Darwinopterus, NOT the Transitional Pterosaur

Google Darwinopterus and you’ll get entries like: “the remarkable transitional pterosaur,” “one hell of an intermediate,” “fills evolution gap.” The news has been out for awhile now. Darwinopterus (Figure 1) has been hailed as the long lost transitional taxon bridging the basal long-tailed pterosaurs and the later short-tailed pterosaurs.

Or was it?

Pterosaurs, the flying reptiles of the Mesozoic, have been known for over 200 years, but until recently no taxon has been put forth as the transitional “link” between the primitive short-handed, long-tailed forms and the more derived long-handed, short-tailed ones. Lü et al. (2009) claimed to fill that gap with an unusual pterosaur, Darwinopterus modularis, which appeared to combine traits from both groups.

The skull and neck were described as “typically pterodactyloid,” while the remainder of the skeleton was considered “identical to that of basal pterosaurs.” This specimen was used to support the hypothesis of “modular evolution” in which the transition from basal to derived pterosaurs was posited to proceed in two phases: (1) skull and neck followed by  (2) post-cervical axial column, limb girdles and limbs.

Darwinopterids

Figure 1. Click to enlarge. Some of the several Darwinopterus specimens now known.

Four big problems arose from the Lü et al. (2009) study: 
1.The cladistic results were not robust. There were no distinct sister taxa and there was a great loss of resolution at the Darwinopterus node. No sequence of basal taxa in the Lü et al. (2009) analysis documented a gradual accumulation of Darwinopterus characters. Instead, four proximal sister taxa nested in an unresolved clade and one of those included four sister taxa in an unresolved clade. Similarly, three derived clades without resolution nested as sisters. A fine mess! And not the way evolution works!

2. The gamut of included taxa was not sufficiently broad ( = inclusive) to recover a single tree result matching evolution’s own single tree. Instead 500,000+ trees were recovered! Such loss of resolution provides little to no recoverable data. Missing from the inclusion set were all the tiny pterosaurs (skulls smaller than 2 cm in length, Figure 3) because they were traditionally considered juveniles and thus unworthy of inclusion. Also missing were several specimens considered congeneric with others but were distinct in several regards.

3. Evolution does not otherwise proceed in modules. Most fossil vertebrates can be identified by just a skull, tooth, pes or pelvis, but “modular evolution” would play havoc with this, creating chimaeras with the head of one taxon on the torso of another, as imagined in Darwinopterus by Lü et al. (2009). There is another, more parsimonious, explanation (see below).

4. Lü et al. (2009) used more than one specimen to score Darwinopterus, Scaphognathus and other pterosaurs. In Darwinopterus, the YH skull was a third longer with a larger antorbital fenestra, smaller orbit and deeper skull. Such characters change scores. Lü et al. (2009) also incorrectly scored several taxa (details on request).

Here’s how to solve all four problems at once:
Increase the size of the cladistic study inclusion set (Figure 4).

I added Darwinopterus to a cladistic analysis of 170 taxa (152 pterosaurs + 18 outgroup taxa, Figure 4)). These were tested against 185 characters. That analysis employed three times the number of taxa employed by Lü et al. (2009) against fifty percent more characters. While the Lü et al. (2009) resulted in 500,000+ trees, my analysis recovered a single tree in which there was not one, but four “pterodactyloid”-grade clades (not including the darwinopterids/wukongopterids). Completely unexpected, but it makes sense when you look at the details in each of the lineages. This new family tree goes a long way to explaining many of the mysteries surrounding pterosaurs.

Darwinopterus and kin nesting between Sordes and Scaphognathus

Figure 2. Nesting between Sordes and Scaphognathus were Pterorhynchus, Kunpengopterus, Darwinopterus and Wukongopterus (not shown), taxa with a longer skull, a longer neck and a reduced to absent naris..

Darwinopterus nested as the sister to Wukongopterus (Wang et al. 2009). Both nested as sisters to Kunpengopterus (Wang et al. 2010) and Pterorhynchus (Czerkas and Ji 2002). This clade nested between Sordes and Scaphognathus (Figure 2.), far from any of the actual transitional pterosaurs (see below). Lü et al. (2009) included Pterorhynchus in their analysis, but they did not score 27 percent of the characters. Of the remaining 80 characters 19 were scored differently in Darwinopterus and Pterorhynchus. Of these 19, I found 10 scores to be valid. Two of these described the longer rostrum in Darwinopterus. Three described the absence of a distinct naris in Darwinopterus. Of the remaining 9 characters all were incorrectly scored. (Details on available on request.)

The Actual Transitional Taxa
There were four transitional lineages (see below and Figure 4) that produced four convergent clades of “pterodactyloid”-grade pterosaurs, none of which came close to including Darwinopterus. At the bases of the four pterodactyloid-grade clades were four separate series of tiny pterosaurs, smaller than their predecessors and smaller than their successors. Each sequence documented a gradual transition from basal to derived.  Two arose from distinct Dorygnathus specimens. Two arose from the small Scaphognathus specimens (Figure 3).

The descendants of Scaphognathus.

Figure 3. Click to enlarge. The descendants of Scaphognathus. Note the size reduction followed by a size increase.

The widely-held hypothesis that small, short-rostrum pterosaurs were juveniles of larger forms is falsified by this study. Those tiny pterosaurs had a short rostrum because their ancestors within Scaphognathus also had a short rostrum. Other tiny pterosaurs did not have a short rostrum. They were basal to other clades leading to the azhdarchids and to the ctenochasmatids. If the tiny pterosaurs were juveniles, they should have nested with their parents, as in Pterodaustro.

Remember: pterosaurs were derived from a clade of lizards that grew isometrically, not allometrically. Juveniles were close matches to adults. This is never more clear than when looking at pterosaur embryos here, here and here. Even the Darwinopterus embryo was the spitting image of its mom, with a long rostrum and tiny eyes.

Pterosaur family tree

Figure 4. Click to enlarge. The family tree of the Pterosauria.

Other pterosaurs also had tiny transitional taxa. Between Campylognathoides and Rhamphorhynchussize reduction is documented. Preceding Nyctosaurus and Pteranodon a small taxon appears.

The fact that pterosaurs reduced their size during morphological transitions indicates that size reduction was a survival technique producing new morphologies (and sizes!) to keep genetic lineages from going extinct. Eudimorphodontids, dimorphododontids, dorygnathids, campylognathids and rhamphorhynchids did not survive into the Cretaceous, but their reduced descendants did. Thereafter size increases created some of the most spectacular forms now known. The tiny scaphognathids and dorygnathids were their common ancestors. This new heretical hypothesis of pterosaur relations plays havoc with traditional hypotheses, including the one invoking Cope’s rule by Hone and Benton (2007).

So what was Darwinopterus, if not a transitional pterosaur?
Sadly, Darwinopterus and Wukongopterus produced no known descendants. They were derived pterorhynchids about the same size as others. In the context of their phylogenetic nesting, both Darwinopterus and Wukongopterus inherited their so-called “basal” traits from a sister to Pterorhynchus, which also had a rather short metacarpus, long tail, long pedal digit 5 and long tail. The elongation of the skull and reduction of the naris were well underway in Pterorhynchus. The elongation of the neck was relatively slight. These traits were convergent with those in “pterodactyloid”-grade pterosaurs. A sister taxon, Wukongopterus, did not have an elongated neck. Kunpengopterus had a longer neck, but not an elongated skull (see above).

As always, I encourage readers to see the specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References:
Czerkas SA and Ji Q 2002. A new rhamphorhynchoid with a headcrest and complex integumentary structures. In: Czerkas SJ ed. Feathered Dinosaurs and the Origin of Flight. The Dinosaur Museum:Blanding, Utah, 15-41. ISBN 1-93207-501-1. 
Hone and Benton 2006. Cope’s Rule in the Pterosauria, and differing perceptions of Cope’s Rule at different taxonomic levels. Journal of Evolutionary Biology 20(3): 1164–1170. doi: 10.1111/j.1420-9101.2006.01284.x
Lü J, Unwin DM, Jin X, Liu Y and Ji Q 2009. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society London B  (DOI 10.1098/rspb.2009.1603.)
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605
Wang X, Kellner AWA, Jiang S and Meng X 2009. An unusual long-tailed pterosaur with elongated neck from western Liaoning of China. Anais da Academia Brasileira de Ciências 81 (4): 793–812.
Wang X, Kellner AWA, Jiang S-X, Cheng X, Meng Xi & Rodrigues T 2010. New long-tailed pterosaurs (Wukongopteridae) from western Liaoning, China. Anais da Academia Brasileira de Ciências 82 (4): 1045–1062.
Zhou C 2009. New material of Elanodactylus prolatus Andres & Ji, 2008 (Pterosauria: Pterodactyloidea) from the Early Cretaceous Yixian Formation of western Liaoning, China.Neues Jahr. Geo. Paläo. Abh. (DOI: 10.1127/0077-7749/2009/0022.)

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