Variation within the genus Pterodaustro

Wiki reports that there are over 750 specimens of Pterodaustro known with 288 of those cataloged. Only Pteranodon is known from more specimens (more than 1000). In both cases the vast majority are incomplete or scraps.

Earlier we looked at a new specimen of Pterodaustro with stomach stones and a wingtip claw MIC-V263 (Codorniú et al. 2013). Today we’ll compare the new specimen to a complete specimen PVL 3860 (Fig. 1) to see if it was identical or displayed variation in its morphology.

Figure 1. Pterodaustro specimens compared. Click to enlarge. In the middle, the stomach stones Pterodaustro. On the left to the same scale, the PVL 3860. On the right scaled to equal tibial length, PVL 3860 again. Tucked in on the right, the embryo Pterodaustro.

Figure 1. Pterodaustro specimens compared. Click to enlarge. In the middle, the stomach stones Pterodaustro, MIC-V263. On the left to the same scale, the PVL 3860. On the right scaled to equal tibial length, PVL 3860 again. Tucked in on the right, the embryo Pterodaustro.

Compared to the PVL specimen …
Scaled to a similar tibia length the MIC specimen had a relatively longer and more gracile humerus and metacarpus. Manual 4.2 and m4.3 were slightly longer. The femur was longer with a longer “neck.” The pes was longer with a relatively longer metatarsus and longer lateral toes. Metatarsal 3 was ever so slightly longer than mt1 and substantially longer than mt4. In the PVL specimen the metatarsals are ever so slightly shorter from mt1 to mt4. Digit 4 was longer than 3 in the MIC specimen, but subequal in the PVL specimen. Pedal digit 5 and its metatarsus have distinct proportions.

The anterior dentary teeth were relatively longer in the MIC specimen.

To the same scale…
The MIC specimen is the taller skinnier one with longer feet.

Individual variation?
Seems so. Similar variation is seen among gracile and robust humans and other animals. Such individual variation in genetic isolation is the engine that leads to new species and genera. Interesting that the feet appear to show the greatest variation, acting like fingerprints in identifying distinctions.

Compared to the embryo…
The MIC specimen has a more robust dentary, a shorter more gracile humerus, a shorter, more gracile antebrachium, a longer, more gracile metacarpus and a shorter femur. In the embryo the lateral metatarsals are longer but digit 4 is shorter than digit 3 and digit 5 is relatively longer. So once again, there’s a distinct pedal morphology.

It bears mentioning…
Pterodaustro is the only pterosaur genus currently known in both embryo and adult forms. Other pterosaur workers with traditional views on allometric growth have hypothesized that hatchlings would have a large orbit and short rostrum (Welnhofer 1970; Bennett 2006). The Pterodaustro embryo (as well as the others) falsifies that hypothesis and supports the isometric growth hypothesis demonstrated by Huehuecuetzpalli, Zhejiangopterus and Pteranodon, in which juvenile and adult forms are known. Even so, the Pterodaustro embryo has distinct morphological differences with published adults that suggest there was some isometric growth in this genus in the fore limbs and hind limbs (but the rostrum or orbit!), likely brought on by the different feeding strategies each of them had.

Hatchlings were too little to wade.
Very young Pterodaustro would not have been able to handle even a small wavelet and could not have waded into deep enough waters to feed. So, in this genus parental care might have taken place, vomiting partially digested food into whatever pterosaurs considered a nest. Perhaps their restricted diet made them good parents. We cannot ascribe the same behavior to other more generalized pterosaurs yet. Earlier we considered the possibility that most tiny pterosaurs had to live within damp leaf litter in order to avoid desiccation, as modern tiny lizards can experience (Hedges and Thomas 2001). So that would have been a separate niche with those pterosaurs feeding on tiny insects and such in the undergrowth.

Do individual bird species show such variation in their skeletons?
I don’t know. All of the Pterodaustro specimens were found in the same bone bed. This variation would make a good PhD project chronicling the variation and similarity in the unique Pterodaustro bone bed.

References
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Codorniú L, Chiappe LM and Cid FD 2013. First occurrence of stomach stones in pterosaurs, Journal of Vertebrate Paleontology, 33:3, 647-654.
Hedges SB and Thomas R 2001. At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

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