Ambulocetus BW.jpg

''Ambulocetus'' ( Latin ''ambulare'' "to walk" + ''cetus'' "whale") is a genus of early amphibious cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene ( Lutetian). It contains one species, ''Ambulocetus natans'' (Latin ''natans'' "swimming"), known solely from a single, near-complete fossil. ''Ambulocetus'' is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. ''Ambulocetus'' is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes '' Himalayacetus'' and ''Gandakasia'' (also from the Eocene of the Indian subcontinent).

''Ambulocetus'' had a narrow, streamlined body, and a long, broad snout, with eyes positioned at the very top of its head. Because of these features, it is hypothesised to have behaved much like a crocodile, waiting near the water's surface to ambush large mammals, using its powerful jaws to clamp onto and drown or thrash prey. Additionally, its ears possessed similar traits to modern cetaceans, which are specialised for hearing and detecting certain frequencies underwater, although it is unclear if ''Ambulocetus'' also used these specialised ears for hearing underwater. They may have instead been utilised for bone conduction on land, or perhaps served no function for early cetaceans.

It is thought to have swum much like a modern river otter, tucking in its forelimbs while alternating its hind limbs for propulsion, as well as undulating the torso and tail. It may have had webbed feet, and unlike its modern relatives, lacked a tail fluke. On land, ''Ambulocetus'' may have walked much like a sea lion.

''Ambulocetus'' inhabited the Indian subcontinent during the Eocene. The area had a hot climate with tropical rainforests and coastal mangroves, and ''Ambulocetus'' may have predominantly inhabited brackish areas such as river mouths . It lived alongside requiem sharks, catfish and various other fishes, turtles, crocodiles, the amphibious hoofed mammal ''Anthracobune'', and the fellow cetaceans ''Gandakasia'', ''Attockicetus'', '' Nalacetus'', and '' Pakicetus''.

Taxonomy

Discovery

In December 1991, Pakistani palaeontologist Mohammad Arif and Dutch–American palaeontologist Hans Thewissen were jointly funded by Howard University and the Geological Survey of Pakistan to recover land mammal fossils in the Kala Chitta Hills of Punjab, Pakistan. On 3 January 1992, they recovered a small, thick rib fragment. Later in the field season, while surveying the upper Kuldana Formation, Thewissen discovered a femur (thigh bone) and proximal portion of the tibia (upper portion of the shin) which clearly belonged to a mammal. An hour later, Arif discovered the rest of the skeleton, and the two began excavation the next day. At first, Thewissen speculated the fossils belonged to an anthracobunid (a large semi-aquatic mammal), until he found the teeth near the end of the field season, which were characteristically cetacean (living cetaceans are whales, dolphins, and porpoises). Thewissen, at the time, could not afford to excavate and store everything, so he took the skull with him to the United States, while Arif kept the rest in two crates which used to hold oranges. In October 1992, Thewissen presented his research of the skull to a vertebrate palaeontology convention in Toronto, Canada. The next year, American palaeontologist Philip D. Gingerich paid for the rest of the skeleton to be shipped to the United States. In 1994, the formal description of the remains was published by Thewissen, mammal palaeontologist Sayed Taseer Hussain, and Arif. They identified the remains as clearly belonging to an amphibious cetacean, and so they named it ''Ambulocetus natans''. The genus name comes from Latin ''ambulare'' "to walk" and ''cetus'' "whale", and the species name ''natans'' "swimming".

The Kuldana Formation is constrained to sometime during the Lutetian stage of the Early Eocene, and the remains may date to 48–47 million years ago. The holotype specimen, HGSP 18507, is a partial skeleton initially discovered preserving an incomplete skull (missing the snout), some elements of the vertebral column and ribs, as well as portions of the fore- and hind-limb. Other specimens initially found were HGSP 18473 (a second premolar), HGSP 18497 (a third premolar), HGSP 18472 (a tail vertebra), and HGSP 18476 (lower portion of a femur). The holotype was found in a silt and mudstone bed over a area. Further excavation recovered most of the holotype's skeleton—most notably the hip, sacrum, and most of ribcage and thoracolumbar series (the spine excluding the neck, sacrum, and tail). These left the holotype about 80% complete by 2002, making it the most completely known cetacean from the time period. In 2009, some more elements of the holotype's jawbone were identified from a then-recently prepared matrix block.

Though it was known that cetaceans descended from land mammals before the discovery of ''Ambulocetus'', the only evidence of this in the fossil record was the 52-million-year-old (fully terrestrial) '' Pakicetus'' and the Paleocene mesonychids (as there was a hypothesised link between cetaceans and mesonychids). The limbs of more aquatic Eocene cetaceans did not preserve very well. ''Ambulocetus'' demonstrated that cetaceans swam by flexing the spine up and down (undulation) before they had evolved the tail fluke, forelimb propulsion evolved relatively late, and that cetaceans went through an otter-like phase with spinal undulation and hindlimb propulsion. These had already been hypothesised to have occurred in the earliest aquatic cetaceans, but were impossible to test without more complete remains. The describers noted that, "''Ambulocetus'' represents a critical intermediate between land mammals and marine cetaceans."

Classification

Modern cetaceans (Neoceti) are grouped into either the parvorders Mysticeti (baleen whales) or Odontoceti (toothed whales). Neoceti are descended from the ancient Archaeoceti, whose members span the transition from terrestrial to fully aquatic. Archaeoceti are thus paraphyletic (it is a non-natural group which does not comprise both a common ancestor and all of its descendants). ''Ambulocetus'' was an archaeocete. By the time ''Ambulocetus'' was discovered, archaeocetes were classified into the families Protocetidae (which included what are now the terrestrial Pakicetidae, and the rest were amphibious), Remingtonocetidae (amphibious), Basilosauridae (aquatic), and Dorudontidae (aquatic, now a subfamily of Basilosauridae). The earliest cetaceans were thought to be the mesonychids, proposed before any firm early cetacean fossils were identified. In the original description, ''Ambulocetus'' was preliminarily placed into Protocetidae, until the further description of the holotype prompted Thewissen and colleagues to move it into its own family Ambulocetidae in 1996. At the same time, they also erected the family Pakicetidae. They also proposed that some members of Pakicetidae, Protocetidae, and Ambulocetidae were the other two archaeocete families' ancestors. They suggested that mesonychids gave rise to pakicetids, which gave rise to ambulocetids, which gave rise to both protocetids and remingtonocetids.

Though middle-to-late-Eocene archaeocetes are also known from North America, Europe, and Africa, most of these are found only on the Indian subcontinent. Therefore, it is thought cetaceans originally evolved in that region. Based on molecular data, cetaceans are most closely allied with hippos ( Whippomorpha), and they split approximately 54.9 million years ago. They are all placed in the order Cetartiodactyla alongside terrestrial even-toed ungulates (hoofed mammals). This puts mesonychids as a distant relative of cetaceans rather than an ancestor, and their somewhat similar morphology was possibly a result of convergent evolution. The oldest known cetacean is the ambulocetid '' Himalayacetus'' identified in 1998 and dated to 52.5 million years ago (predating the terrestrial pakicetids), though the exact dating of ''Himalayacetus'' and ''Pakicetus'' is debated. Ambulocetidae also includes ''Gandakasia''. ''Himalayacetus'' and ''Gandakasia'' are known only from partial jaw fragments. Ambulocetidae are endemic to the Indian subcontinent, and span the early to middle Eocene.

Description

Size

Upon description, Thewissen and colleagues suggested the holotype specimen may have weighed the same as a male South American sea lion — about — based on the size of the vertebrae, ribs, and limbs. They also estimated a length of roughly . For comparison, the holotype of ''Pakicetus attocki'' may have been long. In 1996, they estimated weight of ''Ambulocetus'', using the cross-sections of the long bones, as . Alternatively, they estimated about by using the length of the second upper and lower molars compared to trends between this length and ungulate body mass. They obtained the same result comparing the skull size to those of similarly sized carnivores. In 1998, based on vertebral size, Gingerich estimated a body mass of , similar to modern cetaceans. In 2013, Thewissen suggested that this may be an unreliable mass determinant as the vertebrae are unusually robust in ''Ambulocetus''.

Skull

Like other archaeocetes which preserve this element, the base of the skull has an undulating contour, probably related to the shape of the nasal canal (and its passage to the throat) and the narrow infraorbital region (the area below the eyes). The base of the skull is wide compared to other archaeocetes, more like that of modern cetaceans. The narrow infraorbital space, made of primarily the pterygoid processes, also occurs in '' Remingtonocetus'' and ''Pakicetus''. The pterygoids connect as far back as the middle ear, much farther than other archaeocetes including the more ancient ''Pakicetus''. Most modern cetaceans have a falcate (sickle-shaped) process which juts out prominently halfway between the hypoglossal canal and the ear; ''Ambulocetus'' has a similar process continuous of the pterygoid, but it runs alongside and behind the hypoglossal canal. Like many other archaeocetes, the pterygoids, sphenoids, and palatines form a wall lining the bottom of the nasal canal, which causes the palate to extend all the way to the ear. Like other cetaceans, ''Ambulocetus'' lacks the postglenoid foramen, which usually is one of the main passageways for veins into the skull in placental mammals. The ectotympanic bone which supports the eardrum is similar to that of ''Pakicetus'', about as long as wide, whereas later archaeocetes have more elongate ectotympanics. The ectotympanics of all archaeocetes, nonetheless, are much different than those of terrestrial mammals. The ectotympanics of all cetaceans, including ''Ambulocetus'', possess an involucrum (thickened lump of bone) at the medial lip. Unlike ''Pakicetus'', but like later archaeocetes, the tympanic made close contact with the jaw. Like later archaeocetes, ''Ambulocetus'' seems to have possessed an air sinus in the pterygoids. It may have also had paranasal sinuses. The parietal bones on the braincase sides are more perpendicular than in ''Remingtonocetus'', which makes the cheeks appear less flared. Like ''Remingtonocetus'', ''Ambulocetus'' appears to have had a small brain.

The snout was quite broad, but the end of the holotype's snout is missing, so it is unclear how long it would have been. The snouts of '' Basilosaurus'' and ''Rodhocetus'' are short and make up about half the skull's length. Remingtonocetid snouts are quite narrow, which was clearly not the case for ''Ambulocetus''. The mandibular symphysis of most mammals is restricted to the midline of the jaw, but extends much farther in archaeocetes; in ''Ambulocetus'', it reaches the back end of the first premolar. Snout robustness and symphysis length suggest reinforcement of the jaw to withstand a strong bite force. Similarly, the strongest biting muscle in ''Ambulocetus'' seems to have been the temporalis muscle involved in biting down. Like other cetaceans, there are embrasure pits (a depression between the teeth), preserving the tooth positions for the fourth premolar, the first molar, and the third molar. Unlike later archaeocetes, the molars' roots do not extend to the cheek bones, and the third molar is not as nosewards as in remingtonocetids. The coronoid process of the mandible (where the lower jaw connects with the skull) in ''Ambulocetus'' is steep. In contrast, it is low and slopes gently down in basilosaurids and later cetaceans. The mandibular foramen opens below the coronoid process, and is around midway between terrestrial mammals and toothed whales in size. Like other cetaceans, the body of the hyoid bone (the basihyoid bone) is about as long as wide. Unlike other archaeocetes, the eyes are quite large and are placed near the top of the head facing upwards.

Unlike modern toothed whales which only have one kind of tooth (homodont), archaeocetes are heterodont. Judging by tooth root size, the lower canine was larger than the incisors. The teeth are more robust than those of ''Rodhocetus'' and ''Basilosaurus''. The premolars were double rooted, whereas most archaeocetes have single-rooted first premolars. The enamel of the lower premolars is crenulated (has scalloped edges). The fourth premolar is a high triangular shape. Like other ancient cetaceans, and most pronouncedly in ambulocetids, the lower molars are shorter than the back premolars. The lower premolars are larger than those of ''Pakicetus'' and are separated by wider gaps ( diastemata). The molars had distinct trigonid and talonid cusps (these cusps are lost in basilosaurids), and the upper molars were trituberculate like ancient archaeocetes and ancient placental mammals, meaning they had a large protocone, distinct paracone

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