Azhdarchid Fossil Distribution and Taphonomy

Many of you have heard about the recent paper by Mark Witton and Scibling Darren Naish called A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. The paper has received a large amount of press due, mainly, to the conclusion that the Azhdarchids were terrestrial stalkers like some species of modern stork and ground hornbills. Ordinarily, I would be blogging about the osteological aspects of the paper – and there are plenty of them – but I want to focus on something different (and just as important for the paper). In the meantime, if you want to know more about the osteology Greg Laden sums it up for you. What I would like to focus on is the paleoecological and taphonomical aspects of the paper.

Since Witton and Naish end up arguing that azhdarchids were terrestrial stalkers, they need to demonstrate the environment that azhdarchid fossils occur in bears that out. To that end they examined the environmental distribution of fossils:

An assessment of azhdarchid trace fossils was used to evaluate their terrestrial competence. Additionally, we tested the notion that azhdarchids were, as has been suggested for other pterosaurs, predominately shore-dwelling animals with a quantitative assessment of their geological context. Using sedimentary and other fossil remains as evidence of paleoenvironment, azhdarchid fossils were scored as occurring in settings that were fully terrestrial, coastal, marine with terrestrial input or fully marine with no terrestrial input. This dataset of 33 azhdarchid-bearing localities was then assessed along with consideration of the completeness and abundance of their remains to develop an understanding of preferred azhdarchid habitat.

Interestingly enough, most azhdarchid fossils are found in continental fluvial deposits or alluvial sediments. Additionally, all but five of the marine occurrences of marine fossils have terrestrial biota associated with them and are usually isolated fragments of bones such as vertebrae or limb bones. Both of which come from Voorhies groups I and II, respectively, and are easily transported by fluvial processes. Factors that influence fluvial transport include size, shape, and structural density. Typically, one also examines the sedimentary matrix surrounding the bones and determines the settling velocity of the particles that compose it to aide in the determination. One can also look for signs of abrasion, fluvially transported bones tend to be evenly abraded across their entire surface and somewhat rounded. The amount of abrasion is dependent on sediment size, distance transported, presence of soft tissue, and the condition of the bone (i. e. fresh, weathered, whole, broken, etc.).
Consequently, Witton and Naish argue that this is a valid indicator of habitat preference and it is this preference that the rest of the paper is devoted to exploring. You can go here for more details, and, of course, you can read the paper as well. Finally, and in some ways this is just as important, since the paper is also something of a review paper, it is exhaustively referenced and can serve as an entry point into the literature on azhdarchids. If you want to learn more, just start tracking down the references…