Looking at skulls and teeth

This is an excerpt from Australian Backyard Naturalist, published by the National Library of Australia, now only available in second-hand shops, or in my revised edition, available through Polymoth Books, or from Amazon, as an e-book, or as print. The book shared the 2012 W. A. Premier's Prize for Children's Literature. It is a very different look at Australian life forms.

2.02 Mammals at a glance

Mammals are warm-blooded animals like us. They have fur or hair (sometimes not very much), they give birth to live young (except for platypuses and echidnas, which lay eggs), and they all feed their young with milk. Some mammals like whales and dolphins live in the sea, and polar bears live in extremely cold places. Camels can live in very dry deserts, and humans are able to live just about anywhere.

Most people think Australia’s mammals are all marsupials—mammals that have pouches, but even before European settlement, Australia had more placental mammals (that means mammals like us, whose babies are nourished before birth by an organ called the placenta, attached to the wall of the mother’s uterus).

Australia’s placental mammals include the bats and native rats and mice on the land, and the whales, dolphins and seals in our oceans. Evolution sometimes deals animals a nasty hand, and that is certainly the case with the marsupials. Putting it simply, the ‘plumbing’ of a female marsupial is such that they cannot give birth to large babies.

The solution is sensible: the joey is born as a tiny speck, little more than an appetite with two legs that it uses to haul itself to a pouch, where it attaches to a teat, and proceeds to develop a full body, just as a lamb, a kitten or a human child does, inside the mother, before it is born.

The odd thing about evolution is that any system that works and lets a new generation survive is allowed to repeat itself. Sometimes, there are even advantages, like the mother being able to store a fertilised embryo when conditions were bad, but that, as they say, is another story.

A possum skull, a rabbit skull and a piranha jaw: which is which? (The 32 mm coin is there for a scale: you will see quite a lot of it.)

2.03 The teeth of mammals

Then there are the mammals that lay eggs, but I will come to them later. First, we need to talk about teeth (which platypuses and echidnas don’t have). Teeth are amazingly tough and they last much better than bones. The teeth of mammals are worth studying because they are all different, and can often tell you what animal they belonged to, and what it ate. The teeth of dead animals are safer to look at than those in live animals, so this is mainly about the teeth of dead mammals.

Dentition tells us that this mandible or jaw bone, found on a deserted beach on Thursday Island, came from a dugong. Local hunters butcher their catch there, far from their homes because they don’t want to attract crocodiles and sharks to where they live and swim.

Teeth are classed as incisors, canines, premolars and molars, and the patterns of these teeth, in this order, are called ‘dentition’. To get a proper identification, you often need to see and count the teeth (or the sockets, in which the teeth once sat) in one half of each jaw.

The incisors (I) are the front teeth, the cutting teeth, and in humans we write this as 2/2, meaning you have two incisors on each side of the upper and lower jaw (we only count the teeth on one side). Our canines (C) are coded 1/1, premolars (Pm) are 2/2 and molars (M) are 3/3 (children have molars as 2/2 until their ‘wisdom teeth’ emerge in their later teen years). Zoologists write the dentition in a formula like this for human beings: I2/2 C1/1 Pm2/2 M3/3. Now take a look at the five Australian skulls below.

Because they are close relatives, it’s not surprising that chimpanzees, gorillas and orangutans have the same dental formula as us. A wombat, on the other hand, is I1/1 C0/0 Pm1/1 M4/4.

An eastern grey kangaroo is I3/1 C0/0 Pm2/2 M4/4. A koala is I3/1 C1/0 Pm1/1 M4/4. A ringtail possum is I3/2 C1/0 Pm3/3 M4/4. A Tasmanian devil is I4/3 C1/1 Pm2/2 M4/4. The dentition of Australian marsupials, such as kangaroos and Tasmanian devils is clearly quite unlike that of humans.

You only need two terms to work with skulls, along with a bit of careful hygiene (dead meat can be germy, so use latex gloves and wash up afterwards). Those terms are dentition and dental formula. Just fire up your favourite search engine, and off you go.

This skull on the right, found near Sydney Harbour was easy to identify by its dentition as having once belonged to a possum.