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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
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
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
"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
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
"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
"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
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.