Extremely rare juvenile pterosaurs: no cute little baby faces here!

Contra traditional thinking (Witton 2013, Unwin 2005), there are very few pterosaur juveniles known and certainly there are no “cute” ones. The term “cute” here means having a baby face = short rostrum and big eyes, as in baby mammals, crocs and bird, distinct from the longer faces with relatively smaller eyes found in adults.

Among those verified pterosaur juveniles are Pteranodon (see Ptweety), Pterodaustro (see embryo and hatchling, we’re still waiting to see mid-size juveniles published, but we hear (Chinsamy et al. 2008) they are known) and Zhejiangopterus (Fig. 1, Cai and Wei 1994). We also know of four other embryos: Darwinopterus, the IVPP basal anurognathid embryo, the JVMP basal ornithocheirid embryo and the aborted embryo of Anurognathus.

None of these have a short rostrum and large eyes when compared to adults of the same genus. Rather, they are virtually identical to adults, only smaller. Note, however, the IVPP specimen and the Anurognathus embryo both have a short rostrum because that is typical for adult anurognathids. Even so, their eyes remain small.

Figure 1. Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured  in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.

Figure 1. Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest ones at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen. The skull is unknown in the largest specimens, but it is difficult to envision any skull longer than those shown here. Known bones from each specimen are shown in white.

All other purported juveniles (small to tiny chiefly Solnhofen pterosaurs), phylogenetically nest with a variety of other small to tiny pterosaurs succeeding larger primitive taxa and preceding larger derived taxa. Since this pattern repeats itself a half dozen times within the Pterosauria, this appears to be the most common method by which certain pterosaurs evolved smaller to genetically survive whatever was killing their unreduced brethren.

The other well known azhdarchid
Earlier we guessed what the embryo of another azhdarchid, Quetzalcoatlus, might look like and we restored the skull of Q. sp. based on all known specimens. Here (Fig. 1) we do the same with a hatching Zhejiangopterus. It’s easy because hatchlings and juveniles were virtual copies of adults. We call this method of maturation isometric development.

Isometry: the growth rates in different parts of a growing organism are the same. This is appropriate for pterosaurs. They hatch ready to fly.

Allometry: the growth rates in different parts of a growing organism are not the same. This is appropriate for mammals, birds and crocs who are fed by their parents and are stimulated to do so by their “cute” appearance.

Today’s focus
will be on a family of Zhejiangopterus individuals Figs. 1-4) that were described together (Cai and Wei 1994).

Figure 2. Zhejiangopterus juveniles. Neither of these two juveniles has a short rostrum or large eyes. Note the soft tissue impressions. Rostrum restored on M1324 based on M1330 by simple enlargement.

Figure 2. Click to enlarge. Zhejiangopterus juveniles to scale (see figure 1). Neither of these two juveniles has a short rostrum or large eyes. Green = maxilla. Yellow on skull = premaxilla. Note the soft tissue impressions. Rostrum restored on M1324 based on M1330 by simple enlargement. In M1324 the ascending process of the premaxilla is displaced.

Here (Figs. 1, 2) it is plain to see (we call this “evidence” in paleontology) that even young juveniles of Zhejiangopterus had a long skull and tiny eyes. And, wonder of wonders, this follows in the pattern of all other known pterosaur juveniles and embryos (listed above), rare though they are. The evidence in Zhejiangopterus also falsifies any claims made by traditional researchers that pterosaur young had “cute” features.

Moreover, we determined, through phylogenetic analysis, that skeletal bones did not co-ossify with maturity. Rather all the evidence indicates that bone fusion was a phylogenetic, rather than an ontogenetic signal.

Many of the small, chiefly Solnhofen specimens belong to clades of adults, like Scaphognathia, that also feature a short rostrum and large eyes. These traits were emphasized in ever smaller descendants by allometric development within the eggshell. That’s the only explanation that fits the evidence for morphological change (evolution) when juveniles display isometric ontogeny. Note, however, many other tiny pterosaurs had a long rostrum and small eyes, matching the traits of their clade and sisters, as demonstrated earlier.

There’s one other issue:
Cai and Wei (1994, Fig. 3) published a reconstruction of the small Zhejiangopterus skull that is inaccurate with regard to skull sutures. Unwin and Lü (1997) reprinted the same image without commenting on the problem.

Figure 3. Zhejiangopterus according to Cai and Wei 1994. No other pterosaur and no other azhdarchid has such a deep premaxilla. The back of the skull is more difficult to reconstruct. A better reconstruction, based on DGS, is shown in figure 4.

Figure 3. Zhejiangopterus according to Cai and Wei 1994. No other pterosaur and no other azhdarchid has such a deep premaxilla with such a long ventral margin. Moreover, no pterosaur has such a short maxilla.  The back of the skull is more difficult to reconstruct due to crushing. A better reconstruction, based on DGS, is shown in figure 4.

Cai and Wei 1994 gave Zhejiangopterus a huge premaxilla (Fig. 3) and a tiny maxilla, both atypical for azhdarchids and pterosaurs in general. Unwin and Lü (1997) accepted this error without comment.

The actual premaxilla (Figs. 2, 4) is a small strip along the dorsal rim (Fig. 4) and it extends along the jawline just a short length. The maxilla actually comprises the great majority of the rostrum with the nasal and jugal adding laminated layers (Fig. 4). This may be yet another example of how experts have disfigured pterosaurs and how DGS has brought more accuracy to tracings than experts have provided with the fossil in front of them. More precision in tracing along with referencing other closely related taxa would have solved such problems.

Figure 4. Zhejiangopterus skull. Two scale bars are shown for the large and small specimens.

Figure 4. Reconstructed Zhejiangopterus skull. Two scale bars are shown for the large and small specimens, but the reconstruction is based on the two smallest juveniles (Figs. 1, 2). Here the premaxilla is a small strip on the dorsal rim of the rostrum. The nasal and jugal laminate the large maxilla . The nasal is much larger here and the postorbital is more traditionally triangle shaped.

All that aside, if you have evidence for errors or more precision, please send them along.

The Independent Hatchling Issue
In Zhejiangopterus and Pterodaustro various size specimens were found together. So at least some pterosaurs enjoyed families or flocks, despite being able to fly independently shortly after hatching.

References:
Cai Z and Wei F 1994.
 On a new pterosaur (Zhejiangopterus linhaiensis gen. et sp. nov.) from Upper Cretaceous in Linhai, Zhejiang, China.” Vertebrata Palasiatica, 32: 181-194.
Hwang K-G, Huh M, Lockley MG, Unwin DM and Wright JL 2002. New pterosaur tracks (Pteraichnidae) from the Late Cretaceous Uhangri Formation, southwestern Korea. Geology Magazine 139(4): 421-435.
Unwin D and Lü J. 1997. On Zhejiangopterus and the relationships of Pterodactyloid Pterosaurs, Historical Biology, 12: 200.

wiki/Zhejiangopterus

PS an update was just added to the pterosaur wings as gills hypothesis with reference to gas exchange in bat wings here.

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