The Family Tree of the Pterosauria 15 – Pteranodon

We just looked at the base of the Protopteranodontia and Nyctosaurus. Today we’ll finish up the Pteranodontia with everyone’s favorite, Pteranodon, then later move over to the Cycnorhamphus/Ornithocheirid clade.

Figure 1. Click to enlarge. A family tree of Nyctosaurus and Pteranodon. Note the gradual size increase followed by, in one lineage, a size decrease. 

No one has contributed more to our understanding – and misunderstanding – of Pteranodon (Marsh 1876) more than Dr. S. Chris Bennett. His detailed and unprecedented 1991 PhD thesis on Pteranodon was subsequently split into several published papers (Bennett 1992, 1993, 1994) culminating with a reprinting of his PhD thesis in two parts (2001). Bennett’s hypotheses on pterosaur juveniles, growth, bone fusion and family trees have been the basis for many current studies. Unfortunately, his use of statistics, rather than phylogenetic analysis and his reliance on the outdated archosaur paradigm, rather than the new lizard model, has created problems  in his assessments of growth patterns, gender and speciation in Pteranodon and other taxa.

Two species or several?
Bennett (1991, 1992,  1993, 1994, 2001) reduced Pteranodon to two species: the holotype P. longiceps and the high-crested P. sternbergi. All other variations (Figure 1) Bennett ascribed to gender and immaturity. Phylogenetic analysis tells a different story. I analyzed the specimens above (Figure 1) and found a huge variety that nested in a phylogenetic sequence. Smaller taxa generally were more primitive (but the exceptions (R-V and Z4 in Figure 1) tell another story, see below).  P. occidentalis YPM 1179 (Marsh 1876) is the one closest to the outgroup, the private specimen of Germanodactylus, SMNK PAL 6592. Here a gradual increase in size and crest size, among many other traits is documented followed by a size decrease in a clade descending form a sister to P. ingens.

Figure 2. Click to enlarge. The Triebold specimen and UALVP 24238, the two most complete Pteranodon known. These two are nearly sister taxa and still the morphological variation is striking.

Kellner assigned some Pteranodon specimens to new genera
Kellner (2010) reassigned the Alberta specimen, UALVP 24238, (Z in Figure1) to a new genus and species: Dawndraco kanzai because the upper and lower margins of the rostrum were nearly parallel to one another, rather than tapering, as in all other specimens. Bennett (1991, 1994, 2001) had a assigned it to P. sternbergi. Here the Alberta specimen was derived from USNM 12167 (W in Figure 1) and is at the base of a clade of two other tall but slender crested Pteranodon specimens, AMNH 5099 and YPM 2473 (Z2 and Z3 in figure 1), both of which lack a rostrum for comparison. So a new genus does not appear warranted, only a new species: Pteranodon kanzai. Kellner (2010) also reassigned Pteranodon sternbergi FHSM VP 339 (Harksen 1966) to a genus previously erected by Miller (1972), Geosternbergia sternbergi. Again, considering the wide variation in species within several other widely recognized pterosaur genera (Rhamphorhynchus, Pterodactylus, Campylognathoides, Dorygnathus, etc.), such an assignment appears to be unwarranted until all these other genera are similarly split apart. Actually there is greater morphological disparity in several other Pteranodon specimens, but this was overlooked.

Gradual Increase in Size Followed by Gradual Decrease
We’ve already seen what happens in various other pterosaur clades as size increase is followed by decrease and followed again by increase. We see the same pattern in Pteranodon. Phylogenetically the largest specimens in the P. ingens clade were followed by smaller specimens. Of greater interest, these specimens had a shorter rostrum and an unfused scapulocoracoid, a pattern seen in other pterosaurs lines. Bennett (1991) and others ascribed a short rostrum and lack of fusion to immaturity (following the archosaur model). But that is false. Pterosaurs were lizards with lizard-like growth patterns. In living lepidosaurs, Maisano (2002) reported, “no terminal fusion universally coincides with the achievement of either sexually or skeletally mature size and that some squamates continue to grow long after the fusion of various elements.” Thus pterosaurs, as squamates, could have fused bones early in ontogeny. They could also have never fused certain bones before they died of old age. Under the squamate paradigm, “immature” bone texture could have been retained in phylogenetically smaller adults. This is a key fact that has been previously overlooked or ignored by Bennett and others. And that’s why you only find unfused scapulocoracoids in these smaller derived taxa (R, S, T and Z4 in Figure 1), not in the more primitive taxa (L – N ) of similar size.

Figure 3. Click to enlarge and identify. Various Pteranodon specimens known from post-crania. Note the yellow box includes one of the largest specimens, but it has an unfused extensor tendon process, which may mean it is a very large Nyctosaurus with fingers, following the pattern in primitive Nyctosaurus.

Over 1000 Specimens, Few Even Close to Complete.
Unfortunately, most Pteranodon corpses did not stay intact on their way to the sea floor. That makes it more of a puzzle with smaller pieces to work with. Very few skulls are associated with post-crania. Skull-only and skull-less taxa are the rule. Can they be correlated? The answer is yes, at least somewhat. Fortunately we know more about the P. sternbergia clade because we have two relatively complete specimens from that clade (Figure 2). When we employ skull and skull-less taxa together we get much less phylogenetic resolution because there are no traits certain sisters have in common. However if we run two analyses, one with skulls and one without, then we don’t have that problem. What we get our two very similarly split trees that enable us to match certain post-crania with skulls by the process of elimination and phylogenetic analysis (Figure 4).

Figure 4. Click to enlarge. Comparing Pteranodon skull and skull-less taxa. The three main clades (basal, P. ingens and P. sternbergi) are recovered either way.

What We Can Learn from a Juvenile Pteranodon
Several years ago, a juvenile Pteranodon (informally known as “Ptweety”, Figure 3C) was found and (unfortunately) mounted with lots of putty into a standing reconstruction and sold to a retailer. These actions removed this specimen from the possibility of appropriate academic study. Even so, I was able to see photographs of the in situ specimen, examine casts and talk with the preparator. Ptweety turned out to be a Nyctosaurus.

Lack of Fusion Means Immaturity?
Bennett (1991, etc.) reported on a very large proximal wing phalanx (Figure 3o) without a fused extensor tendon process (YPM 2501), but this specimen belongs to a very large Nyctosaurus.

Female Pteranodon?
Bennett (1991, 1992, 2001) reported on a disassociated pelvis that he believed belonged to a female Pteranodon because it had a deeper pelvic opening than all other Pteranodon pelves, including the Triebold specimen, which has a small crest (which meant it should have also been female). The only problem is, the pelvis in question, KUVP 993, is identical to smaller Nyctosaurus pelves (Figure 5). Here is yet another clue that a big Nyctosaurus roamed the airspace over the Niobrara Sea.

Figure 5. Pteranodon and Nyctosaurus pelves in left lateral view. A. Pteranodon ingens YPM 1175, reconstructed from Eaton (1910). B. Pteranodon sp. UNSM 50036. C. The TRIEBOLD specimen of Pteranodon, NMC 41-358, tracing from in situ specimen. D. Pteranodon sp. UALVP 24238, tracing from in situ specimen. E. Nyctosaurus bonneri, FHSM VP 21 reconstructed. F. Nyctosaurus gracilis, FMNH 25026 in situ G. Nyctosaurus sp. UNSM 93000, reconstructed. H. The same enlarged 1.85x to reflect the hypothetical pelvis size of the largest known Nyctosaurus (Bennett 2000). I. Dubious Pteranodon? KUVP 993 tracing from extricated specimen and slightly reconstructed. While larger than any other known Nyctosaurus pelvis, KUVP 993 has a pelvic aperture deeper than any Pteranodon (Fig. 3a) and morphologically more similar to Nyctosaurus specimens. The obturator foramen rivals and surpasses the acetabulum diameter in Nyctosaurus, not Pteranodon. Thus KUVP 993 does not represent a female Pteranodon pelvis, but a very large Nyctosaurus pelvis. Scale bar = 10 cm. Black circles are approximate egg diameters able to pass through each pelvic opening. Note C and D are associated with skulls (Fig. 2) in the P. sternbergi lineage.

As always, I encourage readers to see 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:
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1993. The ontogeny of Pteranodon and other pterosaurs. Paleobiology, 19:92-106.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Eaton GF 1910. Osteology of Pteranodon. Memoirs of the Connectictut Academy of Arts and Sciences 2:1-38.
Harksen JC 1966. Pteranodon sternbergi, a new fossil pterodactyl from the Niobara Cretaceous of Kansas. – Proceedings of the South Dakota Academy of Science 45: 74–77.
Kellner AWA 2010. Comments on the Pteranodontidae (Pterosauria, Pterodactyloidea) with the description of two new species. Anais da Academia Brasileira de Ciências 82(4): 1063-1084.
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrate Paleontology 22:268-275.
Marsh OC 1876. Notice of a new sub-order of Pterosauria. American Journal of Science, Series 3, 11:507-509.
Miller HW 1972. The taxonomy of the Pteranodon species from Kansas. Transactions of the Kansas Academy of Science 74: 1–19.