Another look at “Pterodactylus” spectabilis, actually a tiny dorygnathid

Today another tiny pterosaur
traditionally considered a juvenile (Wellnhofer 1970). And indeed, TM 10341 does look like a baby. But not a baby Pterodactylus, but a baby Dorygnathus (Figs. 1-3).

Figure 1. I didn't realize the teeth were so long in ?Pterodactylus spectabilis, TM10341,  n1 in the Wellnhofer 1970 catalog. This is no Pterodactylus, but a tiny dorygnathid. Click to learn more.

Figure 1. I didn’t realize the teeth were so long in ?Pterodactylus spectabilis, TM10341, n1 in the Wellnhofer 1970 catalog. This is no Pterodactylus, but a tiny dorygnathid from the Late Jurassic and the ancestor to the mighty azhdarchid, Quetzalcoatlus. Click to learn more.

?Pterodactylus spectabilis in situ, traced and reconstructed.

Figure 2. ?Pterodactylus spectabilis in situ, traced and reconstructed. The specimen is crushed and the premaxilla is twisted so that its right side is exposed, the opposite of the rest of the skull. Several of the anterior teeth make a jumble in the front.

TM 10341 (no. 1 in the Wellnhofer 1970 catalog) is a key taxon tying Dorygnathus to Beipiaopterus and the long metacarpal pre-azhdarchids like no. 44 and no. 42 in the large pterosaur tree. Note that TM 10341 does NOT have such a long metacarpal, so it’s lagging int this trait and is distinct from other short-tail pterosaurs. This lineage also gives rise to flightless pterosaurs, Huanhepterus, a ctenochasmatid- mimic and other tall, skinny pterosaurs like Quetzalcoatlus. They all had their origin in the cute, big-headed TM 10341 (Late Jurassic, Solnhofen).

TM 10341 had a reduced tail, a short antebrachium (radius + ulna) and a big head, but it was a tiny pterosaur. It looks like an adorable infant Dorygnathus, but we know from embryos that they actually have the proportions of adults, so TM 10341 is something new.  Since pterosaurs grew isometrically from hatchlings on up, so the changes that made TM 10341 “cute” all occurred in the egg or the gene.

This is one of the surviving remnants of Dorygnathus in the Late Jurassic. Others include the similarly reduced Scaphognathus and all of its ancestors from a different line of dorygnathids along with Ctenochasma and kin from a third lineage of dorygnathids.

Phylogenetic analysis nests TM 10341 after Dorygnathus and before Beipiaopterus (Fig. 3). No other tested taxa (202 at last count) are closer to TM 10341.

Figure 3. Three sister taxa to scale. At left, Dorygnathus SMNS 50164, middle Jurassic. Middle, ?Pterodactylus spectabilis, late Jurassic. Right, Beipiaopterus, Early Cretaceous. Size reduction was a driver in the evolution of new morphologies.

Figure 3. Three sister taxa to scale. At left, Dorygnathus SMNS 50164, middle Jurassic. Middle, Pterodactylus spectabilis, late Jurassic. Right, Beipiaopterus, Early Cretaceous. Size reduction and enlargement were drivers in the evolution of new morphologies.

Size reduction is what drives pterosaur evolution, bringing with it the largest changes in structure and proportion. Most of these changes are retained as subsequent taxa become larger and larger. Size reduction takes place when sexually mature half size adults lay half-size eggs over several to several thousand generations. TM 10341 was about 7.5 cm tall. Hatchlings of TM 10341 would have been only 1 cm tall.

References
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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