously bothered some proponents of the bird-to-dino hypothesis.
The new scheme aims to fix this annoying problem. Under the new classification, theropods are now grouped with dinos that used to be within the bird-hipped Ornithischia, such as Stegosaurus and Triceratops. This new designation, “Ornithoscelida,” is supposed to create a fundamental group of dinosaurs that is less hostile to the dino-to-bird theory.
The grouping of course pleases longtime dino-to-bird advocates like Kevin Padian, president of the National Center for Science Education. He defends the classification in a News & Views article in Nature, stating that “their results cannot be dismissed as simply a different opinion or speculation.”
Indeed, The Guardian reports that people are already using this new classification scheme to imagine feathered dinosaurs where they don’t exist — on totally non-feathered types of dinosaurs like Stegosaurus and Triceratops! Consider these comments by the study’s lead author, Cambridge University graduate student Matthew Baron:
The findings also support the possibility that dinosaurs such as Stegosaurus and Triceratops, traditionally portrayed as tank-like armoured beasts, may have been feathered.
“Maybe we did have fluffy Triceratops and fluffy Stegosaurus,” said Baron. “It could be that the feathers would have been poking out between the scales, it could have been a beautiful fluffy colourful plumage … or scales covered in downy feathers. It’s possible.”
Such unwarranted speculation hints at what agendas may truly be driving this new classification scheme.
In any case, the official reason for the new view of dinosaurs is that it better explains the distribution of traits among various dinosaur species. For example, the technical paper explains that the old Ornithischia/Saurischia division required convergent evolution to clarify the hand anatomy of early dinosaurs — a problem the authors claim to solve:
Recent studies have led to a general consensus that the earliest dinosaurs were relatively small and bipedal, and this idea finds further support within our hypothesis, as both basal sauropodomorphs and basal ornithoscelidans are small bipeds. Manus anatomy in many early dinosaurs also appears to be very similar, with supinated, non-weight-bearing, ‘grasping’ hands appearing in basal saurischians such as Herrerasaurus and basal ornithoscelidans such as Heterodontosaurus and Eoraptor. As pointed out in several previous studies, these similarities were often considered to represent convergences given the supposedly distant relationship between taxa such as Heterodontosaurus and Herrerasaurus. Within our new framework, the supinated, grasping hands seen in some early taxa are interpreted as the primitive dinosaurian condition.
But solving one problem sometimes creates another, and it does so here. By reorganizing major parts of the dinosaur tree, evolutionary paleontologists are now confronted with the prospect of rampant convergent evolution among traits found in various carnivorous dinosaurs as required by their new phylogeny. The technical paper in Nature explains these difficulties:
This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians. … Herrerasauridae is recovered as the sister clade to Sauropodomorpha, suggesting that some of the theropod-like features of their anatomy have evolved independently of those found in theropods. This is most likely a direct result of their fully carnivorous feeding strategy; in our hypothesis a fully carnivorous feeding strategy is not recovered as the plesiomorphic condition for Dinosauria and we are forced to interpret some of the anatomical similarities between herrerasaurids and theropods as convergences. The convergent evolution of hypercarnivore morphology within Dinosauria raises interesting questions about the drivers of early dinosaur evolution. For example, did a dentition composed exclusively of sharp, recurved and serrated teeth, such as those that are present in representatives from both of these clades, evolve independently of each other? The earliest representatives of each of the major dinosaur clades often possess at least some recurved, serrated teeth, most commonly as part of a heterodont dentition. However, no known members of Sauropodomorpha or Ornithischia exhibit dentitions that are exclusively composed of recurved, serrated teeth, nor does the early theropod Eoraptor. Hence, it seems probable, within our new framework, that at least some of the recurved, serrated teeth that make up the dentition of derived theropods and herrerasaurids have convergently adopted this morphology. Furthermore, the rostral extension of the dentary tooth row appears also to be convergent between theropods and herrerasaurids; in members of both clades, the dentary tooth row extends to the rostral tip of the dentary.
And then of course there is the fact that lizard-hipped dinosaurs are now separated into two different groups. Presumably that also would require convergent evolution.
Convergent evolution is a problem for Darwinian evolution because it means that biological similarity does not necessarily result from inheritance from a common ancestor. This undermines the basic logic used to construct phylogenetic trees, and casts into doubt the entire project of tree-construction.
The reality is that no matter what classification scheme you use, a dinosaur tree is going to require convergent evolution. This is because key dinosaur traits are not distributed in a tree-like manner.
Because of the convergent evolution it requires, the new hypothesis has already proven controversial. As Nature News reports:
Hans-Dieter Sues, a vertebrate palaeontologist at the Smithsonian Institution’s National Museum of Natural History in Washington DC, says the study should stoke discussion. “But I caution against totally reorganizing the dinosaur family tree just yet,” he says. For one thing, palaeontologists’ analyses of relations among species are keenly sensitive to which species are considered, as well as which and how many anatomical features are included, he says.
The discovery of new dinosaur species or more complete specimens of those already known might also drive future analyses back toward more currently accepted arrangements of dinosaur lineages, Sues says.
What’s fascinating is that this whole kerfuffle was started by the discovery of a new species of dinosaur named Saltopus elginensis. But when you consider the poor quality of this fossil, it casts more doubt on the proposal, as The Guardian, again, explains:
Langer argues that, while Saltopus might be statistically a good candidate for a common ancestor, given the patchy nature of the fossil it is a poor choice. Rather than attempting to identify the true ancestor of all dinosaurs – which can never be known — scientists’ aim is to find an animal that is a decent approximation of the general form and traits displayed by that ancestor we know must have existed.
The fossil, found in a Lossiemouth quarry, comprises a pair of legs, some hip bones, and vertebrae, all of which have been badly squashed.
“It looks like a chicken carcass after a Sunday roast,” Baron acknowledges.
The Guardian finds scientists who are skeptical of the new proposal:
As anticipated, the conclusions have been met with robust criticism from some rival scientists, including Max Langer, a respected palaeontologist at the University of São Paulo in Brazil.
“There’s nothing special about this guy,” he said. “Saltopus is the right place in terms of evolution but you have much better fossils that would be better candidates for such a dinosaur precursor.
Vinther, whose background is in mollusc research, said that unlike most dinosaur scientists he was not invested in any particular result, but added: “I’ve heard a bit of murmuring already from people who are not too thrilled about this hypothesis.”
Given the controversy that’s already brewing, it seems likely that over time critics will adduce further reasons to doubt this new dinosaur classification scheme.