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Description
A good demonstration of Alter Earth's marine diversity would be the fossil faunal communities of the Jewett Formation, California. Dating to the Late Oligocene approximately 25-23 mya, the shale heavy deposits present at the formation preserve the remains of a wide range of ocean dwelling animals that once inhabited the warm shallow seas of the western North American coastline. The most common animal remains found at the site belong to a variety of shark species, albeit almost entirely in the form of teeth. However, these were so distinctive in form that assignment to specific families was a relatively simple task, especially given that many of the genera present at this site are also found in Our World. Overall, the shark fauna present at the Jewett Formation were highly diverse, with species ranging from large ram feeding predators to small benthic foragers and fast swimming open ocean hunters. Up to 8 distinct species have been described an named, with a selection of four genera present in the image above:

1. Pseudotodus hesperus: A 6m member of the Lamniform family Otodontidae, this ram feeding predator possessed distinctive robust teeth that were broad based and moderately serrated. Likely descended from the successful genus Cretalamna, which Pseudotodus outlasted into the Oligocene, this animal would have superficially resembled a Great White in size, appearance and ecological niche despite being only distantly related. An ambush hunter capable of feeding on a wide variety of prey, including large bony fish, turtles, other sharks and marine reptiles, P. hesperus utilized it's serrated teeth to grip and slice through flesh. Approximately twice the size of Cretalamna, this genus appears to have become larger and more robust in response to the extinction of the archaic serpentine Mosasaurs at the end of the Eocene. At least 6 species are known, which were present from the Late Eocene to the Early Miocene and inhabited the seas surrounding western North America, Japan, Morocco, Egypt and Jordan. Despite it's impressive appearance, Pseudotodus was not the apex predator across its range, with the Tiamatoid Plesiosaurs dwarfing it in terms of size.

2. Isurus kernensis: A 3m member of the familiar genus Isurus also known from Our World, this particular species was on the smaller end of size spectrum when compared to it's relatives. Known from instantly recognisable teeth that appear tall, slim and pointed when viewed from the front, I. kernensis would have been a highly agile and fast swimming Lamnid capable of reaching speeds of up to 25mph. Prey would have included open ocean dwelling bony fish and cephalopods, which were attacked from below with lunging strikes. Although non-dental fossil material remains unknown, it is probable that this animal possessed a very powerful bite utilized in effectively disabling prey by targeting the fins and tail. Although this particular species died out during the Miocene, members of the genus are widespread in Alter Earth's Holocene, with three species inhabiting tropical and temperate waters. Isurus kernensis is known only from fossil bearing deposits in California, Oregon and coastal Peru from the Early Oligocene to the Late Miocene.

3. Echinorhinus sparksi: This unusual animal was a member of the mysterious family Echinorhinidae, also known as the Bramble Sharks. Phylogenetic studies have placed this group either within Sqauliformes or as close relatives of Sawsharks and Angel Sharks. Measuring approximately 3.2m long, E. sparksi tooth fossils are rare in comparison to other species of shark present in the Jewett Formation, a fact explained by the deep water environments favoured by this species. Sticking close to the sandy sea floor at depths of 400–900 m (1,300–3,000 ft), this shark fed on benthic fish, cephalopods, crustaceans and smaller sharks. The teeth somewhat resembled thin razor blades with three spiked peaks, while the mouth was wide and enabled E. sparksi to engage in a form of suction feeding. A slow moving and sluggish hunter, the skin would have been rough, covered by large protective denticles and was capable of exuding a foul smelling mucus to deter potential predators.

4. Leptogaleus campagnoi: This 1m Triakid is the smallest known genus of shark recovered from the Jewett Formation. Although the familiar Triakis Hound Shark is present on Alter Earth, Leptogaleus is a form unique to this parallel universe, first appearing in the fossil record during the Late Paleocene and surviving into the Holocene. An inhabitant of shallow coastal waters, L. campagnoi preyed on small bony fish and invertebrates either on the seabed or in midwater. The teeth were slim and knife-like, suggesting a diet composed of relatively soft bodied animals. Living species within this genus possess somewhat cryptic coloration characterized by dappled patterns and it is probable that L. campagnoi was similar, being useful in camouflaging this small shark from a wide array of predators.

Description
A good demonstration of Alter Earth's marine diversity would be the fossil faunal communities of the Jewett Formation, California. Dating to the Late Oligocene approximately 25-23 mya, the shale heavy deposits present at the formation preserve the remains of a wide range of ocean dwelling animals that once inhabited the warm shallow seas of the western North American coastline. The most common animal remains found at the site belong to a variety of shark species, albeit almost entirely in the form of teeth. However, these were so distinctive in form that assignment to specific families was a relatively simple task, especially given that many of the genera present at this site are also found in Our World. Overall, the shark fauna present at the Jewett Formation were highly diverse, with species ranging from large ram feeding predators to small benthic foragers and fast swimming open ocean hunters. Up to 8 distinct species have been described an named, with a selection of four genera present in the image above:

1. Pseudotodus hesperus: A 6m member of the Lamniform family Otodontidae, this ram feeding predator possessed distinctive robust teeth that were broad based and moderately serrated. Likely descended from the successful genus Cretalamna, which Pseudotodus outlasted into the Oligocene, this animal would have superficially resembled a Great White in size, appearance and ecological niche despite being only distantly related. An ambush hunter capable of feeding on a wide variety of prey, including large bony fish, turtles, other sharks and marine reptiles, P. hesperus utilized it's serrated teeth to grip and slice through flesh. Approximately twice the size of Cretalamna, this genus appears to have become larger and more robust in response to the extinction of the archaic serpentine Mosasaurs at the end of the Eocene. At least 6 species are known, which were present from the Late Eocene to the Early Miocene and inhabited the seas surrounding western North America, Japan, Morocco, Egypt and Jordan. Despite it's impressive appearance, Pseudotodus was not the apex predator across its range, with the Tiamatoid Plesiosaurs dwarfing it in terms of size.

2. Isurus kernensis: A 3m member of the familiar genus Isurus also known from Our World, this particular species was on the smaller end of size spectrum when compared to it's relatives. Known from instantly recognisable teeth that appear tall, slim and pointed when viewed from the front, I. kernensis would have been a highly agile and fast swimming Lamnid capable of reaching speeds of up to 25mph. Prey would have included open ocean dwelling bony fish and cephalopods, which were attacked from below with lunging strikes. Although non-dental fossil material remains unknown, it is probable that this animal possessed a very powerful bite utilized in effectively disabling prey by targeting the fins and tail. Although this particular species died out during the Miocene, members of the genus are widespread in Alter Earth's Holocene, with three species inhabiting tropical and temperate waters. Isurus kernensis is known only from fossil bearing deposits in California, Oregon and coastal Peru from the Early Oligocene to the Late Miocene.

3. Echinorhinus sparksi: This unusual animal was a member of the mysterious family Echinorhinidae, also known as the Bramble Sharks. Phylogenetic studies have placed this group either within Sqauliformes or as close relatives of Sawsharks and Angel Sharks. Measuring approximately 3.2m long, E. sparksi tooth fossils are rare in comparison to other species of shark present in the Jewett Formation, a fact explained by the deep water environments favoured by this species. Sticking close to the sandy sea floor at depths of 400–900 m (1,300–3,000 ft), this shark fed on benthic fish, cephalopods, crustaceans and smaller sharks. The teeth somewhat resembled thin razor blades with three spiked peaks, while the mouth was wide and enabled E. sparksi to engage in a form of suction feeding. A slow moving and sluggish hunter, the skin would have been rough, covered by large protective denticles and was capable of exuding a foul smelling mucus to deter potential predators.

4. Leptogaleus campagnoi: This 1m Triakid is the smallest known genus of shark recovered from the Jewett Formation. Although the familiar Triakis Hound Shark is present on Alter Earth, Leptogaleus is a form unique to this parallel universe, first appearing in the fossil record during the Late Paleocene and surviving into the Holocene. An inhabitant of shallow coastal waters, L. campagnoi preyed on small bony fish and invertebrates either on the seabed or in midwater. The teeth were slim and knife-like, suggesting a diet composed of relatively soft bodied animals. Living species within this genus possess somewhat cryptic coloration characterized by dappled patterns and it is probable that L. campagnoi was similar, being useful in camouflaging this small shark from a wide array of predators.