Mammaliformes: Overview

Introduction

The mammaliforms are, of course, the run-up to mammals. Not that they were "destined" to become us, or anything at all. The whole lineage could quite easily have disappeared during the Mesozoic without a trace. But, from our perspective, they're the group that produced us; so it is natural to wonder how that happened.

All of the early mammaliforms looked more or less like rodents and were about the same size as that most successful group of modern mammals. The mammaliform story is about internal, structural developments, many of which we have only begun to be able to study in the last decade. Understanding these changes unavoidably requires us to look at technical anatomical details.  Some of the things to look for are these:

1) The dentary-squamosal jaw joint: All terrestrial vertebrates except mammaliforms form the jaw hinge between the articular and the quadrate. Mammaliforms form the joint between the dentary and the squamosal. This transition was well under way before mammaliforms got started.

2) The post dentary bones -- the articular and angular -- weren't lost. They, and the quadrate, became incorporated into the middle ear as the auditory ossicles. In mammals they are called the malleus, incus and stapes.

3) The inner ear was also re-engineered. The otic capsule became somewhat separated from the rest of the braincase as a pair of petrosals. Within the petrosal, one of the organs of hearing (the maculae) became first enlarged, and then coiled.

4) The teeth of mammals are almost unique in a number of respects. Instead of having lots of simple teeth that were replaced frequently, mammals have only one set of adult teeth which meet (occlude) in a very precise fashion.

5) Most importantly, mammals developed separate, specialized molars. Molars not only have points (cusps) which shear past each other, but have a certain cusps which grind food on relatively flat regions of the opposite molar. This system seems to have evolved separately at least three times and probably more often. Since most mammaliform remains consist of teeth, we have a great many examples of tooth forms. Unfortunately, the degree of convergent dental evolution has also greatly confused the picture.

6) The brain itself, and the surrounding bone, seems to have undergone some profound reorganization. After hundreds of millions of years of gradual retreat, the anterior part of the old palatoquadrate (the epipterygoid of reptiles) makes a strong come-back in mammaliforms in vatious guises such as the alisphenoid and orbitosphenoid -- structures that provide central support for the anterior skull.

7) these changes in the organization of the skull are accompanied by soft tissue changes in the distribution of the cranial nerves and the major blood vessels. These transitions remain poorly understood.

8) Post-cranially, as in reptiles, the limbs tended to move under the body. Surprisingly, mammals seem to have been very slow to acquire a truly erect stance; and the fore- and hind-limbs seemed to have evolved erect postures almost independently.

Of course, a great many other things were going on at the same time: lactation, increased body metabolism, body hair, and so on. But these are almost impossible to study directly in the fossil record. They remain, for the most part, matters of speculation.

Phylogeny and Origins

Some time in the Middle Triassic, the mammaliforms derived from near one of two branches of the cynodont family: either the tritylodonts or the tritheledonts. See Rubidge & Sidor (2001) for a nice review. It is even possible that both statements are true. The Allotheria multituberculates) are so different from anything else that one can almost imagine their deriviation from an entirely separate line of cynodonts.

The Mammaliaformes, as current fashion spells it, are defined as: the last common ancestor of Sinocodon and modern mammals and all of its descendants. Luo et al. (2002). Sinocodon + Smilodon is one way to remember it. However, for our purposes, we have used a working definition anchored on Haramiyavia, assuming that the unknown intersection between multituberculates and modern mammals is the appropriate break point. This assumes that Haramiyavia really is a primitive allotherian, which is hotly debated.

Assuming all this -- quite a lot to swallow -- we can envision the Mammaliaformes as made up of three main groups: the Allotheria, the Docodonta and close relatives (e.g. Morganucodon), and the Symmetrodonta. The Allotheria consist largely of the multituberculates. They outlasted the dinosaurs and are the single longest lived branch of mammaliforms. The docodonts were relatively short-lived, but left us with a superb fossil record. The Symmetrodonts (e.g. Kuehneotherium) are no longer bel ieved to be a separate branch at all, and include the living mammals. ATW020316.