The Good, the Bad & the Ugly
Updated: Apr 23
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?
This “genus” of sharks lived between 290 - 259 million years ago. Helicoprion were absolutely remarkable because the dentition was completely different to modern-day sharks.
Extant sharks 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. However, Helicoprion’s baby teeth were not shed, whilst new, larger teeth grew from the back of its jaw, to constantly push the front teeth forwards into an incredible whorl. This unusual adaptation is thought to have been beneficial because it conferred the shark a unique, chopping bite, making it an effective hunter (Wheeler, 1939).
This is first true shark, which is known to have existed 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 relatively impenetrable. However, what might be perceived as a weakness, was actually advantageous. During a time when the mighty armoured fish known as Dunkleosteus was roaming the oceans, being armourless made Cladoselache nimble and quick, allowing it to evade the larger, heavier predators (Timita, 2015).
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).
The males of this species had a dorsal fin which was adapted to be anvil-shaped, rather than pointed.Topping this unusual feature Stethacanthus males had a crest of enlarged “dermal denticles”, which were probably inflatable. As these sharks were “sexually dimorphic” (meaning these unusual features were only found in the males), it is thought that the anvil and crest structures evolved by “sexual selection”, as they were used during courtship displays.
At a glance you may assume that this creature was an eel, as its extended, ribbon-like dorsal fin is not at all similar to 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).
Edestus, commonly known as the scissor-toothed shark, lived between 300 - 320 million years ago.
As a relative of Helicoprion, these strange jaws also formed by teeth growing from the back, but in Edestus this caused the jaw to extend and project forwards. It is thought this made Edestus perfectly adapted to hunting. Unlike any other shark known, Edestus used its projecting teeth in a thrashing manner; moving its body up and down to traumatise its prey by slashing (Itano, 2015).
This strange creature, known as Falcatus, lived some time during the early Carboniferous period.
The incredible, elongated first dorsal spine, whilst looking bizarre to us, might actually have been used for attracting a mate. Sexual dimorphism suggests that the males evolved the spine to display their strength while courting females. Over time, sexual selection (favouring larger spines), caused the spine to become more and more pronounced (Lund, 1985).
At first glance, Ptychodus might look quite similar to modern day sharks, but what set them apart were the unusual jaws and teeth.
Sharks of the Ptychodus genus lived between 100 - 85 million years ago and, despite only feeding on relatively small shellfish, could grow up to 11 m in length! They were perfectly adapted to their "niche" because the teeth evolved to become flattened, which was perfect for grinding the shells of its food in enormous quantities (Hamm, 2008).
If you enjoyed learning about the strange extinct sharks, you can learn about bizarre sharks which are alive today, at the The Weird and the Wonderful
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.