The Tiniest Pterosaur: No. 6

Figure 1. Click to enlarge. Scaphognathus and its shrinking descendants through No. 6 (the tiny one in the middle) at the base of the increasingly larger Germanodactylus clade.

A Little Background
Earlier we looked at the tiny pterosaurs that phylogenetically succeeded the smaller specimens of Scaphognathus (Figure 1). Analysis (see below) indicates that these tiny taxa, not Darwinopterus, were the true transitional taxa appearing at the bases of every major pterosaur clade. While many of these tiny pterosaurs have been traditionally labeled “Pterodactylus,” not all of them actually nested with Pterodactylus, as noted earlier. While virtually all tiny pterosaurs were considered juveniles (Wellnhofer 1970, Bennett 2006), phylogenetic analysis indicates they were not.

In preparation for our look at the Germanodactylus clade (coming soon and previewed in Figure 1), we’ll examine the pipsqueak at its base. Wellnhofer (1970) considered this diminutive specimen a hatchling Pterodactylus prior to the discovery of embryo pterosaurs in their eggs that disprove the hypothesis of allometric growth in pterosaurs.

Meet B St 1967 I 276 (or No. 6 in the Wellnhofer 1970 catalog).
No. 6 is the single tiniest of all pterosaurs. It stands no taller than the shoulders of other tiny pterosaurs including several ready-to-fly embryos (Figure 2). No. 6 is about the size of the smallest bird, the bee hummingbird (Figure 2). Both are several times larger than the smallest living lizard, Sphaerodactylus ariasa (Hedges and Thomas 2001).  If No. 6 is not an adult, we know from the examples of embryo pterosaurs, that it is a good copy of an adult. It’s sisters are only slightly larger than No. 6, so it unlikely to be a hatchling of an eight times larger taxon. Perhaps No. 6 would have become an adult upon reaching the size of the other tiny pterosaurs that were its sisters. No. 6 is smaller than the four pterosaur embryos now known (Figure 2).

Figure 2. From left to right, the smallest known bird (a bee hummingbird), the smallest known pterosaur (No. 6) and and the IVPP pterosaur embryo all to scale. Below them is the smallest known lizard, a living gecko, Sphaerodactylus ariasa.

No. 6 in Detail
Other than its size, there was nothing particularly “juvenile” about No. 6. Yes, it had large eyes, but these were inherited from its ancestors, sisters to Scaphognathus and Ornithocephalus.  The anterior teeth were procumbent (tilted forward), with the two medial teeth merging to become one (or else losing one tooth in the process).  The quadrate was set at a low angle, perhaps as a buttress to some sort of pecking motion. Descendant taxa, like No. 12, No. 23 and Germanodactylus rhamphastinus (Figure 1) successively developed a longer, sharper set of jaws, ideal for spearing or pecking. Distinct from its phylogenetic predecessor, No. 31, the cervicals and torso were longer. The deltopectoral crest was squared off. The humerus was straight. The metacarpus and ulna were longer. The hind limb was more robust. The unguals were shorter. Pedal digit 5 was straight and shorter. PILs (parallel interphalangeal lines) indicate a digitigrade pes was more likely.

Figure 1. The smallest of all adult pterosaurs, B St 1967 I 276 or No. 6 in the Wellnhofer (1970) catalog. At left is the foot plantigrade and with metatarsals slightly raised, which simplifies and aligns the PILs (parallel interphalangeal lines). The gray oval is a hypothetical egg based on the pelvic opening. The sternal complex is also shown separated from the lateral view reconstruction.

The Problem With Being So Small
Recent studies of the world’s smallest lizards have revealed a problem with desiccation due to their high surface/volume ratio. With a snout/vent length under 18mm, Sphaerodactylus specimens dry up and die when removed from their moist leaf litter environment (Hedges and Thomas 2001). The younger, smaller juveniles of No. 6 would have been likewise faced with desiccation — only more so with their wings and uropatagia outstretched. With a snout/vent length around 5 mm, No. 6 hatchlings would have been smalller than house flies. It appears likely that such tiny pterosaurs were not ready to fly when hatched unless they had a novel method for conserving moisture with their wings out and flapping. Rather, they may have found their food by walking on all fours through moist leaf litter until large enough to take wing.

Figure 3. Click to enlarge. The family tree of the Pterosauria. No. 6 appears midway down the right column.

Why Did Pterosaurs Shrink?
We can look at the family tree of pterosaurs to see why pterosaur transitional taxa were much smaller than their predecessors and successors. After the appearance of the tiny pterosaurs, the larger predecessors became extinct. Evidently there were environmental pressures that were not favorable to larger pterosaurs. Evidently a faster maturation at a smaller size became a survival advantage.

 

Figure 4. Ginko leaf and the smallest pterosaur and its hatchling to scale.

How Did Pterosaurs Shrink?
Chinsamy et al. (2008) reported that Pterodaustro specimens were sexually mature at half their full size. If eggs were proportional to the pelvic passageway, then such young mothers would have produced smaller eggs and smaller embryos. This provides a method for rapid size reduction and generational turnover after just a few generations. The ultimate disappearance of pterosaurs may be blamed on their inability to shrink enough after achieving their greatest sizes by the end of the Cretaceous. Note there was no pterosaur genus that existed unchanged throughout the Mesozoic. Rather a succession of taxa, both larger and smaller, survived through that era.

Hone and Benton’s Three Mistakes
Hone and Benton (2006) reported, “The remarkable extinct flying reptiles, the pterosaurs, show increasing body size over 100 million years of the Late Jurassic and Cretaceous, and this seems to be a rare example of a driven trend to large size (Cope’s Rule).” They arrived at this “result” by drawing a straight line from early pterosaurs, like Anurognathus, to the Late Cretaceous pterosaur, Quetzalcoatlus over time and by deleting all purported juveniles. They did not realize that 1) there were four pterodactyloid-grade lineages; 2) the purported juveniles were actually adults; and 3) any sort of a roller-coaster effect of size increase/decrease/increase/decrease over time would be negated by drawing a straight line.

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 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.SMNS
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
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.
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
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.