The word “Madagascar” conjures up images of lemurs and baobob trees—indeed, a great portion of vertebrate paleontological research has pursued questions on the origin of the island’s unique land-dwelling life forms. Yet, there is also a story in the prehistoric waters around Madagascar. A new paper adds to that increasingly detailed picture, published today in PLOS ONE.
Sharks and their close relatives form an important part of past and present-day marine ecosystems. The teeth of these animals are also pretty durable, and so can be common finds in some deposits. Despite this (or perhaps because of this), shark teeth intimidate me. There’s lots of them (sometimes thousands from a single locality), and it can be tough to parse out variation. As anyone who has ever watched a shark documentary probably knows, sharks have an endless conveyor belt of disposable teeth in their jaws. These teeth come with a seemingly limitless range of shapes and sizes, even within the jaw of a single animal. Couple that with evolutionary time spans, and sharks present simultaneously difficult and fascinating study subjects. Beyond species classification, sharks are useful for establishing connections between marine bodies of water as well as past environmental conditions.
Paleontologist Karen Samonds and a team including researchers from Madagascar, Saudi Arabia, and the UK just published their work describing fossil shark teeth from some marine rocks exposed along the coast of northwestern Madagascar. This has been a real slow-burn project—the first specimens of the sample were collected nearly 20 years ago! The rocks are pretty hard, which means that fossils weather out at a sluggish pace. As Samonds told me over email, “We have been able to piece it together by being patient, and screening the material in the field.”
The published sample includes a grab bag of seven shark and three ray species. Although all but one of the species were previously known to science, the paper presents the first published fauna of sharks and rays from the Eocene-aged rocks of Madagascar. The precise age of the rocks has been a mystery, and still remains elusive. However, the types of sharks and their overall geological context suggests that they were mid- to late Eocene, between 45 and 34 million years old. It will probably take microfossils–the tiny remains of ancient single celled organisms–to provide a more precise date. Some species of microfossils had a very short duration in the rock record, and so can be used to pin a date on the rock by correlating them with rocks of known age.
Every fossil study gives us a better picture of Madagascar’s past. The sharks and geological context from the work by Samonds and colleagues show a shallow marine habitat that was fairly warm and close to the prehistoric shoreline. Furthermore, the fossils have a close connection with the more than 100 sharks and rays that swim the waters around Madagascar today. As Samonds noted, “Some of the taxa we find in Madagascar today…were also living at this site during the Eocene, which I find pretty impressive.”
Samonds KE, Andrianavalona TH, Wallett LA, Zalmout IS, Ward DJ (2019) A middle – late Eocene neoselachian assemblage from nearshore marine deposits, Mahajanga Basin, northwestern Madagascar. PLoS ONE 14(2): e0211789. https://doi.org/10.1371/journal.pone.0211789