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Comments on Thulborn's new paper

Thulborn, R.A. 2006. Theropod dinosaurs, progenesis and birds: homology of digits in the manus. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 242(2/3):205-241.

One of Thulborn's recent papers is based on the idea birds developed from other dinosaurs via progenesis, wherein the reproductive organs mature faster than the rest of the animal, leaving adults with juvenile characteristics. While I have no problem with that idea in general, the application of it to the digital homology problem seems unecessary.

Thulborn is antiquated in his ideas on homology. He writes "An understanding of homology will assist the reconstruction of phylogeny, but a reconstruction of phylogeny will not advance understanding of homology." This is backwards, is it not? Indeed, Thulborn subscribes to Owen's (1843) concept of homology, which seems uselessly vague to me- "the same organ in different animals under every variety of form and function". This is determined without reference to phylogeny. I don't see how one could distinguish analogy and homology without a phylogeny. Moreover, it ignores the genetic and developmental aspects of homology, which seem to be important to the avian digit issue.

Thulborn's ideas on phylogeny and nomenclature are also backward. Oddly, he writes as if the only "stem-group birds" are theropods, with no mention of sauropodomorphs, ornithischians or pterosaurs. "In some instances an elastic concept of a 'total-group' (not the total-group s. s.) has been expanded to such a degree that an avian 'stem' of indefinite extent is purported to include dinosaurs outside the Theropoda, and even pterosaurs (e. g. NORELL et al. 1993)." And yet Thulborn previously defined the avian total group as "all tetrapods, living and extinct, that possess at least one of the characteristics differentiating extant birds from all other tetrapods." Pterosaurs and non-theropod dinosaurs fit this definition though, as they share characters like an erect gait, elongate tibia and absent dermal armor.

"Such practices are potentially confusing, as they maintain, inter alia: that all birds are theropods whereas some theropods are not birds; that Aves is not synonymous with birds (GAUTHIER 1986); and that the avian 'stem' includes animals which do not possess even a single feature differentiating the crown-group Aves (and, thus, have no meaningful resemblance to birds)."

Yes, exactly. All birds are theropods, but not all theropods are birds. And as noted above, pterosaurs and non-theropod dinosaurs DO possess features differentiating crown-group Aves. The definition of Aves is a separate matter. Just because Thulborn prefers to call all theropods Aves (birds), doesn't mean it's a problem if more distantly related avemetatarsalians fall into his crown-group Aves. Thulborn's the only one calling any non-maniraptoran dinosaurs Aves/birds anyway.

"The increasingly common description of stem-group birds as 'non-avian theropods' is not merely cumbersome but unjustified and misleading, because the relationship between birds and theropod dinosaurs should be construed as just the opposite. Birds are not surviving dinosaurs, though some dinosaurs are primitive birds. This latter view, which provides more convenient and intelligible nomenclature, is not pedantry or a semantic quibble."

It really is just semantics. Thulborn goes on and on about it, but it's all just his personal problems with the nomenclature. Almost everyone else treats Aves as a subgroup of Theropoda and Dinosauria, and most workers explicitly define the taxa so that it works out that way (using Phylocode). Thulborn's problems are imaginary and his terminology is not accepted by anyone else.

Thulborn proposes growth was interrupted in theropods at the point where digit I would form, leaving only digits II, III and IV developed. In addition, there wouldn't be enough mesenchyme to give each digit its expected number of phalanges, leading to the standard 2-3-4 digital formula. This means that the change would have to occur prior to Avepoda, as the three large coelophysoid digits are clearly homologous with bird digits. The placement of Herrerasaurus as a basal theropod or theropod relative would destroy Thulborn's scheme (as it has pentadactyl hands with digits I-II-III seemingly homologous with bird digits), so he tries hard to disregard it.

"Herrerasaurus and a possible relative, Staurikosaurus, have proved 'difficult to place within either the Saurischia [Sauropodomorpha + Theropoda] or the Ornithischia and probably stand outside the common ancestor of these two [dinosaurian] orders' (PADIAN 1986: 364; see also BRINKMAN & SUES 1987: 493). In some parts of its anatomy Herrerasaurus exhibits very primitive proportions, by dinosaurian standards, and occasionally it has been likened to, or even classified among, the prosauropods. Herrerasaurus is sometimes excluded from the Dinosauria or is regarded as the sister-taxon of a clade comprising all other dinosaurs (e. g. GAUTHIER 1986; BRINKMAN & SUES 1987; NOVAS 1992), though some studies have placed it close to, or within, the Theropoda (e. g. CARRANO & WILSON 2001; RAUHUT 2003). Even then, Herrerasaurus is noticeably different from most early theropods in its relatively great size, with an overall length estimated at about 3.9 M (PAUL 198'8). Persistent uncertainties about the status and affinities of Herrerasaurus were clearly expressed by LANGER et al. (1999: 154), who equivocated by presenting alternative schemes of dinosaurian phylogeny. In one scheme Herrerasaurus and Staurikosaurus were united in the monophyletic family Herrerasauridae, which was designated the sistergroup
of the Dinosauria. In the radically different alternative scheme Staurikosaurus and Herrerasaurus were situated crownward of the Ornithischia, within the Dinosauria, and were identified as successively more derived sister-taxa of the Saurischia (Theropoda + Sauropodomorpha)."

Ack!! None of the studies advocating non-dinosaurian herrerasaurids are in any way meaningful, since future studies placing them inside Saurischia contain many more characters and taxa. Padian's 1986 opinion has no relevence in 2007, nor do the OLD assignments of herrerasaurids to Prosauropoda by Cooper (1981) and Colbert (1970). It's laughable such texts would even be cited as evidence of alternative phylogenetic possibilities. Langer et al.'s (1999) analysis has been superseded by Langer (2004) and Langer and Benton (2006), which use 107 and 98 characters respectively, as opposed to 40. They also use newly discovered taxa such as Guaibasaurus and Silesaurus. Both of the latter studies support herrerasaurids as eusaurischian saurischians.

"In short, Herrerasaurus is a problematical archosaur that may transpire to be something other than a theropod or a theropod relative, and its manus which has been used as the standard of reference for identifying digits in theropods -is not known from a complete and naturally articulated example."

Wrong. PVSJ 407 is an articulated forelimb lacking phalanges that shows metacarpal V, and PVSJ 373 is complete and articulated as well (Sereno, 1993). To call Herrerasaurus a "problematical archosaur", as if its affinities could be reasonably claimed to lie outside Saurischia, is dishonest.

"In fact, some comparisons do omit the questionable evidence of Herrerasaurus, but the inclusion of more distantly related forms (e.g. prosauropods, omithischians, crocodilians) still conveys the impression that a tendency to reduce digits 5 and 4 was widespread among archosaurs and was merely carried to completion in the theropods (e. g. GAUTHIER 1986; SHUBIN 1994). Even so, skeletal remains and fossil tracks reveal that the hands and feet of early archosaurs were ectaxonic, with the largest and strongest digits towards the lateral margin (digit 4). Subsequently in archosaur history there was a common, though far from universal, trend to the mesaxonic condition, where digit 3 is the most prominent."

Sauropodomorphs are the SISTER GROUP of theropods. Even without Herrerasaurus, they do show avepod digits are I-II-III-IV. Ornithischians such as Heterodontosaurus also show this. Early archosaurs are simply irrelevent. Thulborn's idea depends on his "polyphyletic" Dinosauria which is in turn based on spurious ichnological evidence and refuted by osteology. As David has noted, theropods and their saurischian ancestors could not pronate their hands. So Thulborn's supposed basal theropod trackways showing reduced manual digit I simply cannot be theropods. They are more likely basal dinosauromorphs or shuvosaurids, both of which have theropod-like pes.

Thulborn has a rather large section devoted to the frame shift hypothesis, but it is mostly semantic. Digital frameshifts occur (e.g. Chalcides). The digits develop like one sequence (II-III-IV), but have the anatomy of another sequence (I-II-III). It doesn't matter if they are homologous, homeotic, homeocratic, etc.. I have a feeling developmental biologists would be as amused by him citing Bateson (1894) as an authority when it comes to whether homeotically shifted organs are homologous, as I am by citing Padian (1986) as an authority when it comes to whether herrerasaurids are saurischians. Indeed, the former is much more ridiculous, as evo-devo and genetics wouldn't emerge for many decades. I could comment more, but it's rather unecessary. The osteological evidence firmly shows theropods' successive sister groups had reduced manual digits IV and V, not I and V. And this is enough to discredit his progenesis hypothesis for the origin of the theropod hand.

In conclusion, Thulborn's paper is anachronistic. Its concepts of nomenclature and homology are not accepted by modern workers, nor are its ideas on dinosaurian phylogeny. Overall it reminded me of a work by Martin or Feduccia- clinging to obsolete methodologies and citing outdated and superceded references. His idea may have been deemed relevent in the 1970's, but it has no place here in 2007.

Mickey Mortimer