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Re: Phylogeny and evolutionary history of tyrannosauroid dinosaurs (free pdf)
Mickey Mortimer <email@example.com> wrote:
> It's only been mentioned as a possibility in Brusatte's thesis so far, not
> demonstrated with evidence. As I write on the Database-
> "Finally, Brusatte (2013) strongly suspects Bagaraatan is a chimaera of
> tyrannosauroid and other coelurosaurian elements, which
> will be the subject of a paper written by Makovicky, himself and others.
> Based on general morphology and the above results, it
> may end up that the mandible, vertebrae and pelvis are tyrannosauroid while
> the hindlimb is troodontid." Though it would be
> suspicious not to have any duplicated elements (unless they weren't mentioned
> by Osmolska).
20 years ago, Halszka Osmolska was convinced that the elements all
belonged to a single individual:
"The specimen, as found in the field, had all bones anterior to the
pelvis strongly weathered and represented only by scraps. The
mandible, however, judging by its position and the arrangement of the
weathered postcranial skeletal fragments, certainly belonged to the
same individual as the hind quarters and tail."
She accordingly classified all elements as one specimen number (= the
holotype). 21 caudals at least constituted a series, of the very
John D'Angelo <firstname.lastname@example.org> wrote:
> If we really wanted to get technical about it, wouldn't it be
> Dryptosauroidea, not Tyrannosauroidea?
A nomenclatural purist might say it should be Deinodontoidea! I'm
happy with the names used by Brusatte & Carr: Tyrannosauroidea,
Tyrannosauridae, Alioramini, etc. When it comes to 'family-level'
taxa (superfamilies, families, subfamilies, tribes) I don't see any
reason for strict enforcement of arbitrary ICZN 'rules'. In other
words, I prefer _not_ to get technical about it. :-)
> I don't understand what the reason to split paraphyletic genera is, unless
> one were to argue for a total abolition of the system of
> binomial nomenclature as currently conceived.
Under phylogenetic taxonomy, a genus should be a clade (all
descendents of a common ancestor = monophyletic). A paraphyletic
group does not include all descendents of a common ancestor; it is a
"grade". This is why paraphyletic groups have been abandoned, such as
Thecodontia and Pelycosauria; or re-defined such that they are clades,
such as Theropoda (which now includes birds) and Therapsida (which now
includes mammals). Genera are treated the same way.
We can adhere to binomial nomenclature AND ensure genera are
monophyletic. If _Daspletosaurus_ spp. comprise a grade of
tyrannosaurid evolution between _Lythronax_ and _Zhuchengtyrannus_ (as
in Fig 2 of Brusatte & Carr), then there is justification for
splitting up genus _Daspletosaurus_. Of course, it's not compulsory.
Given that _Daspletosaurus_ comes up as monophyletic in the parsimony
analysis and paraphyletic in the Bayesian analysis, there is good
reason to keep both species (one not yet named) in the same genus.
> This analysis is almost totally limited to Tyrannosauroidea (aside from four
> non-tyrannosauroid avetheropods used as outgroups),
> so it isn't an appropriate matrix to test the relationships of anything not
> certain to be a tyrannosauroid. This is also why they
> excluded megaraptorans from consideration, so I am unsurprised that far more
> fragmentary possible tyrannosauroids were not
Fair point. _Timimus_ was posited by Benson &c to be a fairly derived
tyrannosauroid (though outside Tyrannosauridae), close to the position
recovered by some analyses for Megaraptora. Several poorly known
(including fragmentary) theropods have been touted as potential
tyrannosaur(oid)s over the years (e.g., _Santanaraptor_, _Iliosuchus_,
_Labocania_, _Calamosaurus_), and for this reason I can understand why
_Timimus_ was excluded. All we have is a single femur.