Ontogenetic crest development in Tupuxuara

Let’s set the stage
Earlier we looked at the isometric growth of the azhdarchid pterosaur, Zhejiangopterus and the juvenile Pteranodon. Earlier we falsified unsupported claims that tiny Solnhofen pterosaurs were allometric juveniles of larger forms. Earlier we reported on four pterosaur embryos that demonstrated isometric growth. None had the larger eyes and shorter rostrum that typify hatchling and newborn mammals, birds and crocs (contra Bennett 2006). Even so, Witton 2013 continues to promote the false hypothesis of allometric growth during ontogeny in pterosaurs (Fig. 1).

But we haven’t yet touched on the crest size question
Today we’ll show evidence that yes, crest size does increase during ontogeny. And, really, how else would you ever expect to get such big crests inside an eggshell? However, rostrum length and eye size does not change during ontogeny, no matter how much Bennett and Witton want that to happen. Witton unwittingly demonstrates this himself with a juvenile Tupuxuara (Fig. 1).

Figure 2. Witton 2013 promotes the myth that small Rhamphorhynchus specimens were juveniles of larger specimens. Actually the small ones were ancestral to the larger ones as recovered in phylogenetic analysis.  Below: An actual juvenile Tupuxuara, matched to another species of Tupuxuara with a longer rostrum. Note the juvenile eyes are not large. Crest size does increase with ontogenetic age.

Figure 1. Witton 2013 promotes the myth that small Rhamphorhynchus specimens were juveniles of larger specimens. Actually the small ones were ancestral to the larger ones as recovered in phylogenetic analysis.
Below: An actual juvenile Tupuxuara, matched to another species of Tupuxuara with a longer rostrum. See figure 3 for a better match to an adult. Note the juvenile eyes are not large. Crest size does increase with ontogenetic age.

Today
we have three size of Tupuxuara (Fig. 2) based on the Goshura specimen including two likely juveniles morphologically close to it. Each has a distinct skull showing some variation, but together they shed some light on crest development with maturation in these large Cretaceous pterosaurs. Note that none have the traditional short rostrum and large eyes Bennett (2006) predicted. Neither the juveniles nor the adult have the long rostrum of the Tupuxuara longicristatus specimen that Witton (2013) substituted (Fig. 1) for an adult version of this species. Note that the middle colorized specimen has a relatively longer rostrum than the Goshura specimen. This may represent individual variation grading into speciation.

Figure 1. Ontogenetic skull and crest development in Tupuxuara. Note the eyes are small and the rostrum is long in juveniles. Only the crest expands and only posteriorly.

Figure 2. Ontogenetic skull and crest development in Tupuxuara. Note the eyes are small and the rostrum is long in juveniles and adults, even longer in the middle juvenile. Only the crest expands  posteriorly.

TMM 42489-2, the tall crested Latest Cretaceous large rostrum and mandible. It's a close match to that of Tupuxuara, otherwise known only from Early Cretaceous South American strata.

Figure 3. TMM 42489-2, the tall crested Latest Cretaceous large rostrum and mandible. It’s a close match to that of Tupuxuara longicristatus, otherwise known only from Early Cretaceous South American strata. Note the big difference in rostral length is a phylogenetic difference, not an ontogenetic one. If you were hell bent on proving allometric growth, this is the skull I would use, like the one Witton did. 

We also have a juvenile Tapejara with a crest
And the crest not much smaller than an adult crest. Skull proportions are virtually the same between adult and juvenile. And yet, look, there’s another long rostrum adult, just another variation on a theme.

Figure 4. Juvenile Tapejara with two adults of distinct species. Note the rather large crests on the juvenile, but otherwise the skull had adult proportions.

Figure 4. Juvenile Tapejara with two adults of distinct species. Note the rather large crests on the juvenile, but otherwise the skull had adult proportions.

Along the same lines:
Witton 2013 also attempted to promote the Bennett (1995) idea that small Rhamphorhynchus specimens were actually juveniles (Fig. 1). Neither worker used phylogenetic analysis which demonstrates that known small Rhamphorhychus specimens were actually primitive, closer to the outgroup taxon, Campylognathoides.

You’ll recall
Bennett (1991, 2000) said that short-crested, smaller specimens of Pteranodon represented juveniles and females. That was falsified using phylogenetic analysis that recovered short-crested Pteranodon specimens closer to short-crested outgroup taxa, such as Germanodactylus. We do know of Ptweety, the juvenile Pteranodon, but it had a long rostrum and, unfortunately, the crest was broken off

Nevertheless, it’s obvious that long and tall-crested pterosaur specimens could not have fit such long and tall crests into an eggshell. So there must have been some sort of crest growth during ontogeny in large crested specimens. And this, so far, is the evidence for it.

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 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
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.SMNS
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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