SURPRISES IN THE CLASSIFICATION OF LIVING ORGANISMS, 8; Big double mammal brains trumped the dinosaurs (Or Why Killing Sapient Dolphins and Whales and Elephants Is Murder, and Why Humankind is Getting More Intelligent)

BY DR. JOSE PALU-AY DACUDAO

(Part 1)

WHAT is the main reason why mammals took over most of the medium-to-large terrestrial animal niches over dinosaurs (which includes all birds) 65 million years ago at the end of the Cretaceous? (Note that birds which are regarded as dinosaurs, also survived the extinction event.)

There are numerous hypotheses. One of the more popular ones is that the small burrowing mammals could resist catastrophic environmental changes better than large exposed dinosaurs that needed more food.

However, in my opinion, much of the answer lies in intelligence. Mammals are much more intelligent than birds. This allows mammals to develop more complex adaptive behavior.

You might ask why did dinosaurs never evolve to be as intelligent as mammals?

Both dinosaurs and mammals evolved from amniotes, which in turn evolved from ancient amphibians during the Carboniferous Period (around 360 to 300 million years ago). Amniotes are the first vertebrates to evolve an amnion, a covering of the embryo, which protected the eggs from drying up on land. This enabled the first amniotes to invade the land decisively, living off the numerous arthropods and land plants that preceded them.

Afterward, the first amniotes diverged into two clades, the Sauropsids and the Synapsids, in the late Carboniferous. It was from the first Sauropsids that all modern traditional ‘reptiles’ (a paraphyletic group) and birds are descended from.

A more sensible way of putting it is that both traditional ‘reptiles’ and birds are Sauropsids. Thus, all non-mammalian terrestrial amnion-forming vertebrates belong to Clade Sauropsida.

It would probably be better to just redefine ‘reptile’ as sauropsid. However, it was Sauropsida’s sister group, the Synapsida, that actually dominated the medium-to-large terrestrial niches in the succeeding Permian Period (from about 299 to 252 million years ago).

Then in a twist of events, only one group, the therapsids, among the descendants of the early synapsids, survived the end Permian extinction event as will be discussed below.

Moreover, early in the succeeding Triassic Period, all other lineages of Clade Therapsida died out except for the cynodonts, from which mammals are derived from. Thus, all mammals are synapsids, or to put it in another way, mammals are the only synapsids extant today.

Class Mammalia was first defined by Carl Linnaeus in 1758 as creatures that possess mamma, the Latinized form of teat.

Female mammals characteristically suckle their young with their mammary glands (teats or mamma). The mammary gland is a type of specialized sweat gland, and so we can also restate the definition of a mammal as creatures that possess the distinctive mammalian sweat gland.

Related to sweat glands is hair. Mammals and only mammals possess mammary glands and hair. It’s pretty hard for mamma and hair to fossilize, but bones and teeth more easily do. So, paleontologists and anatomists generally define any creature that possesses a lower jaw bone that articulates with the squamous bone of the skull (the dentary–squamosal jaw articulation), three bones in the middle ear (the malleus, incus, and stapes), and has upper and lower molars that occlude, as a mammal.0

Fascinatingly, there is another way to define a mammal. Mammals and only mammals of all creatures possess a neo-cortex that covers the cerebral hemispheres. (To be continued)/PN