Merwals of the Early Ultimocene

By the early Pangeacene 228 million years hence, shortly after the canitheres first diverged among the tribbethere clade, one early branch had already begun a new evolutionary path and returned to the ocean as a semi-aquatic predator. Lured from coastal scavenging into finding food in the water itself, the mertribs became the first tribbetheres to return to the ocean - the home of their earliest ancestors.

It may seem strange for a terrestrial fish such as any tribbet to return to the water tens of millions of years after leaving it, yet such is not unusual among living things if one were to still consider seals, whales, and even sea birds to still be lobe-finned fishes - as they remain, by descent, no matter how much evolution may change them. Oceanic environments are highly productive, Serina's shallow waters in particular, and can support a wide variety of often wholly unrelated animal groups, though only so long as they are able to niche partition to avoid the direct competition of the same particular resources. Alas, this is not always the case.

Mertribs may still be extant by the Ultimocene, but only just. Initially spreading widely over the oceans across the Pangeacene, dozens of species went extinct by the end of that epoch. The reasons for their rapid rise and equally rapid decline seem to be linked to competitive exclusion; the first mertribs fed on fish, but appear to have been pushed from this diet by much more nimble dolfinches and other aquatic bumblets and toward a later specialization upon shellfish feeding. Replacing long, extensible jaws of sharp teeth with less flexible mouths adapted to engulf mollusc shells and pulverize them with crushing molars, they became more specialized. In nature specialization is a double-edged sword; it can bring relief from competition with generalists to adapt to exploit a more difficult to access resource, yet the more specialized an animal becomes for one diet the less it will later be able to handle changes to its environment that may limit this resource as well. Sadly, this is the case with the mertribs - an unfortunate double-whammy of direct dietary competition, first with birds and later, other tribbetheres - but that is a subject for another page.

In the last 27 million years the diversity of mertribs has fallen sharply to truly dire straights, and the living species belong to just one fairly modern clade. Though very few in number, the forms that remain belong to a clade quite derived. Known as merwals, these canitheres have become entirely marine, cutting all ties to the land, and are the last of the mertribs.

We meet the merwals for the first and last time in this chapter, for today there are left just about half a dozen species of them, all fairly closely related and members of the most recent derivation event within the last 10 million years. Even a few thousand years ago we might have seen an additional four or five species, a few tens of thousands more and perhaps as many as ten more. Now, though, their days are numbered.

These last of the mertribs only slightly resemble their ancestors, retaining a mermaid-like manner of swimming with their single hind leg has become once again a fluked tail, though now pulsing horizontally and retaining its ankle joint versus the vertical ray-finned tail of their earliest Serinan ancestor. The body of all merwals is more streamlined, devoid of fur, and the front paws have lost their dexterity and become rounded flippers, and adults are more streamlined than the young, whose bodies have less fat and show more clearly the joints still present in their hind legs. The external ears are very small, though not yet lost to time - indeed, they will not get the chance to lose them. The jaw is long and extremely strong, with cone-like teeth at the front used to grip and pluck shellfish from the ocean floor and large square molars in the back of the jaw used to crack them open. The extensible jaws of the canithere have become nutcrackers, stretching wide to accommodate the very large bivalve molluscs they feed upon - organisms that returned to the sea from the last freshwater refugia at the end of the Thermocene and reclaimed much of their former diversity during the Pangeacene.

Resembling crook-backed dolphins, the last remaining merwal species are scattered at low densities around the southern coasts of Serinaustra where their food sources, which include clams, snails, and occasional crustaceans - can still be abundantly found. They combine an enigmatic mix of traits; they are dietary specialists yet highly intelligent, the smartest of all canitheres, with complex lasting social groups and cooperative parental care, and they take many years to mature. Their young are able to swim with their mothers from the moment of birth and are born with a full mouth of teeth, which lets them feed themselves on whole food offered to them by the adults in their social groups.

The last few merwal species are different colors and different sizes, and coexist via slightly different diets. The largest, the ringnecked merwal, grows 11 to 14 feet in length and feeds on very large bivalves and almost nothing else, while other species may preferentially eat crustaceans or weaker shelled molluscs like small snails.