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Why Thulborn's ideas on dinosaur polyphyly are wrong

Thulborn, 2006. On the tracks of the earliest dinosaurs: implications for the hypothesis of dinosaurian monophyly. Alcheringa 30, 273-311.

Thulborn's second recent paper is an investigation of dinosaur monophyly using ichnological data.

Although he notes early on that Carrano and Wilson (2001) found that dinosauroid tracks could be made by non-dinosaurian dinosauromorphs (e.g. Marasuchus), this is soon forgotten and Ladinian dinosauroid tracks are attributed to quadrupedal theropods.

Thulborn reprises his confused and archaic ideas on herrerasaurids (see my earlier post on his digital homology paper), using the same Padian (1986) quote to introduce them. After this muddled grasp of their phylogenetic affinities, the author tries hard to make Herrerasaurus chirotheroid instead of dinosauroid. He illustrates and discusses the partial holotype pes, claiming that perhaps the first ungual was longest and that the fourth digit was much longer than II and subequal to III (these areas are unpreserved in that specimen). Yet Novas (1993) described a complete pes of Herrerasaurus, which has a short first pedal ungual and fourth digit (unpreserved in the illustrated right foot, but preserved on the left). So any statements based on Thulborn's fictional chirotheroid Herrerasaurus such as "the pes would have been rather prosauropod-like, which seems reasonable considering that Herrerasaurus has sometimes been likened to, or even classified among, the prosauropods (e.g. Colbert 1970, van Heerden 1978,Cooper 1980,1981)" or "it implies that any herrerasaurian footprints that do exist might have been classified among chirotherioid ichnotaxa, such as Brachychirotherium, or allied with problematical ichnogenera such as Batrachopus, along with tracks of some prosauropods, thecodontians and 'crocodiliornorph' reptiles" are obsolete and incorrect. His concluding sentence "In view of these chronic uncertainties about the anatomy of the herrerasaurian pes (theropod-like or prosauropod-like?) it seems impossible to identify the tracks of herrerasaurs with any reasonable degree of confidence." is particularly laughable. Amusingly, Thulborn then notes the complete pes described by Novas, but incorrectly states its distal phalanges are unknown (they are even listed in the measurement table on the same page) and that it "does not resolve uncertainties surrounding the form of the herrerasaurian pes." Using the given measurements, we can see that Herrerasaurus has a mesaxonic pes, where digit IV is intermediate in length between II and III, just like the dinosauroid tracks illustrated in Thuborn's figure 4. Herrerasaurids would have left dinosauroid footprints, just like Eoraptor, silesaurs and Marasuchus, while Lagerpeton and Scleromochlus would have left ectaxonic prints due to their elongate digit IV.

Thulborn reminds us that these ideas are better left in the 1970's by referring to coelophysoids as podokesaurids (or procompsognathids). Staying behind the times, he notes "The Ornithosuchidae is often identified as the sister-group of a monophyletic taxon Dinosauria (e.g. Paul 1984, p. 176) or as the sistergroup of a taxon comprising Dinosauria, near dinosaur thecodontians such as Lagosuchus and, sometimes, the Pterosauria (i.e. the clade 'Ornithodira' sensu Gauthier 1986; Benton & Clark 1988, fig. 8.1; Benton 1990, fig. 1.1).", despite the fact all recent analyses have agreed ornithosuchids are crurotarsans. He considers the ichnogenus Sphingopus to be ancestral to the dinosauroid type. With a distally placed pedal ungual I and large pedal digit V impression, these are possibly referrable to poposaurs like Effigia. Effigia would make a track with a dinosauroid-like cross axis, as shown by its metatarsal proportions. This would also explain the pronated manus with large lateral digits. Thulborn concludes "In summary, early dinosauroid tracks are almost certainly the work of theropod dinosaurs." without ever considering basal dinosauriformes.

The author next discusses sauropodomorphs. He uses Blikanasaurus as an example of an early sauropodomorph, but we now know it was a basal sauropod and its enlarged pedal digit I and possible ectaxony are not primitive for sauropodomorphs. Saturnalia (Langer, 2003), Pantydraco and Efraasia show that basal sauropodomorphs had short pedal digit I and were mesaxonic. The transformation to a Blikanasaurus-like pes can be followed via plateosaurs and Anchisaurus. Furthermore, the subdigitigrady of Blikanasaurus is indeed a reversal from the digitigrade condition seen in basal sauropodomorphs (Saturnalia, etc.), despite how unlikely it seems to Thulborn. Citing Charig et al. (1965) as support for primitively subdigitigrade sauropodomorphs is completely worthless, given the recent discovery of basal taxa (e.g. Pantydraco, Saturnalia) and the unresolved relationships of sauropodomorphs through the 1990's, let alone the 1960's. Thulborn agrees with Olshevsky that the enlargement of pedal digits I and V in sauropods are "implausible reversals", yet again without any evidence save personal incredulity. He then claims the enlarged first manual ungual and robust first manual digit were slight modifications of the condition seen in crurotarsans, yet basal sauropodomorphs such as "Thecodontosaurus" YPM 2195 show that these features were less developed than in prosauropods and sauropods. Thulborn uses Tetrasauropus as an example of a basal sauropodomorph track, while doubting the sauropodomorph identity of Otozoum. Yet the most basal sauropodomorphs were mostly bipedal (Langer, 2003) and even prosauropods couldn't walk quadrupedally with a pronated manus (Bonnan and Senter, 2007). Indeed, the latter authors concluded Otozoum was left by prosauropods while Tetrasauropus was left by basal sauropods. Contra Thulborn, basal sauropodomorphs would have left tracks very similar to the dinosauroid type but with a more prominant pedal digit I. Indeed, the pes of Saturnalia would make indistinguishable tracks from Herrerasaurus.

Thulborn then discusses ornithischians. He uses Charig (1982) as support that Pisanosaurus has no ornithischian synapomorphies, contrary to Weishampel and Witmer (1990) and Sereno (1991, 1999), as well as more recent references such as Langer (2004), Langer and Benton (2006) and Irmis et al. (2006). Again the use of such an outdated reference is misleading and inappropriate. Thulborn suspects Pisanosaurus may be a herrerasaurid(!) because "there are some definite resemblances between the hindlimb bones of Pisanosaurus (Bonaparte 1976, figs 5,7) and those of Herrerasaurus ischigualastensis", and Bonaparte (1976) judged some characters to be intermediate between sauropodomorphs and "thecodonts". While the hindlimb elements do exhibit plesiomorphic dinosauriform characters, Irmis et al. identified several ornithischian characters in the jaws- coronoid process; constricted tooth roots; emarginated tooth row. These are not seen in herrerasaurids, and Langer (2004) and Langer and Benton (2006) found Pisanosaurus to clade with ornithischians (not herrerasaurids) with good support. Even if Pisanosaurus turns out to be a basal dinosauriform with ornithischian-like jaws (as in Silesaurus; though it didn't clade with Silesaurus in Langer and Benton's study either), it's certainly no herrerasaur. Thulborn views the elongate saurischian-like manus of heterodontosaurids as autapomorphic, but it may be plesiomorphic based on Butler's (2005) unpublished analysis. Then the "more generalized form" with short digits and IV longer than II (as seen in Lesothosaurus) would be a genasaurian morphology. In fact, heterodontosaurids would have probably made dinosauroid tracks, as their first digit did not contact the ground, and their pes was slender with mesaxonic digits and claw-like unguals. The same could be said of lesothosaurs. Thulborn uses both Otozoum and Batrachopus as examples of early ornithischians, yet the former is prosauropod (Bonan and Senter, 2007) and the latter crocodylomorph (Lockley and Meyer, 2004). Furthermore, he refers Atreipus to the Theropoda because "the manus ... is matched in theropods (Thulborn 1993) and the well-defined digital nodes of the pes resemble those in Grallator." Yet the manus with its elongate digit IV is not matched in theropods, whereas it does resemble those of lesothosaurs. Saurischians (except sauropods) cannot pronate their hands anyway, so Atreipus cannot be theropod. Safran and Rainforth (2004) agree it is ornithischian and distinguish it from Grallator based on a few characters. It's possibly a lesothosaur or related taxon and shows ornithischians evolving from the dinosauroid track type in the Carnian. Another possibility is that it is a silesaur, which have theropod-like pes, quadrupedal stance and possibly pronated manus. Silesaurs are known from the Late Triassic of North America and Europe, matching the distribution of Atreipus.

Based on the above, Thulborn finds the chance of sauropods and ornithischians being secondarily quadrupedal to be implausible, due to the number of changes that would have to occur (e.g. enlarged pedal digit I in sauropodomorphs, redevelopment of pedal digit V in sauropods, semiplantigrade posture). But as we've seen, basal sauropodomorphs and ornithischians actually had pes which would have left tracks basically indistinguishable from the dinosauroid type which characterized theropods and basal dinosauriforms. Thulborn cites several papers in support of dinosaurian polyphyly (Thulbom 1975, Charig 1976, 1982, Maryanska & Osmolska 1983, Zhao 1983, Welles 1986, van Heerden 1997), but none are relevent today. For instance, Zhao (1983) suggests prosauropods, sauropods, carnosaurs, coelurosaurs and ornithischians each had a separate pseudosuchian ancestor. No evidence was given, no particular non-dinosaurian archosaurs were suggested as relatives to any dinosaur clade, and the entire idea is preposterous in an age where we know Zhao's "prosauropods" are a grade leading to sauropods, while his "carnosaurs" and "coelurosaurs" are artificial divisions of theropods. Welles (1986) concerns the origin of theropods, where he concludes that "proterosuchians" must have been their ancestors, since the crurotarsan ankle of "pseudosuchians" is too derived. Yet this says nothing of other dinosaurs, for which such a conclusion would be equally true. Van Heerden (1997) wasn't even a technical article, but rather a chapter in the laymans book "The Complete Dinosaur". Any study from the 1970's or even early 1980's would be plagued with fictional ideas like pseudosuchians sensu lato, teratosaurs, palaeosaurs and the (pre-Gauthier) carnosaur-coelurosaur dichotomy. Not to mention the absence of cladistic methodology and the numerous new specimens and taxa discovered since. In short, such papers are useless today.

Thulborn postulates each dinosaurian clade had an independant origin from a chirotheroid "thecodont" based on ichnological trends, but as we saw above, his identifications are full of errors. His figure 8 is a phylogeny of sorts showing the proposed transformations. At the base is a chirotheroid, probably a crurotarsan. The theropod lineage starts with Sphingopus (possibly a poposaur), then goes through Atreipus (a lesothosaur or silesaur), and ends at Eubrontes (a theropod) but with the manus of Atreipus. Thus Thulborn's idea that theropods retained digits II-III-IV is based on non-theropod tracks. His sauropodomorph lineage begins with Navahopus (a basal sauropod) and goes through Pseudotetrasauropus and Brontopodus (more derived sauropods). It completely skips prosauropods and basal taxa like Saturnalia. Finally, his ornithischian lineage begins with Otozoum (a prosauropod) and continues with Anomoepus and Moyenisauropus (ornithischians). These errors invalidate Thulborn's problems with deriving sauropodomorphs and ornithischians from dinosauroid trackmakers since the morphologies he views as basal for each clade are usually not members of that clade at all, and are not basal members even when they are properly assigned.

He states that each dinosaur clade developed an interlocking tibioastragalar articulation independantly, as shown by different mechanisms. Yet these morphologies (tall ascending process in derived theropods; posterodistal tibial notch in derived sauropods) are not present in basal members of each clade (coelophysoids, Guaibasaurus, Saturnalia). Contra Thulborn, all basal dinosauromorphs have homologous ascending processes, not just theropods. His figure 9 comparing Allosaurus, Diplodocus and Hypsilophodon to illustrate the differences is misleading, and would be ineffective if basal members were used.

For his discussion of the osteological evidence for dinosaur monophyly, Thulborn lists 38 previously proposed dinosaurian synapomorphies. He notes that most are postcranial, "even though cranial structures are conventionally regarded as more conservative than postcranial ones and, thus, more trustworthy as indicators of phylogenetic affinity." Yet until the discovery of Silesaurus (published after anything Thulborn cites on the issue), non-dinosaurian dinosauromorph crania were poorly known, and conventional wisdom regarding conservatism in various skeletal regions is not based on empirical data. Thulborn first concentrates on the absent postfrontal, which is indeed also known in proterochampsids, crocodylomorphs and poposaurids. Yet the fact a character exhibits homoplasy elsewhere in a phylogeny is not a reason to doubt its validity, unless Thulborn is proposing to unite a dinosaur subclade with one of those other three groups of archosauriformes. He then states the expression of the supratemporal fossa on the frontal is a consequence of the absent postfrontal, so should not be considered a separate character. Yet pterosaurs have the former but not the latter, showing he is incorrect. As the supratemporal fossae are exposed on the frontals of pterosaurs and Silesaurus, this is an ornithodiran character, so Thulborn is right to reject it as a dinosaurian synapomorphy, but for the wrong reasons. Of the other cranial synapomorphies noted by Thulborn as being called equivocal by Novas (1996)-
- an ectoptergoid which dorsally overlaps the pterygoid is a dinosaurian synapomorphy (absent in pterosaurs and crurotarsans).
- a laterally exposed quadrate head has a wider distribution, being present in Lewisuchus, some crurotarsans and Euparkeria.
- a reduced posttemporal opening is a dinosaurian synapomorphy (absent in Silesaurus, Marasuchus and pterosaurs) otherwise present in proterochampsids and crocodylomorphs.

Thulborn remarks that Novas found many proposed dinosaurian synapomorphies to be invalid, and yet this is misleading. Most of the rejected characters were rejected because they are also present in dinosaurian outgroups such as Pseudolagosuchus, Marasuchus, Lagerpeton and pterosaurs. These are perfectly valid to use when arguing against dinosaurian polyphyly as advocated by Thulborn, as he states the dinosaurian common ancestor would be plantigrade or semiplantigrade, have a functionally pentadactyl pes and a quadrupedal gait. Ornithodiran, dinosauromorph and dinosauriform characters all support a dinosaurian common ancestor which is a bipedal digitigrade animal with a tetradactyl (though functionally tridactyl) pes. They support dinosaurian monophyly exclusive of the crurotarsan or basal archosauriform taxa Thulborn advocates theropods, sauropodomorphs and ornithischians separately evolved from. Of the ten postcranial characters Thulborn claims Novas "found to be equivocal or of doubtful validity"-
- postaxial cervical epipophyses are a dinosaurian synapomorphy (absent in Silesaurus, Pseudolagosuchus and Marasuchus) otherwise present in pterosaurs and teratosaurids.
- an elongate deltopectoral crest is a dinosaurian synapomorphy (absent in Silesaurus, Marasuchus, most pterosaurs and Scleromochlus) otherwise present in campylognathoidids.
- fewer than four phalanges on manual digit IV is a dinosaurian synapomorphy (absent in pterosaurs and crurotarsans) reversed in derived sauropodomorphs.
- a brevis fossa is a Dinosauria+Silesaurus synapomorphy (absent in Marasuchus, Lagerpeton, Scleromochlus and pterosaurs) reversed in herrerasaurids. An analogous condition is present in teratosaurs and poposaurs.
- an ischium with a shallow medioventral lamina is a Dinosauria+Silesaurus synapomorphy (absent in Marasuchus, Lagerpeton, Scleromochlus and pterosaurs) otherwise present in poposaurids.
- a reduced medial tuberosity of the femur is a Dinosauria+Silesaurus synapomorphy (absent in Pseudolagosuchus, some Marasuchus and Lagerpeton), that is also present in some Marasuchus specimens, and it absent in some Plateosaurus and ornithopods.
- a prominant anterior trochanter is a Dinosauria+Silesaurus synapomorphy (absent in Pseudolagosuchus, Marasuchus, Lagerpeton, Scleromochlus and pterosaurs) reversed in Staurikosaurus.
- a tibial descending process which fits caudal to the astragalar ascending process is a Dinosauria+Silesaurus synapomorphy (absent in Pseudolagosuchus, Marasuchus, Scleromochlus and pterosaurs).
- a flat to concave proximal calcaneum is a Dinosauria+Silesaurus synapomorphy (absent in Pseudolagosuchus and Marasuchus) otherwise present in Lagerpeton.
- a proximally flat distal tarsal IV is a Dinosauria synapomorphy (absent in Marasuchus, Lagerpeton and pterosaurs).

Claiming that "Eventually Novas (1996) found that dinosaurian monophyly could be supported by only two or three seemingly unequivocal synapomorphies" is simply a lie, as Novas supported Dinosauria with seventeen characters, six of which were unequivocal. Oddly, these six unequivocal characters include only one of the "two or three" listed by Thulborn (dorsosacral added to sacrum), while another (perforated acetabulum) was equivocal due to uncertainty in Pseudolagosuchus, and the third (mesotarsal ankle) was never used by Novas (1996) as a dinosaurian synapomorphy.
- a dorsosacral added to the ancestral two sacral vertebrae is present in Eoraptor, theropods, Efraasia, basal prosauropods, sauropods and ornithischians. It is absent in herrerasaurids, Saturnalia, Thecodontosaurus(?), Plateosaurus+Sellosaurus and immediate outgroups (Silesaurus, Pseudolagosuchus, Marasuchus, Lagerpeton). Poposaurs, Pterosaurs and Scleromochlus all have a dorsosacral as well. It is thus equally parsimonious as a dinosaurian synapomorphy reversed in a few taxa or a character which convergently developed in several dinosaurian clades.
- a perforated acetabulum is a dinosaurian synapomorphy (absent in Silesaurus, Marasuchus, Lagerpeton, Scleromochlus and pterosaurs) otherwise present in poposaurids and ornithosuchids.
- a mesotarsal ankle joint is generally agreed to be an avemetatarsalian synapomorphy instead.

Far from being equivocal or of doubtful validity, nearly all of these characters unequivocally support Dinosauria or Dinosauria+Silesaurus. A few cannot be determined among basal dinosauromorphs, but they still support a monophyletic origin of dinosaurs separate from chirotheroids. Numerous additional characters diagnose nodes between Avemetatarsalia and Dinosauria (Novas, 1996; Benton, 1999; Langer and Benton, 2006). Also important is that the many characters diagnosing Saurischia and Eusaurischia support a Dinosauria more monophyletic than advocated by Thulborn, where sauropodomorphs and theropods have separate origins. The occasional reversals and convergences are of little consequence, as no one dinosaur clade exhibits many reversals and the only non-dinosaurian taxa that share more than a few of the characters are poposaurids (e.g. Effigia). As usual for 1970's dinosaur polyphyly proposals (and BAND proposals), Thulborn never addresses which non-dinosaurian taxa are most closely related to theropods, sauropodomorphs or ornithischians. This leaves his hypothesis untestable. His statement "In short, the evidence for dinosaurian monophyly is weak, inconclusive and no more compelling than it was 20 years ago (Charig 1982)." is of course completely inaccurate. To the contrary, newly discovered basal theropods and sauropodomorphs (Guaibiasaurus, Saturnalia) are extremely similar to each other, while more complete remains of herrerasaurids show them to have many saurischian characters.

In conclusion, Thulborn's ideas on dinosaur polyphyly are based on misidentification of tracks, personal incredulity regarding character transformations, a near complete disregard for basal dinosauromorphs and basal sauropodomorphs, and a dependance on outdated sources. Furthermore, he is extremely misleading in regard to herrerasaurid relationships and pedal morphology, Pisanosaurus' relationships and morphology, dinosaur tarsal morphology, and the number and quality of proposed synapomorphies supporting dinosaurian monophyly. Many of his ideas are disturbingly archaic, and like Feduccia and Martin, he cites outdated articles and those which tangentially deal with a topic as evidence of controversy or validity when in actuality all modern articles dealing with that topic in depth have reached a consensus. Just like his digital homology paper, this one belongs back in the 1970's.

Mickey Mortimer