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Re: [dinosaur] Dinosauria reclassification joins Ornithischia and Theropoda in Ornithoscelida
> But I'm uneasy about the revised definition of Saurischia, which has
> only one internal specifier (a theropod).
My mistake - here I meant to write "a sauropod". Having Saurischia
anchored solely on a sauropod taxon (in this case, _Diplodocus_) is
bad policy IMHO. It's better for Saurischia to be tied to the
hypothesis that Theropoda and Sauropodomorpha form a clade to the
exclusion of Ornithischia. If this is not the case, and Theropoda is
recovered as closer to Ornithischia (Ornithoscelida hypothesis), or
Sauropodomorpha is recovered as closer to Ornithischia
(Phytodinosauria hypothesis), then clade Saurischia should be void.
David Marjanovic <email@example.com> wrote:
> Excluding Sauropodomorpha from Dinosauria would have had the
> interesting effect of returning to Owen, though. When Owen coined
> the name Dinosauria, he knew about *Cetiosaurus* but didn't include
> it in the group; as its name says, he thought it was a marine "reptile"
> (though not a plesio- or ichthyosaur).
IIRC, Owen originally interpreted _Cetiosaurus_ as a (very large)
I don't think it's such a bad idea that the definition of Dinosauria
is tweaked such that it ensures that theropods, sauropodomorphs, and
ornithischians are all included. If individual basal taxa fall out of
the Dinosauria (like _Pisanosaurus_ or herrerasaurids), then this is
not such a big deal. But when a topology results in _Plateosaurus_ or
_Brontosaurus_ or _Diplodocus_ being excluded from Dinosauria, then it
shows that the definition of Dinosauria needs to be updated. And if
future phylogenies happen to recover taxa that are currently not
considered dinosaurs as members of the dinosaur clade (like
silesaurids or even pterosaurs), then so be it.
Thomas Yazbeck <firstname.lastname@example.org> wrote:
> The discussion of forelimbs also struck me as questionable. I wonder if the
> tendency of basal 'ornithoscelidans' to be bipedal arose for reasons
> unrelated to freeing up the manual appendages for grasping.
All phylogenies are consistent with dinosaurs being ancestrally
bipedal. One hypothesis is that bipedalism arose in dinosaurs for the
purpose of enhanced cursoriality - see Persons & Currie (2017; J Theor
Biol 420: 1-7.)
Although it's pretty much settled that the first dinosaurs were
bipeds, their dietary preference(s) have been more ambiguous. Baron
et al. favor the hypothesis that the first dinosaurs were omnivorous.
So they *might* have used their forelimbs for predation. But I think
the evidence is weak that this was the driving force behind
bipedality; I think the cursorial hypothesis has more going for it.
One interesting consequence of this new dinosaur phylogeny is that the
filamentous integumentary structures preserved in _Tianyulong_ and
_Psittacosaurus_ are likely primitive for Ornithoscelida.
> The biogeographical remarks too are very much up for debate. It seems
> unlikely to me that there would be distinct regionalism of
> dinosauromorphs-active, mobile animals- in the mid Triassic.
> I guess the real take-away from Baron et al is that the base of Dinosauria
> is a mess.
Given the topology given by
> Baron et al., a better idea might have been to drop Saurischia
> altogether, rather than salvage it as the name for the new
> Herrerasauridae+Sauropodomorpha clade. Firstly, this rump Saurischia
> overturns an established tradition that Saurischia as a group should
> include theropods; I'd say that no Saurischia at all is better than a
> Saurischia sans Theropoda. Secondly, the sister clade to
> Ornithoscelida could simply be called Sauropodomorpha, with
> herrerasaurids simply considered basal sauropodomorphs; the prevailing
> stem-based definition of Sauropodomorpha allows for this. Instead
> Baron et al. re-define Sauropodomorpha to exclude herrerasaurids, such
> that the two are sister taxa within a 'new' Saurischia.
> Baron et al.'s phylogeny implies that a supinated, grasping hand is
> primitive for dinosaurs, and they further suggest that "the ability to
> grasp with the manus played an important role in early dinosaur
> evolution". It's an entirely reasonable hypothesis; but I'm
> skeptical. I lean toward the view that theropod forelimbs weren't
> really all that useful for grasping (especially prey capture). Like
> Persons & Currie (2017; dx.doi.org/10.1016/j.jtbi.2017.02.032), I
> suspect that the tendency to ascribe great adaptive significance to
> the freed hands of dinosaurs at least partly reflects our own human
> bias. Most Triassic theropods and herrerasaurids had short forelimbs;
> as Persons & Currie (2017) put it, it's "difficult to envision them
> grappling with any prey that could not have already been seized by the
> jaws". So I doubt the 'grasping' manus conferred on early dinosaurs
> some evolutionary edge over other archosaurs (including other bipeds).
> _Eoraptor_ (which Baron et al. put back in the Theropoda, as the most
> basal theropod) and basal sauropodomorphs ('prosauropods') share a
> specialized medially ‘twisted’ phalanx 1 of manual digit I
> (pollex/thumb) . This 'twisted' pollex is a convergent feature
> acquired by _Eoraptor_ and sauropodomorphs independently, or it is
> primitive for dinosaurs. Either way, the function of the twisted
> pollex is unclear; it might have been used for defense or
> intraspecific combat. Once freed from a function in locomotion and
> weight-bearing, dinosaur hands might have been employed for defensive
> purposes, more so than grasping.
> On Thu, Mar 23, 2017 at 5:13 AM, Ben Creisler <email@example.com> wrote:
>> Ben Creisler
>> A new paper:
>> Matthew G. Baron, David B. Norman & Paul M. Barrett (2017)
>> A new hypothesis of dinosaur relationships and early dinosaur evolution.
>> Nature 543: 501–506
>> Free pdf of supp:
>> For 130 years, dinosaurs have been divided into two distinct
>> clades—Ornithischia and Saurischia. Here we present a hypothesis for the
>> phylogenetic relationships of the major dinosaurian groups that challenges
>> the current consensus concerning early dinosaur evolution and highlights
>> problematic aspects of current cladistic definitions. Our study has found a
>> sister-group relationship between Ornithischia and Theropoda (united in the
>> new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the
>> redefined Saurischia) forming its monophyletic outgroup. 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.
>> Kevin Padian (2017)
>> Dividing the dinosaurs.
>> Nature 543: 494–495
>> The standard dinosaur evolutionary tree has two key branches: the
>> 'bird-hipped' Ornithischia and the 'reptile-hipped' Saurischia. A revised
>> tree challenges many ideas about the relationships between dinosaur groups.
>> See Article p.501