Modern beluga whales and narwhals are the only living representatives of the monodontid lineage, found only in cold Arctic and sub-Arctic waters. But this whale family actually first evolved in much warmer climates – and some of them were downright tropical.
It seems to have had a larger number of functional teeth than modern monodontids, and probably didn’t suction feed like its modern close relatives. Instead it may have fed more like most porpoises and dolphins, relying more on speed and snapping jaws to capture prey.
It inhabited the Mediterranean at a time not long after the sea there had mostly dried up and then been rapidly refilled. The presence of warm-water marine species such as bull sharks, tiger sharks, and dugongs in the same fossil beds as Casatia indicates the local climate at the time was hotter than it is today, with tropical temperatures – and suggests that this whale’s ancestors must have originally moved into the replenishing Mediterranean from lower latitudes alongside these other warmth-adapted animals.
This tropical monodontid was also much closer related to modern belugas than modern narwhals are, which raises the possibility that the two living monodontid species actually specialized for colder conditions completely independently of each other rather than descending from a cold-adapted common ancestor. Instead modern belugas and narwhals may have originated from separate warm-water monodontid ancestors who evolved similar cold-tolerant adaptations in parallel as the climate cooled during the onset of the Quaternary ice age, while the rest of their relatives all went extinct.
Known just from fossilized lower jaws and teeth, with some teeth up to nearly 13cm long (~5″), its full life appearance and size are uncertain – but it may have been slightly larger than a modern bottlenose dolphin at around 4.5m long (~14’9″). It’s traditionally been considered to be part of the kogiid family, closely related to modern pygmy and dwarf sperm whales, but some studies disagree with that classification and instead place it in the true sperm whale lineage.
I’ve reconstructed Kogiopsis here as a kogiid-like animal, with a similar sort of shark-like head shape and “false gill” markings. In the background a second individual is depicted displaying “inking” behavior, releasing a defensive cloud of reddish-brown fluid from a specialized sac in its colon.
Living during the mid-Eocene, about 43 million years ago, in a shallow sea-covered region that is now part of Egypt‘s Western Desert, Phiomicetus was an early protocetid – an amphibious foot-powered swimmer, at a transitional point in the evolution of whales from deer-like terrestrial animals to fully aquatic screaming torpedoes.
About 3m long (~10′), it had large jaw muscles and sharp teeth with wear patterns that suggest it was a raptorial hunter grabbing and snapping at prey with powerful bites. It would have probably tackled fairly big prey compared to other protocetids, hunting things like large fish, turtles, and even smaller whales in an ecological role similar to that of modern orcas.
Along with the distantly-related long-snouted Rayanistes it’s one of the earliest known whales from Africa, giving us further glimpses at a time period when early cetaceans were first dispersing out from the South Asian subcontinent via the ancient Tethys Sea.
It was one of the earliest known large-bodied members of the group, and shows that these animals must have increased in size very rapidly during their early evolution, going from rabbit-sized to pig-sized within just a couple of million years.
Unlike modern baleen whales it was small, about the size of a modern porpoise at around 2m long (6’6″), and the wear on its multi-cusped teeth suggest it was a predator taking slicing bites of fish – possibly using suction-assisted feeding like its close relatives the aetiocetids.
Its fossilized remains are also a rare example of an ancient whale fall, with characteristic bore holes in its bones from Osedax worms.
Living about 30 million years ago in shallow coastal waters around the southeast USA, in what is now South Carolina, it was a member of one of the very earliest groups of toothed whales known as the xenorophids. Although only very distantly related to modern forms, xenorophids show evidence of being able to echolocate, suggesting the ability was probably ancestral to all toothed whales.
Estimated to have measured about 1m long (3′3″), Inermorostrum had a very short downturned snout and was completely toothless – specialized adaptations for suction feeding on small soft-bodied creatures on the seafloor.
Globicetus hiberus, a 5m long (16′4″) beaked whale from the Atlantic coast of Portugal and Spain. Its fossils can’t be easily dated since they were fished up from the seafloor, but it was probably around Early-to-Mid Miocene in age (~20-14 mya).
Its skull sported an odd bony sphere at the base of its snout, just in front of the melon, which appears to have been larger and more prominent in males than in females. Many modern beaked whales also have sexually dimorphic crests, ridges, and domes in their skulls, and these structures may function as sort of “internal antlers” – a display structure the whales can “see” via echolocation that signals their size, strength, and health to each other.
Although it was certainly a big animal, it may not have been the giant “super predator” it’s often depicted as – its teeth aren’t particularly specialized and resemble those of entelodonts, suggesting it may have been more of an opportunistic omnivore than a dedicated carnivore.
Without more material we just don’t know for certain. So, frustratingly, the rest of Andrewsarchus’ body remains a mystery.
I’ve reconstructed it here based on one of its more obscure possible relatives: the anthracotheres, a group which may have been closely related to modern hippos. Scaling its body proportions to these animals produces rough measurements of about 1.45m tall at the shoulder (4′9″) and 3m long (9′10″), or about the same size as some of the big entelodonts or large modern bears.