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Darwinopterus paper: misrepresentations and red flags

Misrepresentations and Red Flags:

Darwinopterus is a fantastic find with grand implications. Nevertheless, the following statements were found in the new Darwinopterus paper by Lü et al. 2009. The Misrepresentations (Misreps) appear to play politics with the facts. The Red Flags are characters that should have prompted more investigation on the part of the Lü team.

“The 65 Myr long Late Triassic–Late Jurassic interval seems to have been almost completely dominated by basal clades consisting of small to medium sized seemingly piscivorous or insectivorous pterosaurs (Wellnhofer 1975, 1978, 1991; Kellner 2003; Unwin 2003a,b, 2005).”

Misrep: Huanhepterus, Ctenochasma, Cycnorhamphus, Pterodactylus and Germanodactylus are all Late Jurassic.

“Typical characters of these [basal] pterosaurs include: separate nasal and antorbital openings in the skull, elongate cervical ribs, short metacarpus (less than 80% humerus length) and a fifth toe consisting of two elongate phalanges that supported a flight membrane (cruropatagium) stretched between the hind limbs (Unwin & Bakhurina 1994).”

Misrep: No membrane stretched between the hind limbs. Membranes trailed each hind limb. In any case, Figure 4 of the Darwinopterus paper shows the membranes stretched between the tail and hind limbs.

“…as shown, for example, by the differing allometric relationships evident in growth patterns for the skull, neck and post-cervical axial skeleton (Bennett 1996).”

Misrep: Pterosaur embryos are matches for parents. Bennett’s juvenile/ adult “pairings” were not congeneric.

“The skull is unusually large, almost twice the length of the dorsal sacral vertebral series (DSV). This value is substantially greater than for any basal pterosaur and is high even for pterodactyloids (figure 3a).

Red Flag. A basal pterodactyloid-grade pterosaur should be a close match (incrementally different) to its predecessor in most regards, including the skull. That’s the way evolution works. The alternate view supports and demonstrates that.

“…the tail of Darwinopterus is identical to that of most basal clades, consisting of more than 20 caudals which, apart from the first three or four vertebrae, are long and rod-like and enclosed by a sheath of bony filaments composed of highly elongated ossified extensions of the zygapophyses and hypapophyses (figure 2e) (Wellnhofer 1975, 1978).”

Misrep: Pterodaustro likewise has an elongated tail. We can suspect its predecessors also did. Just because preparators haven’t prepped out more long tails, doesn’t mean they’re not there. A little digging will reveal more. That the tail of Darwinopterus is more robust than the tail of Scaphognathus is notable.

“The glenoid is located on the scapula rather than equally shared by the scapula and coracoid as in pterodactyloids (Wellnhofer 1978).”

Misrep: Also shared by scapula and coracoid in basal pterosaurs too. IF the glenoid is only on the scapula in Darwinopterus (hard to see) , that would be a derived character, not a transitional character.

“The metacarpus is less than 70 per cent the length of the humerus, a universal feature of basal pterosaurs (figure 3c),”

Misrep: This is Nature at her trickiest. This species has an atypically short metacarpus for reasons aside from phylogenetic transition, as determined by its placement higher in the family tree. This is a reversal.

“Darwinopterus corresponds to the condition in all long-tailed pterosaurs where phalanges two and three are of similar dimensions and exceed the length of the first and fourth phalanges (Wellnhofer 1975, 1978; Kellner 2003).”

Misrep: M4.2 not < m4.1 also in Elanodactylus, Ninchengopterus, two Pterodactylus specimens, Wellnhofer (1970) No. 6, No. 12, No 23, all ctenochasmatids. M4.2 < m4.1 among basal pterosaurs in three basal Rhamphorhynchus specimens, Raeticodatylus and eight higher anurognathids.

“Finally, the fifth toe consists of two elongate phalanges (figure 2d; electronic supplementary material, figure S3), as in all basal pterosaurs, and the distal phalanx is sharply recurved, a condition only met with in some rhamphorhynchids (Kellner 2003; Unwin 2003a,b).”

Misrep: Such a toe appears in a wide variety of pterodactyloid-grade pterosaurs, unrecognized until now. Preparators understood that a big fifth toe was not to be found in certain pterosaurs, so they didn’t bother to even try to prep them out.

“These contrasting results emphasize two key aspects of Darwinopterus: the complete absence of ‘intermediate’ character states that fall between those states found either in basal pterosaurs or in pterodactyloids (figure 3), and the almost perfect modularity exhibited by the mosaic pattern of character state distributions found in this pterosaur.”

Red Flag: Evolution, by definition, is a series of small intermediate steps. The alternate view demonstrates small steps.

“Darwinopterus provides, to our knowledge, the first detailed insights into the transition from basal pterosaurs to pterodactyloids and, when combined with our rapidly increasing knowledge of the pterosaur fossil record (Barrett et al. 2008), helps to pinpoint several key features regarding the nature and timing of this event (figure 4c).”

Misrep: Well, it wouldn’t be the first time the alternate view was ignored. Both Lü and Unwin heard and read the alternate view in Munich in Sept. 2007. The paper provides few insights in that no predecessor genus is identified and no successor genus is identified. The alternative view has these predecessors and successors.

“Two distinct phases are recognized. In the first, elongation of the skull, breaching of the bony bar separating the nasal and antorbital opening, reconfiguration of the cranium leading to an increase in the relative size and volume of the braincase and simplification of the dentition, together with changes to the shape of the cervical vertebrae and loss of the cervical ribs, culminated in the monofenestrate skull and modified neck inherited by Darwinopterus and all pterodactyloids figure 4b).

Misrep: Simplificaton of the dentition? You can’t get simpler than Scaphognathus. And ctenochasmatids do not have simplified dentition.

> The ctenochasmatid problem was the key to my searching for a new answer to the pterodactyloid evolution issue. You can’t start from Darwinopterus and get Ctenochasma or Gnathosaurus. It’s more parsimonious to start with basal pterosaurs that have elongated teeth and a relatively wide rostrum, like Dorygnathus and Angustinaripterus, which elongates the skull and reduces the naris. When done right, you can see the gradual evolution of characters in all parts of the head, neck and body, including the feet. There is no need to invoke large scale “modular” evolution. Evolution works in increments, sometimes modular increments, but increments nevertheless, and these are all visible when the taxon list is enlarged.

“In the second phase of the transition, modifications were confined to the post-cervical axial column, limb girdles and limbs. Key among these were shortening of the tail, elongation of the metacarpus and reduction of the fifth toe, changes that appear to have significantly improved the locomotory abilities of pterodactyloids (Unwin 2005).”

Misrep: Lü et al. will find with further study that reduction of the fifth toe occurs, not once, but in several pterodactyloid-grade lineages. Shortening of the tail begins with the smaller Scaphognathus specimens.

On the timeline I note long ghost-lineage lines in the Jurassic. Plenty of time for evolution to work its magic.

On the phylogenetic chart I notice several taxonomic bunch-ups at the transition from basal to pterodactyloid-grade. In other words, several taxa are jockeying for positions both on the predecessor and the successor sides of Darwinopterus. This is the reason for the 500,000+ trees. A good phylogeny would have showed which genus was closest to Darwinopterus on both sides, but this study fails to do so. Increased taxon inclusion will help.

David Peters
St. Louis