Some pterosaurs lost their teeth. Some reduced their tail. In ornithocheirid pterosaurs the feet became incredibly small (Peters 2011, Fig. 1).
Among living birds, the apodiformes (swifts and hummingbirds) have the smallest feet. These birds rarely walk.
The Reduction of the Pes in Ornithocheirids
Ornithocheirids are characterized by their toothy beaks and giant wings. The most primitive forms, like Yixianopterus and the JZMP embryo, had relatively typical proportions overall. That changes with Boreopterus in which the feet were greatly reduced. Derived taxa, like Anhanguera (Fig. 2), further reduced the metatarsus, producing feet with atypical proportions. Note the relative size of pedal digit V (Figs. 2-4), which did not phylogenetically shrink in proportion to the rest of the pes. Instead digit 5 regains more primitive proportions not seen since Scaphognathus.
The pterosaur foot with the most atypical proportions belongs to Anhanguera (Fig. 2), the most derived of the ornithocheirids. The metatarsus was reduced to shorter than the proximal phalanges. Metatarsal 4 was shorter than the unguals! This arrangement gave the foot the maximum toe/metatarsal ratio, which was important because the webbed toes were used to automatically extend the horizontal hind limbs while airborne.
Prior to 2010 (see below), ornithocheirid feet were not described. Wellnhofer (1985) and Kellner and Tomida (2000) overlooked the tiny feet of Anhanguera, restoring it with a standardized and standard-sized foot. Wang et al. (2005) described Nurhachius but overlooked the pes except to identify it on the in situ fossil. In the late 1990s Anhanguera was the first ornithocheirid in which I noticed the feet were so small (not published until Peters 2011). Subsequent discoveries confirmed this trait.
Lu (2010) described the best preserved ornithocheirid, Zhenyuanopterus, for the first time noting its tiny feet (Figs. 2 and 3) which were perfectly preserved and fully articulated. This is one of the few ornithocheirids with a shorter metatarsal 1 than mt2.
Plantigrade Tiny Feet Rarely Used
The fragile and tiny size of the feet of ornithocheirids suggests they were rarely used. These prehistoric albatross analogs might have spent days to weeks airborne, perhaps nesting in areas free from predators.
It seems unlikely that pterosaur feet could have evolved to become even smaller. Terrestrial locomotion was probably slow and methodical, reducing the stress of each fragile footfall. Despite reduced feet, the toes remained beneath the shoulder glenoid, the center of balance in all flying reptiles. So it was possible for the wings to be extended for flying, sunning or display while standing erect and very finely balanced on such tiny toes.
Hatchlings with Large Feet?
The JZMP embryo had relatively large (for an ornithocheirid) feet. At present this appears to be a phylogenetic character due to its nesting prior to Boreopterus. Nevertheless it seems reasonable that hatchling ornithocheirids might have had larger feet that did not grow in step with the rest of the body. Counter this notion, the hatchling Pterodaustro had relatively smaller feet than the adult.
Perhaps it is no coincidence that the prepubis is rarely found in ornithocheirids. Where it is found (Arthurdactylus, SMNK PAL 1136), the prepubis is relatively small. The prepubis effectively lengthens the pubis providing anchors for femoral adductors. These would be less useful in a pterosaur that rarely walked, hence the reduction )and perhaps loss) of the prepubis in certain ornithocheirids.
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.
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.
Lü J 2010. A new boreopterid pterodactyloid pterosaur from the Early Cretaceous Yixian Formation of Liaoning Province, northeastern China. Acta Geologica Sinica 24: 241–246.
Peters D. 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605
Wang X, Kellner AWA, Zhou Z and Campos DA 2005. Pterosaur diversity and faunal turnover in Cretaceous terrestrial ecosystems in China. Nature 437 (7060): 875–879. doi:10.1038/nature03982. PMID 16208369.
Wellnhofer P 1985. Neue Pterosaurier aus der Santana-Formation (Apt) der Chapada do Araripe, Brasilien. Paläontographica A 187: 105–182.