Unwin et al. (2014)
describe an increasing number of tail vertebrae in a purported ontogenetic series (hatchling to juvenile to adult in a series of purported Darwinopterus specimens.) Although this is unheard of elsewhere among vertebrates, Unwin et al. link this trait to the origin of pterodactyloid-grade pterosaurs. And it should be mentioned that Unwin et al. are the only workers who nest darwinopterids basal to pterodactyloids. Andres nests anurognathids there. Kellner nests Rhamphorhynchus there. I nest tiny dorygnathids and scaphognathids there by convergence (e.g. Fig. 1) four times.
From the Unwin et al. 2104 abstract:
“The evolution of pterodactyloids from basal pterosaurs in the Early-Middle Jurassic involved a complex series of anatomical transformations that affected the entire skeleton. Until recently, almost nothing was known of this major evolutionary transition that culminated in the Pterodactyloidea, a morphologically diverse and ecologically important clade that dominated the aerial environment throughout the mid-late Mesozoic. The discovery of Darwinopterus, a transitional form from the early Late Jurassic of China, provided the first insights into the sequence of events that gave rise to the pterodactyloid bauplan and hinted at an important role for modularity, but was largely silent regarding the anatomical transformations themselves, or the evolutionary mechanism(s) that underlay them. A series of recent finds allowed us to construct a complete postnatal growth sequence for Darwinopterus. By comparing this sequence with those for Rhamphorhynchus and Pterodactylus, pterosaurs that phylogenetically bracket Darwinopterus, it is possible to map key anatomical transformations such as the evolution of the elongate, complex tail of basal pterosaurs into the short, simple tail of pterodactyloids. In Darwinopterus hatchlings the tail is shorter than the dorsal-sacral series (DSV) and consists of around 18 simple vertebral ossifications. The tail is longer (1-2 x DSV) in juveniles and has a normal complement of about 30 caudals, but only reaches its full length (2-3 x DSV) and complexity in adults. Basal pterosaurs largely conform to this pattern, although some species, including Rhamphorhynchus, have longer tails with up to 40 caudals. Generally, the tail of adult pterodactyloids, including Pterodactylus, resembles that of Darwinopterus hatchlings (≤18 ossified vertebrae; tail ≤0.7 x DSV; vertebrae simple, blocky), but occasionally develops a little further (e.g. in Pterodaustro) corresponding to the condition seen in early juveniles of Darwinopterus and paralleling the developmental pattern observed in long-tailed pterosaurs. The short tail of adult pterodactyloids, and anurognathids, basal pterosaurs that also have relatively short tails, appears to be neotenic, resulting from a sharp decrease in growth rate compared to the rest of the skeleton. This mechanism, heterochrony acting upon a distinct anatomical module to effect a large-scale morphological transformation, can be applied to other modules to generate the derived features (e.g. elongate neck and metacarpus, reduced fifth toe) that typify the pterodactyloid bauplan.”
Among professional pterosaur workers, only Unwin et al. nest Darwinopterus as the stem pterodactyloid. No one else does. Andres nests anurognathids with pterodactyloids. Kellner nests Rhamphorhynchus with pterodactyloids. Readers of this blog and reptile evolution.com know that when you add the sparrow- to hummingbird-sized Solnhofen pterosaurs, you get four clades of pterodactyloid-grade pterosaurs.
Figure 1. Scaphognathians to scale. Click to enlarge.
Are the specimens truly juvenile Darwinopterus? Or do they represent smaller genera or species, perhaps closely related, or not? Currently no two Darwinopterus specimens are conspecific. No two are identical. See them here. By comparing purported Rhamphorhynchus and Pterodactylus juveniles to putative adults I’m afraid Unwin et al. are playing with a pack of Jokers. Those smaller specimens are distinct species and genera, as recovered in the large pterosaur tree. Everyone should know by now that pterosaur juvenile pterosaurs are isometric matches to their adult counterparts, from several well-known examples. Any differences in Darwinopterus likewise mark phylogenetic, not ontogenetic differences.
Rhamphorhynchus and Pterodactylus only phylogenetically bracket Darwinopterus if the inclusion set is reduced to these three taxa. Otherwise they nest several nodes away from each other with lots of intermediate taxa as you can see here.
Unwin et al. claim the caudal count increases with maturity in Darwinopterus (18 in hatchlings, 30 in juveniles and adults). Put these into a cladogram and they probably become disparate taxa. Where else does the vertebral count nearly double during ontogeny? Nowhere. Those caudal counts for the larger specimens have to be estimates. Not every tail is complete. It appears as if the caudal count could vary among the larger specimens as well.
I see no mention of a phylogenetic analysis with regard to the various Darwinopterus specimens. This is a problem as Unwin et al. do not want to test their observations with the only method known to lump and split taxa. In the large pterosaur tree IVPP V 16049 nests with YH2000. 41H111-0309A nests with ZMNH M 8782. All four Darwinopterus taxa nest as a sister clade to Kunpengopterus + Archaeoistiodatylus and this combined clade is a sister to Wukongopterus, then the PMOL specimen of Changchengopterus, then Pterorhynchus. This major clade nests between Dorygnathus and Scaphognathus, both of which ultimately give rise to the two pairs of basalmost pterodactyloids.
I noted earlier that the Darwinopterus clade left no descendants. They also did not produce any small taxa like Dorygnathus and Scaphognathus did. Other workers thought the smaller Scaphognathus specimens were juveniles, despite the morphological differences. I can only wonder if the same situation is happening in the Darwinopterus clade? Perhaps what the Unwin team found are the smaller specimens previously missing from their clade branch. Even so, and sadly, this clade was not able to survive into the Cretaceous, small or not, because no known Cretaceous pterosaurs share darwinopterid traits. They are all accounted for with presently known tiny ancestors.
Unwin D, Lü J-C, Pu H-Y, Jim X-S 2014. Pterosaur tails tell tails of modularity and heterochrony in the evolution of the pterodactyloid bauplan. JVP 2014 abstracts