Two successful designs for tetrapods: the chisel and the shell

(writing in progress)

How can we assess basic designs among vertebrates?

Here I compare rodents with other mammals, and turtles with other reptiles. And then I imagine pitting the former against the latter.

One aspect of this is the global ecological spread of these two extremely successful Orders, namely rodents and testudines. Another aspect is something strangely unheard of in Nature: bringing the gnawing mammals face to face with the shelled reptiles.

Thinking about Orders is a bit like doing zoological yoga. It can stretch the mind into uncomfortable but ultimately rewarding positions.

We normally work with species, genera and families because these are small enough to grasp. The next level, the Order, is usually a taxonomic filing cabinet just beyond our normal thinking.

For example, it boggles the mind to consider all 2 277 species of the world’s rodents in one thought. Yet it is worth attempting. Admittedly, there are only about 300 living species of testudines, a mere 13% of the number of living species of rodents. However, my aim here is to compare rodents with other mammals, and testudines with other reptiles. So, the difference in numbers between the two groups is no obstacle.

A concept that may make the bewildering diversity within zoological Orders more tangible is the basic design.

For example, what unites all rodents is a dentition specialised for gnawing.

A central facet of this dentition is one pair of self-sharpening incisors, deeply rooted within the skull, that grow throughout their lives. These incisors have enamel only on the front, typically impregnated with iron, and are technically milk teeth-gone-permanent.

Then, there is a cheek-toothrow containing molars1, but no premolars. This dentition is extremely modified by mammalian standards, and has proved so versatile that myriad niches have evolved around it, enabling rodents to spread far and wide with relatively little variation in body shape. They are globally successful as seed- and insect-eaters and herbivores, quite capable of killing trees, but not – for some strange reason – turtles.

Like the Rodentia, the Order Testudines is extremely successful in its own way, based on a different design.

The testudines (turtles and tortoises) of the world have also made one set of radical changes to their anatomy, and used it to inhabit the world. All testudines have modified the spine and ribcage into a skeletal box in a way not attempted by any mammals. For example, armadillos have dermal, not rib-based, armour. The odd thing about testudines is that they have relocated the pectoral and pelvic girdles within the box of armour and in partial fusion to this box on the inside.

However, testudines have done more than just armouring themselves with bone and cartilage: they have turned themselves partially outside-in. They have compromised their ability to flee for cover in exchange for a static defence of their vulnerable innards and head. This fortification is aided by the biting power of the toothless mouth, and the hard-scaled forelegs that block the main access to the innards.

Both rodents and testudines have done something strange skeletally. Rodents have converted the skull to a chisel, and testudines hide the limb girdles within the rib cage.

While the fundamental design concepts of rodents and turtles are quite different, each Order maintains a common denominator among a wide range of families, genera and species. Both basic designs have pros and cons, and could have proved useful in only a few special environments. Instead, they have proved useful in diverse environments. Clearly, both ideas were just crazy enough to work in a versatile way. But how does each of these Orders compare with its Class – the Class Mammalia and the Class Reptilia – in terms of the range of environments it has colonised?

Well, rodents are so widespread on land that the only terrae firmae lacking a native fauna of rodents were ice deserts, New Zealand, and various remote islands, such as Hawaii.

They have even accompanied humans to those lands formerly beyond them, such as the most remote islands. If there is just one native mammal on a land surface, it will be a rodent. The exception is New Zealand, where that place was taken by a scurrying bat. Rodents also live across the full vertical range possible for non-flying mammals on land, from treetops down to below ground. For example, they are the only animals known to use their teeth to dig deep tunnels through hard earth. There are even gliding forms, namely the flying squirrels.

Rodents are widespread, in part, because they can use their wear-defying dentition to open hard food items (e.g. nuts), dig, demolish plants, hew construction material, and defend themselves against predators. However, one surprising aspect of the Order is that few of them are carnivorous, one set of exceptions being the amphibious species of Australia and New Guinea. Although they should be suited to breaking open the shells of molluscs, their puzzling failure to exploit shellfish has meant that rodents have been less successful in water than at least five other mammalian orders2. There are rodent species with a wide range of body sizes that are amphibious, from voles to the capybara, but this Order has no member as profoundly aquatic as, for example, the seals in the Order Carnivora.

Now let us turn to the testudines.

Several oceanic archipelagos have tortoises as their largest native animals on land. In addition, all non-polar continents and all larger islands, other than New Zealand, have at least one species of freshwater testudine at latitudes up to about 36 degrees (South America), 40 degrees (Australia), 44 degrees (east Asia) and 50 degrees (Europe and North America).

Among reptiles, testudines are more widespread on Earth than even the snakes. This is mainly because of the unrivalled success of a few species in water, and to a lesser extent in crossing vast expanses of water with only a rudimentary ability to swim3. Marine turtles are widespread in the oceans, including the Atlantic where there are no sea snakes. For example, the leatherback turtle penetrates waters too cold and deep for any other living reptile, even in the oceans where sea snakes do occur. So, turtles exceed sea snakes in distribution.

In comparison with turtles, the penetration of the sea by lizards is negligible except in one place: the Galapagos. In contrast, the leatherback turtle reaches the seas of British Columbia, beyond the north of Scotland, and to the South Island of New Zealand.

However, on dry land, the testudines fall short of the Order Squamata in coverage, being absent today from large areas where snakes and lizards do occur. These areas include the high-lying central Asian plateau and the low-lying Taklamakan Desert, most of Arabia and much of Syria, the Sahara, Cyprus, the whole of Australia, New Guinea and New Zealand, the whole of Chile, and California except for the Mojave Desert.

4 aestivating or hibernating

It would be interesting to make a careful comparison of the native incidences of rodents and testudines on the ocean islands of the world. Although it seems safe to say that rodents were more widespread than testudines before human interference, there are noteworthy exceptions. For example, the Seychelles and Mascarene archipelagos were originally home to testudines but not rodents.

So, rodents have penetrated beyond other terrestrial mammals on land, and testudines have penetrated beyond other reptiles in water. What could it be about their basic design that makes them so successful in their respective media?

One reason why the testudine basic design works better in water than on land is that the partial withdrawal of the limbs into the body is less of a handicap for swimming than it is for running. Just picture the typical fin compared to the typical limb.

I realise that certain testudines do achieve more versatile mobility on land than the typical tortoise would suggest. For example, gopher tortoises dig surprisingly deep and long burrows, a few amphibious turtles can climb trees and cliffs, and the pancake tortoise can scurry as rapidly as a slow lizard to jamb itself among rocks. However, my point is that lizards and snakes do all these things much more proficiently.

Meanwhile, in water the tables are turned: marine turtles use flipper-like forelimbs to qualify as the supreme reptilian travellers in terms of both speed and distance. The important observation here is that the propulsive flipper, whether in seals or in turtles, has most of its limb skeleton within the body line to aid streamlining. So the internal girdles of the testudines’ basic design are no handicap in this medium.

Which brings us to a novel exercise of intellectual yoga for readers.

If rodents are the gnawers par excellence, and tortoises are the shelled vertebrates par excellence, then why do rodents not eat testudines, at least as occasional prey on an opportunistic basis?

Porcupines eat bones; why don’t they kill tortoises? Particularly when dormant4, tortoises and freshwater turtles would seem to be sitting ducks for a hypothetical gnawing carnivore.

What is it about the two Orders – other than the obvious fact that bone is harder to gnaw than wood – that has led to an apparent stalemate between them?

Please post possible solutions to this riddle in the comment stream below. The answer is somewhere out there among iNaturalists.

1 The molars have no milk teeth at all.

2 Soricomorpha e.g. desman, Carnivora e.g. seals, Cetartiodactyla e.g. hippos and whales, Sirenia e.g. dugong, Proboscidea e.g. elephants

(writing in progress)

Publicado el 21 de junio de 2022 por milewski