A New Old Shark
Updated: Apr 27, 2021
Arguably, one of the most exciting aspects of working in palaeontology, is discovering an especially interesting fossil specimen... a fossil which teaches us something new about a species or the environment at the time; how they lived, what they looked like... More exciting still is the discovery of a whole new species. Remarkably, a research team working in the "Paleogene deposits" of the southeastern United States, have recently described not one, but two new species of sharks, they believed lived between 43 and 66 million years ago!
The Paleogene deposits in the USA are an especially fossil-rich region, so they allow us to learn about the animals that were alive during a period of deep history. The researchers in the study found their fossils at two sites: at the Clayton Formation of southern Alabama and the Clinchfield Formation, in central Georgia.
These areas were formed during the Paleogene; a "geologic period" that spans 43 million years between the "Cretaceous Period" (66 million years ago (Mya)) to the "Neogene Period" (23.03 Mya).
Whilst this region is within a land-mass now, millions of years ago the area was in fact a marine environment! Plate movements and the formation of mountains have slowly shifted the coastline and changed the topography of the area over time (Hart, 2017).
The fossils in the area were formed by dead animals, which had sunk to the ocean floor, being covered with silt and sediment, and the space created by their bodies eventually being mineralised. This leaves us with a rock in the shape of the animal, or a part of the animal (like the bones), which taxonomists, paleobiologists and paleoecologists can use to learn about their physiology, life-history and how they interacted with other species within their environment when they were alive.
Tissues in the body which degrade rapidly, are often not fossilised. This is why we commonly have to infer how an animal might have looked based upon its bones. As sharks have skeletons made of cartilage, rather than bone, which rapidly degrade after death, they very rarely form fossils. The only part of ancient sharks that can commonly be found are the teeth, because their hardened layer of enamel slows their decomposition, allowing for fossil formation to occur.
The researchers found two new species they named Mennerotodus mackayi and M. parmleyi. They were able to differentiate this species from other previously described Mennerotodus species, due to differences in the size and shape of their teeth; they were broader, with relatively smooth enamel, and had distinctive "cusps" (small, hooked protections at the base of the tooth) (Cicimurri et al, 2020).
The older of these species, M. mackayi was recovered from the Clayton Formation in Alabama and could be dated to the lower Paleocene period; between 65 and 55 Mya. The second species, M. parmleyi was discovered in layers in the Clinchfield formation in Georgia, which could be dated to the middle Eocene; between 33 and 55 Mya. Based upon what is known about the geology of the region, the scientists theorised that these sharks lived in shallow, coastal marine areas and maybe also in estuarine habitats (Cicimurri et al, 2020).
Millions of years later, these ancestors have given rise to modern-day sharks, like the sand tiger shark (C. taurus). These sharks still have remarkably similar dentition to their Mennerotodus ancestors, but their teeth are slightly less curved in profile. This reminds us that how long sharks have been around; potentially remaining quite similar for over 65 million years!
References
Cicimurri DJ, Ebersole JA & Martin G (2020). Two new species of Mennerotodus Zhelezko, 1994 (Chondrichthyes: Lamniformes: Odontaspididae), from the Paleogene of the southeastern United States. Fossil Record, 23, 117–140. Access online.
Hart BJ (2017). Paleoecological Analysis of the Clayton Formation (Paleocene) near Malvern, Arkansas. University of Southern Mississippi, Honors Theses, 513. Access online.
By Sophie A. Maycock for SharkSpeak.