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Metacetacea

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On Terra Alternativa, the absence of cetaceans allowed the evolution of several groups of aquatic tetrapods to fill their niche. One of the most successfull groups was, predictably, a mammalian one, the metacetes.

These mammals evolved from the otter like pantolestids of the Eocene. Their exact relations with other mammals are not well understood, but seem to be early placentals, possibly even non-placental eutherians; the presence of a well developed placenta in metacetes may not be indicative of their relationships, since it could have evolved independently. The earliest known form is the middle Eocene north american form Eocetolutra, a form that occured in both marine and freshwater deposits. It is very similar to the average pantolestid, except for its ear structure, which was already more or less identical to that of modern metacetes. In the rest of the Eocene, metacetes made quick progressions, much like our whales, and by the end of the period recognisable taxa existed. Managing to pass the Eocene/Oligocene extinction with little casualities, they kept diversifying without barriers for the next 35 million years.

Anatomically speaking, metacetes resemble cetaceans in many ways. Their hindlimbs are gone, relying on their fluked tails for propulsion, their remaining front limbs are now flippers, their skin has microscopic structures similar to the ridges in shark and mosasaur scales, and their internal organs are generally quite similar. Their penises are even identical, being muscular organs attached to the remaints of the pelvic grids that, much like the rest of the genitals, are usually inside the body, coming out during intercourse. However, their heads are very different. In cetaceans, the nostrils moved up the skull until becoming a hole at the top of the head, and independent from the pharynx. In metacetes, however, they stayed at the same place, at the tip of the snout (although in most derived groups the bones sorrounding the nasal chamber have receeded away and eventually vanished, leaving only the maxilla, much like in cetaceans). Much like choristoderes and thalattosuchians, this means that the only part of the head that needs to reach the surface is the tip of the snout, thus concealing the rest of the body underwater. This situation is akin to that of our seals (and, to a lesser extent, sirenians), but unlike seals they lost their whiskers (still present in the newly born though, and see below), and being mostly piscivores their dentition returned to an homodont state much like that of cetaceans. Their heads, thus, somewhat resemble those of crocodillians and choristoderes, this being sometimes acknowledged in their vernacular names.

They are very intelligent mammals, most derived species comparable in intelligence to our dolphins. Their complex social behaviour is not clearly understood, but it is known to be extremely diverse. Unlike our odontocete cetaceans, most species lack advanced echolocation (although several can echolocate to some extent), using mostly vision and touch to guide themselves. Because of this most have proportionally large eyes and very sensitive snouts.

Gambo (Gambo gambalo)

This is the most basal living species, similar to long gone Eocene forms. It still has hind flippers (albeit relatively small), and a fluke-less tail. They swim thus in a primitive undulatory motion that uses most of the lower body ("otter style"), instead of using just the tail as more advanced species. Its nose bones barely receeded at all, and its dentition is still somewhat heterodont, with typical piscivore teeth at the front of the jaws and still recognisable molars and premolars at the back. Such a conservative species has survived along the western coast of Africa and associated rivers like Congo; fossils indicate that closely related species managed to survive in south american coastlines until the Pliocene.

The Gambo lives alone or in relatively small groups, its chosen habitat relatively safe of large predators. It is quite nomadic, travelling in long distances in search of food; an individual targetted in what is in our world the coasts of southern Angola was found, a month later, well up the Congo. Vulnerable in the open sea, it rarely ventures into deep waters, although populations do exist on the coasts of Ascension and St. Helena. Whereas these are subspecies or seperate species is still unknown. Gambos are very diverse colour wise; brown, grey, silver, black, white and any of these with stripes of another colour have been recorded. They are very opportunistic regarding their diet, chasing after prey or searching along the bottom with their sensitive snouts; fish, cephalopods, crustceans and shellfish are the most common prey, but they have also been seen scavenging or attacking small crocodiles, choristoderes, various aquatic mammals and aquatic birds. It gives birth year round, gestation lasting as much as our's, and the pup stays with the mother for 3 years.

Pseudomyscetidae

Baleen whale niches are mostly occupied by ray finned fish on Terra Alternativa, such as paddlefish and pachychormids, as well as numerous shark clades. However, minor filter feeders occur here and there, the most notable being the pseudomysceti bhales. 10 species of these mammals occur in the waters of all over the world, some species cosmopolitian in range.

Much like our whales, the bhales traded their teeth for baleen. Said baleen appearently evolved quite early in metacete evolution; Eocene forms show both teeth and what appears to be proto-baleen. The modern Gambo still has proto-baleen among its teeth; they appear to have some function in capturing prey while foraging on the bottom, a function likely shared by the early metacetes. During the Cenozoic, several clades of metacetes specialised in filter feeding, some of which converging very heavily with modern pseudomyscetes. Eventually, this clade emerged triumphant, and has been monopolising the mammalian filter feeding niches since the Pliocene, competing only with the few filter feeding monotremates. The first pseudomyscetes evolved in the Oligocene, possibly even in the late Eocene, but the first forms specialised to filter feeding would only appear in the middle Miocene, under the form of Eumyscetus, already identical to modern species, except for the presence of small upper teeth; earlier pseudomyscetes were more diverse, producing durophagous and macropredatory forms, outcompeted by more advanced metacetes and other marine mammals. They completly lost the bones sorrounding the nasal cannal, the maxilla thus forming a long rostrum with the nasal cannal running on top of it, ending in the nostrils at the very tip of the snout.

Modern pseudomyscetes are divided into 5 genera: Paracaperea, Pseudobalaena, Xenomyscetognathus, Benthicoptera and Pseudoeschrichtius. More information on these will be explored on further explorations of TA. Most of pseudomyscetes feed on the sea bottom, grey whale style, with a few pelagic species going after krill in mid water. Prey items are usually crustaceans and small fish in most forms, with Xenomyscetognathus actively adding sardine sized fish to its diet. They are logically the largest modern metacetes, and among the largest living mammals on Terra Alternativa.

Delphiphocidae

The most diverse (species wise) linage of modern metacetes, they occupy the ecological niches of our dolphins, porpoises and seals (they are all fully marine, obviously). Most species have short snouts, and redeveloped heterodonty and whiskers, ence their resemblance to seals. Being very generalistic animals, they are the most adaptable of all metacetes. They are obviously also the most intelligent, and living in large groups they have an unique and vastly complex social behaviour. Over 60 species have been discovered, and the total number seems to be higher still. Ironically, it was by redeveloping features lost to become specialised to an aquatic lifestyle that allowed their success.
Seal+dolphin+champsosaur=fun!
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