Saturday 25 March 2017

Dinosaur phylogenetics, a radical new proposal.

Phylogenetics is the study of the relationships between organisms, as determined by examining similarities between species and reconstructing family trees. The early origins of the science relied very much on informed guesswork, but modern phylogenetics is a highly mathematical discipline, in which practitioners attempt to measure mathematically differences between the morphologies, or better still DNA, of different species, and allow computer models to estimate the most likely relationships between them. The phylogeny of the Dinosaurs, although it has changed a great deal in detail with the discovery of numerous new species, has remained essentially the same since 1887, with the earliest Dinosaurs appearing in the Middle-to-Late Triassic and rapidly diverging into two distinct groups the ‘Lizard-hipped’ Saurischians and the ‘Bird-hipped’ Ornithischians, and the Saurischians splitting shortly afterwards into the Sauropods and Theropods.

In a paper published in the journal Nature on 23 March 2017, Matthew Baron of the Department of Earth Sciences at the University of Cambridge and the Department of Earth Sciences at the Natural History Museum, David Norman also of the Department of Earth Sciences at the University of Cambridge and Paul Barrett also of the Department of Earth Sciences at the Natural History Museum describe a radically different phylogeny for the Dinosaurs, based upon a new study which incorporated data on 457 morphological characteristics of 74 taxa of Dinosaurs and Dinosauromorph Archosaurs (i.e. Archosaurs thought to be closely related to Dinosaurs). 

Remarkably Baron et al. did not recover the two ‘Saurischian’ Dinosaur groups as being one-another’s closest relatives to the exclusion of the Ornithischians; rather they produced a model in which the Dinosaurs split early into two groups, one comprising the Sauropods plus the Herrerasauridae (a group of early Dinosaurs generally thought to be primitive Theropods) and the other comprising the Ornithischians plus all other Theropods.

Phylogenetic relationships of early dinosaurs. Time-calibrated strict consensus of 94 trees from an analysis with 73 taxa and 457 characters. (A) the least inclusive clade that includes Passer domesticus, Triceratops horridus and Diplodocus carnegii — Dinosauria, as newly defined. (B) the least inclusive clade that includes Passer domesticus and Triceratops horridus — Ornithoscelida, as defined. (C) the most inclusive clade that contains Diplodocus carnegii, but not Triceratops horridus —Saurischia, as newly defined. All subdivisions of the time periods (white and grey bands) are scaled according to their relative lengths with the exception of the Olenekian (Early Triassic), which has been expanded relative to the other subdivisions to better show the resolution within Silesauridae and among other non-Dinosaurian Dinosauromorphs. Baron et al. (2017).

If this is correct, then it presents a number of serious challenges for our understanding of Dinosaur phylogeny. Firstly there is the definition of the term 'Dinosaur' itself. The current definition of Dinosaur is 'Passer domesticus, Triceratops horridus, their most recent common ancestor and everything descended from it'. This made sense because Passer domesticus, the modern House Sparrow, is a highly derived Theropod and Triceratops horridus, is a highly derived Ornithischian, so that a clade comprising all the descendants of their most recent common ancestor and everything descended from it, would, if Theropods and Ornithischians are the most distantly related Dinosaur groups, include everything we would consider to be a Dinosaur. However, if Ornithischians and Theropods are more closely related to one-another than either group is related to the Sauropods, then Sauropods can no longer be considered to be Dinosaurs. Since this goes completely against the common understanding of the term Dinosaur, Baron et al. suggest that rather than exclude the Sauropods from the Dinosaurs, the definition of the group should be amended to 'Passer domesticus, Triceratops horridus and Diplodocus carnegii, their most recent common ancestor and everything descended from it', thereby retaining everything that we would think of as being a Dinosaur within the group.

Then there is the problem of how to split the Dinosaurs into subgroups. The taxon Saurischia, which comprises the Theropods plus the Suaropods, and which has for over a hundred years been seen as one of the major Dinosaur divisions, is no longer valid under this hypothesis. Baron et al. suggest that instead the term Ornithoscelida, first proposed by Thomas Huxley in 1870 to describe a group comprising the Compsognatha (Theropods), Iguanodontidae (Ornithischians), Megalosauridae (Theropods) and Scelidosauridae (Ornithischians), i.e. a group of Dinosaur clades thought to be unrelated since the Ornithischian/Saurischian split was proposed in 1887, but which now appears valid. They retain the term Saurischia to describe the clade that includes the Sauropods and Herreosaurs, as these groups were both fall within the original definition of that group.
This hypothesis also has implications for the origins of the Dinosaurs, and the nature of the earliest members of the group. Since the earliest Therapods (the Herrerasauridae) and the earliest Ornithischians (the Heterodontosaurids) were both thought to have been carnivores, and the earliest members of the most closely related non-Dinosaurian group, the Silesauridae, were also thought to have been carnivores, it was presumed that the earliest Dinosaurs were probably carnovores, but it was unclear what they would have been like. However if the Herrerasauridae are outside the Theropods, then this balance changes. The earliest member of the Theropods become Eoraptor, a small omnivorous Dinosaur with heterodont dentition (i.e. different shaped teeth in different parts of the mouth) and grasping hands, while the earliest Ornithischians, the Heterodontosaurids, were also small with heterodont dentition and grasping hands, as were the Prosuaropods (earliest Suaropods). Only the Herrerosaurs differe from this pattern, being small with grasping hands but having serrated, recurved teeth reminiscent of later Theropods and indicatinve of a hypercarnivorous diet (diet in which little or no vegetable food is ingested), however, this group is not well known, and it is possible that early member of the group did have a different form of dentition. This points to a model where the earliest Dinosaur would have been a small, omnivorous animal with grasping hands and heterodont dentition, a versatile generalist lifestyle and anatomy that could explain how the Dinosaurs were able to diversify rapidly into so many different niches.

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