Jensen and Padian (1989) described the bits and pieces of the Mesadactylus holotype (Fig. 1, right). The type specimen is the synsacrum, distinct in morphology with an anteriorly high and completely fused neural plate. Jensen and Padian (1989) considered Mesadactylus a basal pterodactyloid pterosaur. Bennett (2007) considered the synsacrum anurognathid even though no other anurognathid had such a sacrum.
More recently Smith, Sanders and Stadtman (2004) recovered new material from the same late Jurassic formation. They ascribed these scattered specimens to Mesadactylus despite their distinct size and other differences. Based on their scale bars, these elements are reconstructed for the first time here (Fig. 1, left). Overall they represent a much larger specimen of a different type of pterosaur. Individually there are few similarities among the bones both shared in common.
Both specimens were entered into the large pterosaur tree. The holotype Mesadactylus nested with the other North American anurognathid, Dimorphodon? weintraubi and the early Cretaceous IVPP embryo.
I like to give others credit where credit is due and Bennett (2007) got this one right!
The referred specimen nested away from the anurognathidae, at the base of the azhdarchidae, along with the flightless pterosaur, SoS 2428 (Fig. 2) and its kin, BSPG 1911 I 31 (no. 42 in the Wellnhofer 1970 catalog), alongside Huanhepterus.
Only one cervical vertebrae of the referred specimen was illustrated, so here (Fig. 1) it was duplicated and shortened, producing an elongated restored neck.
That relatively large rib (no doubt the second dorsal) indicates a voluminous torso. Proportionately this rib is much larger than the second dorsal rib in SoS 2428. So either the rib does not belong with the other referred bones or the torso was relatively much larger.
The humerus includes a very large shoulder articulation and a small deltopectoral crest, wider than deep. Manual 4.1 appears to include a fused extensor tendon process and a very short portion of m4.2 with converging margins indicating a short length.
The femur is elongated and S-curved, as in SoS 2428. All pterosaurs have a tibia of greater length, so that gives this restored specimen a stork-like or azhdarchid-like appearance.
Comparisons to SoS 2428 from the Solnhofen
The humerus has a distinct shape in the Colorado specimen, with a semicircular deltopectoral crest and a larger shoulder joint. As mentioned earlier, the rib is much larger in the Colorado specimen. Otherwise most of the elements are comparable.
These clues and the phylogenetic nesting of the referred specimen suggest a close relationship with the flightless pterosaur, SoS 2428. So that makes our second or third (depending on how we count SoS 2179, known only from a skull) flightless pterosaur.
Knowing what to look for now,
I wonder if there is more diagnostic material in the matrix?
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.
Bennett S C 2007. Reassessment of Utahdactylus from the Jurassic Morrison Formation of Utah. Journal of Vertebrate Paleontology 27(1): 257–260.
Jenson J and Padian K 1989. Small pterosaurs and dinosaurs from the Uncompaghre fauna (Brushy Basin Member, Morrison Formation: ?Tithonian), Late Jurassic, western Colorado. Journal of Paleontology 63:364-373.
Smith DK, Sanders RK and Stadtman KL 2004. New material of Mesadactylus ornithosphyos, a primitive pterodactyloid pterosaur from the upper Jurassic of Colorado. Journal of Vertebrate Paleontology 24(4):850-856.