Following the origin of pterosaurs and our look at the most primitive member of the family tree, the Milan specimen (MPUM 6009), today we take a look at the Dimorphodontia, the first clade to branch off from the main lineage of all other pterosaurs. Click on the blue links to see the various pterosaurs described in greater detail and further referenced on reptileevolution.com.
This clade includes several recently discovered Triassic taxa with premaxillary crests, the crestless Dimorphodontidae and finally the typically small, snub-nosed, short-tailed Protoanurognathidae and Anurognathidae (covered in a future blog). All the rest of the pterosaurs, collectively known as the Eudimorphodontia, will be covered over several weeks in future blogs.
Raeticodactylus – The most primitve member of the Dimorphodontia was the Triassic pterosaur, Raeticodactylus. The similarities it shared with MPUM 6009 indicate Raeticodactylus was primitive. However, the many differences not shared provide evidence that the origin of pterosaurs occurred many millions of years prior to the Late Triassic. Those differences also hint at a tremendous variety of Triassic pterosaurs yet to be discovered.
Unique in many ways, Raeticodactylus had a knife-like horn on its snout with the naris displaced posterior to it. The skull was robust with a deep anterior mandible. An extra long retro articular process was present at the rear of the skull. The rest of this pterosaur was extraordinarily gracile, with the thinnest humerus known among pterosaurs. Importantly,Rhaeticodactylus and Eudimorphodon were the most primitive known pterosaurs with the ability to touch the ground without bending over from a bipedal stance.
Regarding the walking abilities of pterosaurs, Dr. David Hone wrote online, “On the ground the ‘rhamphorhynchoids’ were probably pretty poor… the shape of their hips and upper legs meant that could only really sprawl and not walk upright.” Well, this is clearly NOT the case with this very primitive ‘rhamphorhynchhoid’ because the femoral heads were set nearly at right angles to the femoral shafts, convergent with dinosaurs. Raeticodactylus had no trouble walking [really, no pterosaurs did and that myth is dispelled here], butRaeticodactylus might have had trouble sprawling the hind limbs during flight. Instead the femora could likely only manage an inverted “V” configuration while airborne.
Austriadactylus – Two specimens have been attributed to Austriadactylus. Although they do nest next to one another, the differences are substantial enough to deserve a new genus for the second of the two. Both initiate the expansion of the naris that reaches an acme with Dimorphodon. The larger holotype retained a long slender humerus, as in Raeticodactylus. The smaller referred specimen had a shorter, more typical humerus, metatarsal 4 was no longer than 3 and pedal digit 4 was no longer than metatarsal 4. Both retained the curved coracoid initially found in Cosesaurus and straightened out in most later pterosaurs. The size reduction between the two Austriadactylus specimens appears to mark the first time this pattern of size reduction occurs in pterosaurs. It is observable many times within the Pterosauria (contra Hone and Benton 2006).
Preondactylus – One of the first Triassic pterosaurs to be discovered was Preondactylus, preserved as a natural mold with most of its bones washed away. Here Preondactylus serves as a transitional taxon between the Italian Austriadactylus and Dimorphodon. The very short cervicals also demonstrate a transition to the most primitive protoanurognathid, Peteinosaurus. The pectoral girdle was more robust with a straight coracoid. The pelvis and pes were longer with longer metatarsals than digits.
Dimorphodon – One of the first pterosaurs ever discovered, Dimorphodon had the largest skull fenestra of any pterosaur. The naris was larger than its enormous antorbital fenestra. This highly derived early Jurassic pterosaur had large clawed fingers, longer than its hand, which would have been ideal for tree trunk clinging. The robust tail was twice the length of the body and head combined. A recent paper by Nesbitt and Hone (2010) claimed to identify a mandibular fenestra in Dimorphodon and so count pterosaurs in among the archosaurs. Unfortunately what they considered an angular defining the lower rim of that fenestra was actually a displaced pterygoid as documented here. The actual angular was displaced elsewhere (I haven’t found it yet) and the surangular had drifted toward the skull where it appeared to be a very deep and autapomorphic jugal plate.
Peteinosaurus – The Late Triassic Peteinosaurus was at the base of a clade of snub-nosed pterosaurs with a smaller skull, a much shorter neck and a greatly reduced tail, the Protoanurognathidae. Peteinosaurus retained a robust tail only half as long as in Dimorphodon. The terminal caudals were transformed into mere grains the size of most anurognathid caudals (unless this is an artifact of preservation). The sternum was twice as wide as long.
Carniadactylus – Another Late Triassic pterosaur, Carniadactylus, is known from a partial jaw, a pectoral girdle and limb bones. Here the sternum was as broad as deep.
MCSNB 8950 – From the Late Triassic, this tiny pterosaur was originally considered a juvenile Eudimorphodon. MCSNB 8950 was the first pterosaur to show that the sternal complex was composed of three discrete elements, the interclavicle, the sternum and two clavicles that wrapped around the leading edge of the sternum, as in Longisquama. The tail was robust. Unfortunately its length cannot be ascertained due to breakage.
Next time we look at the pterosaur family tree, members of the Anurognathidae will be introduced.
As always, I encourage readers to see the 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.
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
Nesbitt SJ and Hone DWE 2010. An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity 3: 225–233
Other references for each taxon are found at www.reptileevolution.com