Wednesday, 21 September 2011

Pterosaur feeding strategies

Diet and feeding mechanisms are some of the most valuable things we can learn about fossil organisms. In 2010 a crocodile with mammal-like, multicusped teeth and a manoeuvrable jaw was described in Nature (O'Connor et al. 2010). The croc, named Pakasuchus hit the news and was reported in many notable magazines, newspapers and blogs. Pterosaurs, however, have not received such attention. Although later forms lacked teeth (edentulous), or had very uniform dentition (homodont), a large subset of Late Triassic to Early Cretaceous pterosaurs possessed multiple tooth morphologies in their jaws (heterodont). Some tooth morphologies were even multicusped or serrated. Ősi (2011) documented some of this heterodonty, giving a review of feeding-related characters in basal pterosaurs. Ősi's review was comprehensive; however, he did not touch upon the feeding mechanisms of the short tailed, pterodactyloid pterosaurs. This is not surprising; variation in pterodactyloid teeth is usually non-existent in the tooth row itself. Furthermore, the variation between individual species of pterodactyloid usually consists of; short cone to long cone and straight to bent (recurved). In fact, it is nearly impossible to tell the difference between some pterosaur teeth and some very simple croc, plesiosaur or even fish teeth. In some cases careful examination of the tooth structure, or even microstructure must be employed to positively identify a pterosaur tooth (Vidovic, 2010; Vidovic et al. in prep). One particular group of pterodactyloids does break this humdrum cycle though. The ctenochasmatids (Late Jurassic to Early Cretaceous) possess teeth with phenomenal length width ratios; up to 1/75. Gnathosaurus had slender, sinuous teeth which projected horizontally out of the jaw margin, and the smaller Ctenochasma had a similar basket effect with its dentition. Clearly, this particular group of pterodactyloids was sifting sediments in order to filter-feed. By the Early Cretaceous this group of animals had evolved into their role perfectly; Pterodaustro guiñazui, the uncanny cross between a pterosaur, flamingo and whale was certainly filter-feeding directly from the water.

Pterodaustro is one of the most bizarre animals that has ever graced this Earth. It had very elongate forearms and legs, a long neck, big flat feet and its head was longer than its body. Even more bizarre were its jaws, which bowed like those of a whale or flamingo, were slender and elongate like a wading bird, and full of fibrous teeth which are convergent on baleen* (or vice versa). The teeth were so long and slender in fact, a study was conducted to prove, unequivocally, that they were true teeth (Chiappe & Chinsamy 1996). The dentition of the upper jaw is often neglected; it consists of tiny, spatulate, invariable teeth (isodont). As such, Pterodaustro is currently the only pterodactyloid known to have a marked heterodont dentition. So, is Pterodaustro content with being this unusual, well, in a word, NO. Chiappe et al. (2000) reported that the tremendously elongate teeth are not located in sockets, unlike all other pterosaurs, but in an open groove, where the labial ridge (outer side) is lower than the lingual ridge (inside), this is a subthecodont condition. Furthermore, the spatulate teeth of the upper jaw are not in sockets at all. Chiappe et al. (2000) suggested that these teeth and associated bony bits (Yes! Ossicles in the mouth!) were located in a skin integument. I agree with this statement, and I will take it a little further.
* Currey (1999; 2010) noted that the length width ratios of Pterodaustro's teeth were so high that the mechanical properties would allow it to flex a considerable amount without breaking (not necessarily meaning they did).
Modified from Zweers et al. (1995): flamingo beak in the feeding position, showing the keratinous papillae.
It is not unusual for animals to have fleshy or horny (keratinous) fingers (papillae) inside their mouth or digestive system in order to aid certain functions. So far, in the TV series (3) inside nature's giants camels and turtles have been seen with such structures. Flamingos also have a highly developed system of keratinous papillae in their oral cavity (see diagram above), used for selecting specific food types (Zweers et al. 1995). It is likely that Pterodaustro had a similar piston pumping system to that of a flamingo, and hard parts have preserved evidence for such a system in the form of the upper dentition and ossicles. Based on the fossil evidence available and comparative anatomy with flamingos, it seems that the teeth and ossilces in Pterodaustro spanned the palate in ridges, interacting with keratinous papillae on the dorsal surface of the tongue. The teeth in the upper jaw would have been free to move as the horny prongs of the tongue rasped against them, sorting out specific food substances. Meanwhile, the organic pulp created by this action would have been delivered to the inside surface of the teeth (labial surface), collecting against them. With the bowed jaws an even pressure can be applied along the entire oral cavity, using this even pressure, effluent can be squeezed through the filtering teeth of the lower jaw. Once a reasonable quantity of food had been processed a sphincter at the top of the throat would have been opened, the head would tip up so that gravity may aid swallowing, and the tongue would deliver the organic pulp to the oesophagus.
a) Reconstruction of Pterodaustro; teeth and ossicles in the upper jaw (b); slender and elongate teeth in the lower jaw; keratinous papillae on the tongues dorsal surface (c).
In 2009 Codorniú et al. reported the presence of gastroliths in Pterodaustro. Gastroliths are usually pebbles that are swallowed by animals to help them break down tough food, such as cellulose. The diet of Pterodaustro is not typical of animals that use gastroliths. However, it is very likely that Pterodaustro's feeding apparatus would have been tactile, and there would have been a sphincter in place to stop the animal swallowing foreign bodies or too much water, thus it is unlikely that stones would have been swallowed by mistake. Codorniú et al. (2009) noted that small crustaceans are found in the same geological units as Pterodaustro, it was suggested that small angular pebbles of quartz were swallowed to help break down these hard shelled zooplankton. However, the quartz tended to be less than 1 cm in size and it is difficult to see how this grit would have been any more effective in breaking down the crustaceans than the pterosaur's digestive system alone, so impaction (as seen in modern reptiles) cannot be ruled out.

All in all Pterodaustro was a pretty freaky flyer, a winged whale, and certainly worth thinking about in more abstract ways. It seems fitting, to both my research and the aims of this blog, for this to have been the first subject of discussion, but there is much more to come.

COMING SOON… more feeding strategies in pterosaurs (a three part series)…



Chiappe, L. M. and Chinsamy A. 1996. Pterodaustro's true teeth. Nature, 379: 211-212

Chiappe, L. M. Kellner, A. W. A. Rivarola, D. Davila, S. and Fox, M. 2000. Cranial morphology of Pterodaustro guinazui (Pterosauria: Pterodactyloidea) from the Lower Cretaceous of Argentina. Contributions in science, 483: 1-19

Codorniú, L. Chiappe, L. M. Arcucci, A. and Ortiz-Suarez, A. 2009. First occurrence of gastroliths in Pterosauria (Early Cretaceous, Argentina). XXIV Jornadas Argentinas de Paleontología de Vertebrados

Currey, J. D. 1999. The design of mineralised hard tissues for their mechanical functions. The Journal of Experimental Biology, 202: 3285-3294

Currey, J. D. 2010. Mechanical properties and adaptations of some less familiar bony tissues. Journal of the Mechanical Behaviour of Biomedical Materials, 3: 357-372

O'Connor, P.M. Sertich, J.W. Stevens, N.J. Roberts, E.M. Gottfried, M.D. Hieronymus, T.L. Jinnah, Z.A. Ridgely, R. Ngasala, S.E. and Temba, J. 2010. The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana. Nature, 466 (7307): 748–751

Ősi, A. 2011. Feeding-related characters in basal pterosaurs: implications for jaw mechanism, dental function and diet. Lethaia, 44: 136–152.

Vidovic, S. U. 2010. A preliminary analysis of dental microstructure in pterosaurs. Acta Geoscientica Sinica, S1

Zweers, G. De Jong, F. Berkhoudt, H. Vanden Berge, J. C. 1995. Filter Feeding in Flamingos (Phoenicopterus ruber). The Condor, 97: 297-324

No comments:

Post a Comment