Mammals are present in great numbers in Spec, with a surprising number of "modern" (Real Life) groups present and accounted for.  In fact, mammalian diversity in Spec (though not species-count) actually exceeds that of RL, with a number of archaic mammalian clades surviving to the present day.

Spec's North America is home, for instance, to living multituberculates . These mammals, situated somewhere between the rest of the furry creatures and egg-laying monotremes, are greatly reduced from their former glory as "the rodents of the Mesozoic", but a few species still cling to life in North America and Eurasia. Docodonts , the most basal of all living mammal lineages, lay eggs, sprawl, and retain the double jaw joint of our reptilian ancestors. Triconodonts, another group of primitive mammals with sprawling forelimbs (a trait shared by docodonts and multituberculates), may be found in South America.  Also egg-layers, monotremes, have expanded far beyond our world's evolutionary pathways, producing not only a small number of platypus-like forms in Australia, but a variety of giant marine forms, including the walduks, the largest mammals on Spec.

The so-called 'higher mammals' are present on Spec, as well, thought none are as successful as the walduks.  Metatherians (a group which includes marsupials) occupy a very wide range, living on all continents but Africa and Antarctica. Of the eutherians (placental mammals like humans), xenarthrans (anteaters, armadillos, et cetera—order p-Xenarthra) , bats (p-Chiroptera), primates (p-Primates), and other small mammals live on the Spec-world (though greatly altered, in some cases). In addition, certain novel groups, such as the specworld rodents (Xenotheridia ) and the confusing specworld ungulates ( Paraselendontia) make their homes in this timeline only.

A few groups of mammals have even grown to large size during various parts of the Tertiary, most notably during the Pleistocene epoch, when the Ice Age pushed some dinosaur groups out of their accustomed niches. Today, the largest mammals are found at either high latitudes or altitudes, or in the water.

DOCODONTA (Moliarties) Burrowing and swimming egg-layer with an archaic double jaw joint

GONDWANATHERIA (Unrats, mangrove beavers, etc.) Rodent-like basal mammals

MONOTREMATA (Platipi, smoochers, and walduks) Primitively egg-laying mammals with modern jaws and ears

EUTRICONODONTA (Trikes) Sprawling predators

MULTITUBERCULATA (Multies, digga-dumdums, and djads) Burrowing and climbing mammals convergently similar to RL rodents

METATHERIA (Croc-varmits, marsupials, etc.) Mammals without placentas

EUTHERIA (Xenarthrans, primates, etc.) Familiar, live-bearing mammals