Monday, 23 November 2009

Pterosaur Eggs

Speculation about how pterosaurs reproduced has been enhanced over the years by lack of evidence. Many scientists believed that pterosaurs must have laid eggs, but were they hard shelled as in birds, or leathery as in reptiles. To some, the thought of how a long bony wing would work in an egg was a problem to imagine. Bats give birth to live young without the need for eggs, so perhaps pterosaurs could also give birth to, live young.

Speculation became analysis when, in 2004, a pterosaur egg fossil was found in China.

Avodectes pseudembryon (Wang and Zhou 2004), IVPP-v13758, was discovered in the Jehol Biota, being about 121 million years old. It was a complete embryo in a shell. The wings were coiled as they developed (sketch above) and the preservation indicates clearly that the bones were well ossified before hatching. This would enable the newly hatched pterosaurs to use the wings very quickly after emerging from the egg.

Observations of the porosity of the egg shell suggested possible burial during development and the form of the shell was soft and leathery like a reptilian egg, having a shell that was non-laminar and 0.25mm thick. The embryo wingspan was estimated to be 27cm. The bone proportions are unlike any known pterosaur, but show similarities with Anhanguera and Istiodactylus specimens. This should not be taken as an indication of species, since the bone development and proportions may have been subject to changes during juvenile life.

A second egg, JZMP-03-03-2, was somewhat similar, but the skeletal bones were not articulated in the same clear way, so interpretation is a little more complex.
In the same year, a pterosaur egg, MHIN-UNSL-GEO-v246, was discovered in the Lagarcito Formation in Argentina. This formation is well known for the Pterodaustro fossils and this egg was clearly a Pterodaustro egg. The embryo was intact and articulated.
The egg was also from the Lower Cretaceous deposits about 100 million years ago, and the proportions of the embryo closely matched those of known juvenile specimens. Measurements of the shell indicated a very thin (30μm) leathery shell which was long and oval in shape measuring 22mm by 66mm. The estimated embryo wingspan was 27cm. This egg has many differences from the Chinese finds, but it does support the idea that all pterosaurs probably laid eggs.
The questions that are difficult to resolve are;

  1. How many eggs did pterosaurs lay?
  2. Did pterosaurs care for their young?
  3. Where did pterosaurs nest and what nesting structures did they create?
I suspect that the answers to these types of questions will be different for each type of pterosaur. These creatures would have been subject to the same variations and constraints in their habitats as modern animals are subject to today.

Wang, X., and Zhou, Z., 2004, Pterosaur embryo from the Early Cretaceous: Nature, vol.429, p.621.

Chiappe, L. M., Codorniu, L., Grellet-Tinner, G., and Rivarola, D., 2004, Argentinian unhatched pterosaur fossil: Nature, vol.432, p. 571-572. (2 Dec 2004) 

Pterosaur Database Topics - eggs

Sunday, 1 November 2009

The Pterosaur Brain

In 1888, Newton published the first account of a pterosaur brain exposed in a skull from the Lias at Whitby, North Yorkshire. The skull fossil had to be excavated to reveal the exposed brain more fully. Before the work, a number of casts of the original were made and one such cast is shown below.
Perhapsicephalus purdoni, National Geographic Survey, Nottingham.

The brain fossil showed the main lobes clearly and the auditory and semicircular canal structures were identified during excavation of the skull. The analysis applied to the brain by Newton suggested that the brain of the pterosaur was in many ways similar to the brain of a lizard, but in some respects it was charactaristically similar to the brain of birds. His conclusion was that pterosaurs, birds and lizards evolved from common ancestors.

In 1941, Tilly Edinger, a German physiologist, examined two distinct pterodactyl fossils where the brain was exposed. Her findings were similar to those of Newton, but with the advantage of the intervening progress of science since the earlier investigation, Edinger was able to make a more defined conclusion about structure.
Pterodactylus elegans, MCZ, No. 1505

The pterosaur fossils that Edinger worked with had been described by several earlier and well respected German scientists, though the work that they did was descriptive and comparative. This later work looked more closely at the structures and extrapolated the knowledge to a general description of the pterosaur brain for the first time.

Developments in the brains of pterosaurs show similarities with the development of the brain in birds. These changes are attributed to the requirements of flight, with a more developed optic lobe and a fissure (Vallecula Silvii) like that found in the fore brain of birds. At this point there is no evidence for the structure of the base of the pterosaur brain.

These are the two defining works on the nature of the pterosaur brain. With the advance in medical scanning techniques, the destructive analysis of pterosaur brains is a thing of the past. It is now possible in some cases, to examine the cranial cavity of a fossil by electronic means to develop an understanding of its brain anatomy.

Newton E. T., 1888, On the skull, brain and auditory organ of a new species of Pterosaurian (Scaphognathus purdoni) from the Upper Lias near Whitby, Yorkshire. Proceedings of the Royal Society, London. 43, pages 436–440.

Edinger T., 1941, The brain of Pterodactylus. American Journal of Science. 239, 665–682.