The Evolution of Gigantism in Pterosaurs: The Ancestry of Anhanguera

Earlier we followed up on a National Geographic article on how many generations it takes to create a blue whale and an elephant. Yesterday we looked at the evolution of the giant pterosaur Quetzalcoatlus. Today we’ll examine two ornithocheirids.

Others consider toothy ornithocheirids, like Arthurdactylus and Anhanguera (Fig. 1), to be related to toothless Pteranodon based on the shared trait of a warped deltopectoral crest. The large pterosaur tree did not recover that relationship, but found that humerus warp to be a convergence.

The beauty of the large reptile tree and the large pterosaur tree is the ability to trace the ancestry of any listed taxon back to the basal tetrapod, Ichthyostega. Today we won’t go that far back. Rather we’ll start with one of the smallest pterosaurs from the first Late Jurassic in the lineage of Anhanguera and Arthurdactylus. That pterosaur is a tiny specimen inaccurately referred to Pterodactylus? micronyx? TM 13104 (Winkler 1870, No. 34 in the Wellnhoger 1970 catalog), Fig. 1).

The ancestry of Anhanguera and Arthurdactylus includes tiny TM 13104.

Figure 1. The ancestry of Anhanguera and Arthurdactylus includes tiny TM 13104 (no. 34 in the Wellnhofer 1970 catalog).

We’re often taught that as organisms grow larger they also become more robust, with stronger, thicker bones to withstand the effects of their greater mass and weight. Here, there’s more —  and less — of a case for that. Giant Arthurdactyulus was more robust, especially in the wings than, no. 34. However the scapula + coracoid were not. The hind limb was ever so slightly more gracile (certainly not more robust). The torso was shorter with taller vertebral spines. Unfortunately the head and neck are unknown, but sister taxa, like Haopterus and Coloborhynchus, had a more robust neck and a longer skull (which may explain the more robust neck). The feet of Arthurdactylus were comparatively tiny! What a strange combination evolution has wrought here: more wing and head, less body and feet!

The complete TM13104 and the skull of Anhanguera to scale.

Figure 2. The complete TM13104 and the skull of Anhanguera to scale. Also shown in gray is a hypothetical skull of Arthurdactylus based on a stretched out Haopterus.

What can we learn here?
Between no. 34 (which was reduced from early Scaphognathus specimens) and basal ornithocheirds like Arthurdactylus and Haopterus, the proportions changed rather starkly. This is likely due to a distinctively different mode of flight. Over time and millions of generations, the lineage of Arthurdactylus gradually grew and reengineered itself to withstand the increasing stresses imposed by that growth. The wing became more robust. Here the time frame is only from the Late Jurassic, 150 mya to the Early Cretaceous Crato Formation, 112 mya, a time span of 38 million years.

Then Along Comes Anhanguera
Anhanguera was more derived than Arthurdactylus and it shows the “evolution engineers” were hard at work further lightening this aerial predator. Much larger than Arthurdactylus, Anhanguera had a relatively smaller diameter humerus, reduced to not much thicker than the femur at both of their smallest diameters. The rest of the wing followed suit. The pectoral girdle was reengineered with elongated processes to route the wing muscles more efficiently and provide larger areas for their insertion.

Forelimb Take-Off
Dr. Mike Habib (2008) has noted the greater size of the humerus vs femur in Quetzalcoatlus and other large pterosaurs. He didn’t mention no. 34, which does not have a humerus greater in diameter than the femur. Habib considered the more robust humerus a sign that pterosaurs used a vampire bat-like forelimb launch sequence demonstrated here, rather than a bird-like hind limb launch demonstrated here. Unfortunately, its all math at present. We know of no pterosaur take-off tracks, nor any that document the implantation of the wing metacarpal into the substrate. Rather only the fragile first three digits make any impression. Perhaps the increased size of the humerus in Arthurdactylus is a sign that the pectoral engine for wing flapping is much larger to drive the larger wings. The reduced size of the humerus in Anhanguera indicates a reengineered solution to flight: lighter and stronger.

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
Campos, D de A and Kellner AW 1985. Un novo exemplar de Anhanguera blittersdorffi(Reptilia, Pterosauria) da formaçao Santana, Cretaceo Inferior do Nordeste do Brasil.” In Congresso Brasileiro de Paleontologia, Rio de Janeiro, Resumos, p. 13.
Frey E and Martill DM 1994. A new Pterosaur from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 194: 379–412.
Habib M 2008. Comparative evidence for quadrupedal launch in pterosaurs. Pp 161-168 in Buffetaut E, and DWE Hone, eds. Wellnhofer Pterosaur Meeting: Zitteliana B28
Kellner AWA and Tomida Y 2000. Description of a New Species of Anhangueridae (Pterodactyloidea) with Comments on the Pterosaur Fauna from the Santana formation (Aptian-Albian), Northeastern Brazil. National Science Museum, Tokyo, Monographs, 17: 1-135.
Wang X and Lü J 2001. Discovery of a pterodactylid pterosaur from the Yixian Formation of western Liaoning, China. Chinese Science Bulletin 46(13):1-6.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.
wiki/Arthurdactylus
wiki/Haopterus

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