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Description

Here we can see the four genera of Plotosaurian Mosasaurs known from the Pyramid Hill member of the Californian Jewett Formation. Dating to the Late Oligocene, this fossil site preserves the remains of marine animals that dwelt in the warm coastal waters of Western North America. Plotosaurians are highly derived Mosasaurs that superficially resemble Ichthyosaurs and Toothed Whales, inhabiting a diverse array of niches comparable to those of large tuna, beaked whales and dolphins. Members of the more modern clade Tunnimorpha emerged during the Late Eocene and survived the extinction event at the end of the period, going on to great success in the Oligocene and beyond. In the image above, we have:
1. Ilariasaurus princeps: An early and basal member of the clade Cryodyptida, this 6m deep diving animal possessed large eyes, moderately elongated jaws and a fusiform body shape. This genus retained small and conical teeth that would have been difficult to see expect when the jaws were fully open. Feeding on benthic fish and crustaceans caught from the sandy sea floor, Ilariasaurus would have been a relatively slow swimmer that rose to the surface waters at night. Cryodyptidans were common and diverse from the Late Oligocene to the Pliocene, inhabiting all of the world's oceans. Climatic changes would heavily reduce their diversity by the end of the period, with only a single living genus present in the Southern and Atlantic Oceans.
2. Coanosaurus applegatei: A 4m member of the Tengugnathidae family. These were the most basal members of Tunnimorpha and first appear in the fossil record during the Late Eocene. Toothless and long snouted animals, Tengugnathids were suction feeders that fed mostly on soft bodied cephalopods. Preserved skull material indicates that this animal possessed particularly large eyes, suggesting that Coanosaurus was probably a deep diving genus. Additional species have been described from contemporary sites in Japan and Peru.
3. Tunnisaurus gracilis: Within Tunnimorpha, a clade of relatively small, fast swimming forms emerged during the Early Oligocene: Squalisauroidea. These animals tend to be modestly sized and fast swimming piscivores inhabiting similar ecological niches to large tuna, dolphins and Opthalmosaurid Ichthyosaurs. Tunnisaurus was the type genus of the family Tunnisauridae, being a blunt snouted form measuring roughly 2m long. This animal would have been a very fast and agile hunter, capable of swimming at up to 40mph. As large pelagic tuna never evolved on Alter Earth, Tunnisaurids partially filled this kind of ecological niche.
4. Joaquinosaurus latus: The most basal family within Squalisauroidea were the Eosqualisaurids. These animals were commonplace across the pacific during the Oligocene, with members of the group present in California, Oregon, Japan and New Zealand. Joaquinosaurus was a 3m piscivore with a narrow snout and small stout teeth that fed on small fish and cephalopods. Like the contemporary Tunnisaurus, J. latus was a fast swimming open ocean hunter but was capable of feeding on slightly larger prey. Successful during the Oligocene, Eosqualisaurids died out during the Early Miocene to be replaced by their more derived relatives within Squalisauroidea.

Description

Here we can see the four genera of Plotosaurian Mosasaurs known from the Pyramid Hill member of the Californian Jewett Formation. Dating to the Late Oligocene, this fossil site preserves the remains of marine animals that dwelt in the warm coastal waters of Western North America. Plotosaurians are highly derived Mosasaurs that superficially resemble Ichthyosaurs and Toothed Whales, inhabiting a diverse array of niches comparable to those of large tuna, beaked whales and dolphins. Members of the more modern clade Tunnimorpha emerged during the Late Eocene and survived the extinction event at the end of the period, going on to great success in the Oligocene and beyond. In the image above, we have:
1. Ilariasaurus princeps: An early and basal member of the clade Cryodyptida, this 6m deep diving animal possessed large eyes, moderately elongated jaws and a fusiform body shape. This genus retained small and conical teeth that would have been difficult to see expect when the jaws were fully open. Feeding on benthic fish and crustaceans caught from the sandy sea floor, Ilariasaurus would have been a relatively slow swimmer that rose to the surface waters at night. Cryodyptidans were common and diverse from the Late Oligocene to the Pliocene, inhabiting all of the world's oceans. Climatic changes would heavily reduce their diversity by the end of the period, with only a single living genus present in the Southern and Atlantic Oceans.
2. Coanosaurus applegatei: A 4m member of the Tengugnathidae family. These were the most basal members of Tunnimorpha and first appear in the fossil record during the Late Eocene. Toothless and long snouted animals, Tengugnathids were suction feeders that fed mostly on soft bodied cephalopods. Preserved skull material indicates that this animal possessed particularly large eyes, suggesting that Coanosaurus was probably a deep diving genus. Additional species have been described from contemporary sites in Japan and Peru.
3. Tunnisaurus gracilis: Within Tunnimorpha, a clade of relatively small, fast swimming forms emerged during the Early Oligocene: Squalisauroidea. These animals tend to be modestly sized and fast swimming piscivores inhabiting similar ecological niches to large tuna, dolphins and Opthalmosaurid Ichthyosaurs. Tunnisaurus was the type genus of the family Tunnisauridae, being a blunt snouted form measuring roughly 2m long. This animal would have been a very fast and agile hunter, capable of swimming at up to 40mph. As large pelagic tuna never evolved on Alter Earth, Tunnisaurids partially filled this kind of ecological niche.
4. Joaquinosaurus latus: The most basal family within Squalisauroidea were the Eosqualisaurids. These animals were commonplace across the pacific during the Oligocene, with members of the group present in California, Oregon, Japan and New Zealand. Joaquinosaurus was a 3m piscivore with a narrow snout and small stout teeth that fed on small fish and cephalopods. Like the contemporary Tunnisaurus, J. latus was a fast swimming open ocean hunter but was capable of feeding on slightly larger prey. Successful during the Oligocene, Eosqualisaurids died out during the Early Miocene to be replaced by their more derived relatives within Squalisauroidea.