Turtles and Tortoises; the real old-timers

These wonderful animals have been around in pretty much their current form for at least 200 million years. Two hundred and twenty million years ago the first turtles separated off from the other reptiles and have been doing their own thing ever since. It was another hundred million years (would you like a moment to think about that?) before a recognisable crocodile, the other notable vertebrate group which has been around for a very long time in its modern form, made an appearance and doubtless began preying on turtles.

Yellow-spotted River Turtle Podocnemis unifilis, Manu River, Peruvian Amazonia.

For a turtle, the shell's the thing. It comprises a plastron below and a carapace protecting the back. It took them the first twenty million years to get it right, but they had plenty of time and got there in the end. The plastron developed first, to protect swimming proto-turtles from attacks from below. The carapace evolved after turtles came ashore. Segments of ribs and spine were 'borrowed' to form the bony carapace - which is still fused to the ribs and spine - while the plastron developed from the shoulder girdle and breast bones, plus special floating 'dermal bones' found in the belly skin of ancient reptiles including crocodiles. Extensions from the plastron connect the plates at the sides. The bony shield was then eventually covered with tough keratin scales called scutes, based on reptile skin. Modern freshwater and marine turtles have land-dwelling ancestors.

Yellow-spotted River Turtle again - they are seemingly everywhere in the Amazon.
Carapace, plastron plus side extensions, and scutes are all obvious here.
Note too the broadly webbed and strongly clawed feet, plus the nostrils on the very
tip of the nose, for breathing while staying otherwise submerged.

Let's get the 'tortoise or turtle' thing out of the way now, before we get in deeper. It's pretty much a furphy really but there's confusion in Australia in particular because we use the terms differently from elsewhere. Traditionally we've pretty much reserved 'turtle' for the big ocean-going turtles, and 'tortoise' for everything else including the numerous freshwater species. I'm guessing that usage derived in the 19th century from English ancestors, but it may have been home-grown. It is likely that the absence of land-dwelling tortoises here influenced this too. In either case in modern-day Britain as well as elsewhere in the English-speaking world 'tortoise' is reserved for club-footed purely land-dwelling animals and everything else is a 'turtle'. That preference is starting to gain traction here too and it's how I'll be using it today.

 

The turtle world, comprising the Order Testudines for the record, is indeed divided into two, but not like that. As in many other groups of animals and plants, it looks as though this dichotomy had its origins in the times when the world was divided north and south, though it is probably not that simple. For our purposes it is only significant that one Suborder (the Pleuridines, or side-necked turtles) is solely southern today, in Australia, South America and Africa. As the name suggests, they tuck their neck away sideways into the shell where it fits neatly in front of one front leg.

Eastern Long-necked Turtle Chelodina longicollis. Its front is to the left,
and the folded back neck can be seen above my thumb. I've moved very many
turtles (mostly this species) off roads out of harm's way but be wary - they ungratefully
defend themselves with a foul-smelling secretion which you do not want on you or
your clothes!

The other Suborder (the Cryptodines, or hidden-necked turtles) pull their heads straight back in, between their front legs, folding their neck neatly into an S-shape. This, as you may imagine, requires a very different skeletal and muscular design, and the separation between the groups seems to go back to the Jurassic, 150-200 millon years ago.

The Malay, or Southeast Asian, Box Turtle Cuora amboinensis, here at Sepilok,
Sabah, Malaysian Borneo, is an example of the hidden-neckeds. Its short neck
is pulled straight back into the shell when threatened. It is found throughout
southeast Asia. Many species of box turtles are threatened in the wild both
by the demand for wild food in China, and the depredations of the
US pet industry.

However this group also contains all the tortoises and sea turtles, and is found on every non-frozen continent (and ocean) except Australia. Well, not quite. There is one species in Australia, the ancient and unique Pig-nosed Turtle Carettochelys insculpta found in a few Top End rivers, and in New Guinea. It is the only surviving member of its entire family, though fossil relations are widely known. It closest relations seem to be the soft-shelled turtles of Africa, Asia and North America.

Pig-nosed Turtle, Melbourne Aquarium. Unlike any other fresh-water turtle it has
flippers like a sea turtle. Its nostrils are at the end of a long snout, hence the common name.

It is still not well known to science, though of course Indigenous Top Enders have long known it well.

The six species of great sea turtles also belong in this Suborder. They have lost the ability to retract their head, though other factors place them in the group of hidden-neckeds. Their plastron is smaller than that of other turtles, and joined to the carapace only by ligaments. Perhaps their substantial size - carapace length across the species ranges from 70cm to over two metres - enables them to dispense with some protection. And of course they have flippers to enable them to cross vast ocean distances.

Green Turtle Chelonia mydas basking ashore, Galápagos.

By 90 million years ago huge turtles seemingly related to modern leatherbacks were feeding on squid in northern seas. The ancestors of other modern sea turtles arose about ten million years later.

Green Turtle, Galápagos. Adults feed on sea grass beds in shallow water.
However Green Turtles cover huge distances - up to 2500km - to breed,
and can be found across huge areas of the world's oceans where
temperatures are above ten degrees centigrade.

Females determine mating, which takes place in the water.

Green Turtles mating, Galápagos.

After that she comes ashore to dig a hole in the sand and lay up to 300 round white leathery eggs, 40-50mm across, which she buries. The youngsters hatch at night after 8-10 weeks and all head for the water, though few successfully run the gauntlet of predatory birds and crabs. Perhaps only 1% of hatchlings survives to maturity, but if they do they may live for 80 years.

Green Turtle tracks, Lady Elliott Island, Great Barrier Reef.
Within their huge range, Green Turtles are at their most abundant in the
Great Barrier Reef and the Caribbean.

Tortoises, totally land-dwelling animals without any foot adaptations to swimming, all belong to the Family Testudinidae, which is found everywhere except Australia (and of course Antarctica), especially in warmer drier environments.

Angulate Tortoise Chersina angulata, above (Bontebok NP, southwest South Africa)
and below (Augrabies Falls NP, northwest South Africa).
This very attractive tortoise is a South African endemic.

Angulate Tortoises from the drier inland, like this one, tend to be
darker and plainer, but I also wonder if age is a factor in this one.
This, being the only member of its genus, has no close relations.

The lovely Leopard Tortoise Stigmochelys pardalis, despite its apparent similarity
to the Angulate, is not closely related and is much more widely distributed, from
the Horn of Africa to eastern South Africa. Like all tortoises and turtles,
to my knowledge, it buries its eggs in soft soil.

Red-footed Tortoise Chelonoidis carbonarius, crossing the Transpantaneira Highway
in the Pantanal, south-western Brazil. Most tortoises are grazers but forest-dwelling
species like this one also eat fruit, invertebrates and carrion.

Interestingly, the famed Giant Tortoises of the Galápagos belong to the same genus as this, and their ancestors floated - either alone in the open ocean or on floating vegetation rafts - across the Pacific where they came ashore and their descendants gradually grew into giants. This is possible because tortoises can go months without eating or drinking. The same happened to produce the only other living giant tortoises, of the Seychelles in the Indian Ocean (and formerly the Mascarenes), though their ancestors sailed from Madagascar. Indeed until recently there were many giant tortoise species on islands including Malta, Cuba, the Bahamas, the Canaries and Madagascar, but humans destroyed them all. 

Western Santa Cruz Giant Tortoise Chelonoidis porteri, Santa Cruz, Galápagos, munching on grass.
In (the few) moister parts of the archipelago they can rely on this luxury, but elsewhere cactus
flesh and fruit are important food sources.

Until recently it was assumed that there was just one Galápagos tortoise species, but 12 living ones are now recognised, with another two or three recently becoming extinct. The islands rose sequentially from the ocean as they passed over a volcanic 'hot spot'. The oldest islands were colonised first, and later provided new colonists to younger islands as they arose.

One interesting aspect of these giants is the consistent existence within species of 'saddlebacked' and 'dome-backed' shell forms.

Saddlebacked form, Charles Darwin Research Station, Santa Cruz.
At that time different species weren't recognised. The shell rises to the front,
allowing the head to be raised.

Dome-shaped shell on a Sierra Negra Giant Tortoise C. guentheri
crossing the road on Isabela Island.

Saddlebacks tend to be smaller and live on lower drier islands. The dome-backs inhabit higher wetter habitats. It is suggested that the saddlebacks, in a more demanding situation, can reach higher to access tall cactuses, while the dome-backs can easily get enough grass low down. It has also been proposed that the adaptation may allow competing males to stretch higher to intimidate rivals and attract females. In this case it would be an advantage for the smaller animals to be able to reach high. It seems to me that the two explanations are not mutually exclusive.

The tortoises and sea turtles tend to get star billing in this group, but in terms of numbers the more modest freshwater turtles are predominant. Family Geoemydidae, which includes the box turtles we met earlier, has some 70 species across Europe, Asia and North America, with a smaller and presumably relatively recent incursion into South America.

Black River Turtles Rhinoclemmys funerea, here below the bridge at
La Selva Research Station in Costa Rica, are restricted to central America.

Which brings us back to the side-necked turtles, which (apart from the Pig-nosed Turtle) are the only ones we have in Australia; here they all belong to the Family Chelidae. This is an ancient Gondwanan family which first appeared some 100 million years ago and is restricted to Australia, New Guinea and associated islands, and South America. They are carnivores, using long necks and fast reactions to catch fish and small crustaceans. For courtesy I'll start with a couple of South Americans; we've already met the Yellow-spotted River Turtle in the genus Podocnemis but we didn't see its spots before - here they are!

Yellow-spotted River Turtle, Yasuní NP, Ecuador.

South American Snake-necked Turtle Hydromedusa tectifera Iguazu Falls, Argentina.

Our common local turtle is the Eastern Long-necked Turtle Chelodina longicollis, abundant in local waterways and dams. It's often encountered crossing roads, especially on a sunny day after rain, looking for new territory or a somewhere to lay eggs.

Eastern Long-necked Turtle, Tidbinbilla Nature Reserve near Canberra.
The long neck is evident here. They hunt by ambush, combined with a potent
'gape and suck' strategy. As the mouth gets close to the prey, it suddenly
and powerfully opens, creating a powerful suction action.

Elseya is a genus of six Australian species, plus three in New Guinea, known as the Australian snapping turtles (though they are mostly vegetarian, including floating fruits).

Northern Snapping Turtle Elseya dentata Howard Springs, south of Darwin.
It is found across northern Western Australia and the Northern Territory.

Saw-shelled Turtle Elseya (now usually described as Myuchelys) latisternum
Cumberland Dam, inland North Queensland.

Finally, Emydura, the short-necked turtles, of which six are currently recognised in Australia.

Krefft's Turtle Emydura kreftii, Centenary Lakes, Cairns.
It lives in the waterways of east coastal Queensland. The short neck
which characterises the genus is evident in this photo and the next.

Murray River Turtle Emydura macquarii, Melbourne Aquarium.
This is a widespread species in the Murray River basin, including most of NSW,
where it extends to the north coast.

I hope you has as much affection and admiration for these most ancient of vertebrates; but if you don't you've probably not read this far anyway. I find them truly wonderful.

NEXT POSTING THURSDAY 26 AUGUST

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