400 Million Years and Counting
There’s an often-reported statistic that sharks have remained unchanged for 400 million years. What does this really mean? Our familiar sharks weren’t around 400 million years ago, and what do we really know about the extinct shark species that have existed between then and now?
Fischer et al now report on new shark fossils from the middle to late Triassic period (250-200 million years ago). These fossils were found at a site called Madygen, in Kyrgyzstan in central Asia. During the Triassic period, Madygen is believed to have been a large inland freshwater lake.
Unlike bony animals, the cartilaginous skeletons of sharks fossilize poorly, so most extinct shark species are identified from little more than their teeth. The shapes and sizes of the animals belonging to these teeth must be inferred from similar species alive today, and hoping to understand anything about the lives and behaviors of these sharks is usually impossible.
Fischer et al found fossilized shark teeth at Madygen that were different than those that had previously been described, and eventually determined that they belonged to three new species of freshwater shark. The term ‘freshwater shark’ sounds odd today; all but a few of the 400+ species of selachians (sharks, exclusive of rays and skates) currently alive are marine animals. The bull shark can live in both salt and fresh water, allowing it to move far up into river systems, and there is a genus of ‘river sharks’ in Asia, but most sharks live in the ocean. Freshwater elasmobranchs appear to have been abundant in shark history, however, based on large numbers of fossil shark teeth found in the deposits of inland lakes.
In addition to teeth, the embryonic egg cases of sharks can also fossilize, and while the embryonic sharks within them remain only as tiny teeth, studying them can sometimes tell us about the lives of these animals. Madygen is a particularly rich site for fossil shark egg cases and the teeth of the developing young they once held. The researchers found many egg cases, from two different species, in an area that at one time encompassed the shallow near-shore waters of this ancient lake. Few adult teeth were found nearby, however, suggesting only young sharks inhabited the area -- a shark nursery.
Many shark species today use shallow near-shore ocean habitats as juvenile nurseries, where young sharks can grow, with abundant food and reduced risk of predation, until they are large enough to join the adults of their species. It seems things weren’t all that different for sharks more than 200 million years ago.
The paper is: A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries. (2011) Fischer, J, Voigt, S, Schneider, JW, Buchwitz, M and Voigt, S. Journal of Vertebrate Paleontology, 31:937-953.
It can be found at: http://dx.doi.org/10.1080/02724634.2011.601729