Natural selection is the process by which animals change over time; animals which are well adapted to their environment will flourish, whereas those with adaptations which reduce their “fitness” will struggle to survive. When we use the term “extinct”, this means that every individual of a species has died because they became poorly adapted to an environment which has changed around them. Modern day, “extant” sharks are adapted to how the environment is now (or in the recent past), but how did ancient sharks look and why were these (sometimes bizarre!) adaptations advantageous at the time?
Helicoprion
This genus of sharks lived between 290 - 259 million years ago. Helicoprion were absolutely remarkable because their teeth were completely different to modern-day sharks (Wheeler, 1939).
"Extant" sharks (sharks that are still around today) have evolved “revolver dentition”, meaning that rather than having one set of permanent teeth (like we do), they have a conveyor belt of teeth which lie in the jaw in rows. When a tooth at the front is shed, another can pop up from a row behind to replace it. To learn more about revolver dentition you can check out An Endless Supply of Teeth.
However, Helicoprion never shed its teeth. Instead, as new larger teeth continuously grew from the back of its jaw, the teeth in front were constantly pushed forwards into an incredible whorl. This unusual adaptation is thought to have been beneficial because gave the shark a unique, chopping bite, making it an effective hunter (Wheeler, 1939).
Cladoselache
This is first undisputed true shark, which is known to have lived around 380 million years ago.
Cladoselache was unusual because its skin was thin and relatively fragile compared to the scaly “dermal denticles” which make modern shark skin so tough (To learn more about dermal denticles you can check out Sandpaper Shark Skin). But this was not a weakness! On the contrary, it allowed Cladoselache to be nimble and quick. At the time, mighty, armoured fish known as Dunkleosteus were roaming the oceans and their light frames allowed Cladoselache them to evade these larger, heavier predators (Timita, 2015).
Stethacanthus
There is no denying that Stethacanthus is truly bizarre.This species lived between 360 - 340 million years ago, dying out around 298.9 million years ago (Zangerl, 1984).
Stethacanthus had an especially weird dorsal fin. In the males, the fin on their back was adapted to be anvil-shaped, rather than pointed. Topping this unusual feature they also had a crest of enlarged “dermal denticles”, which scientists suspect might have even been inflatable. These sharks were “sexually dimorphic”, meaning these unusual features were only found in the males. Scientists think that the anvil and crest evolved by “sexual selection”, as they were used to attract a mate during courtship displays (Zangerl, 1984).
Xenacanthus
At a glance you may assume that this creature was an eel or some kind of bizarre salamander. That extended, ribbon-like dorsal fin is certainly not at all similar to any modern day sharks.
Xenacanthus lived in freshwater habitats until approximately 202 million years ago. As a small shark (only reaching around 1 m in length), Xenacanthus was adapted to defend itself from predators with an impressive dorsal spine. It is thought this spine was used to inject venom, like Xenacanthus’ modern day stingray relatives (Beck et al, 2014).
Falcatus
This strange creature, known as Falcatus, lived some time during the early Carboniferous period, some 325 million years ago.
Falcatus was equipped with an impressive elongated first dorsal spine that pointed forwards. Whilst looking bizarre to us, this feature might actually have been used for attracting a mate. Like Stethacanthus's anvil, Falcatus's spines were sexually dimorphic. As only male Falcatus boasted the spine, scientists think it evolved as a display of strength that it used to attract a mate. Over time, sexual selection favouring larger spines, caused the spine to become more and more pronounced (Lund, 1985).
Edestus
Edestus, commonly known as the scissor-toothed shark, lived between 300 - 320 million years ago (Itano, 2015).
Like their close relatives Helicoprion, these strange jaws also formed by teeth growing from the back, but for Edestus this caused the jaw to extend and project forwards. Scientists think this made Edestus perfectly adapted to hunting. Unlike any other shark known, Edestus moved its body up and down, using its projecting teeth to slash its prey (Itano, 2015).
If you enjoyed learning about these strange extinct sharks, you can learn about bizarre sharks which are alive today, at the The Weird and the Wonderful.
References
Beck KG, Soler-Gijón R, Carlucci JR & Willis RE (2014). "Morphology and Histology of Dorsal Spines of the Xenacanthid Shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and Palaeoenvironmental Implications". Acta Palaeontologica Polonica, 61:1, 97–117. Access online.
Hamm SA (2008). Systematic, stratigraphic, geographic and paleogeological distribution of the late Cretaceaous shark genus Ptychodus within the western interior seaway. Master of Geosciences Thesis, University of Texas, USA. Access online.
Itano WM (2015). An abraded tooth of Edestus (Chondrichthyes, Eugeneodontiformes): Evidence for a unique mode of predation. Transactions of The Kansas academy of Science, 118:1-2, 1-9. Access online.
Lund R (1985). The morphology of Falcatus falcatus (St John and Worthen), a Mississippian stethacanthid chondrichthyan from the Bear Gulch Limestone of Montana. Journal of Vertebrate Paleontology, 5:1,1-19, doi: 10.1080/02724634.1985.10011842.
Timita T (2015). Pectoral fin of the Paleozoic shark, Cladoselache: new reconstruction based on a near-complete specimen. Journal of Vertebrate Paleontology, 35:5, e973029.
Wheeler HE (1939). "Helicoprion in the Anthracolithic (Late Paleozoic) of Nevada and California, and its Stratigraphic Significance". Journal of Paleontology. 13:1, 103–114. Access online.
Zangerl R (1984). On the microscopic anatomy and possible function of the spine-“brush” complex of Stethacanthus (Elasmobranchii: Symmoriida). Journal of Vertebrate Paleontology, 4:3, 372-378, doi: 10.1080/02724634.1984.10012016.
By Sophie A. Maycock for SharkSpeak
