[Don_1 (OP)]

There are estimated to be around 260 species of Testudines in existence. I won't (and can't) name them all, but they range from the small Egyptian Tortoise (Testudo Kleinmanni) to the Isabella Island Galapagos Giant Tortoise (Geochelone Elephantopus) in the terrestrial species and from the tiny Bog Turtle (Glyptemys Muhlenbergii) to the purported 70 inch Narrow-headed Asian Softshell Turtle (Chitra Indica) in the freshwater aquatic species. There are terrestrial tortoises and terrestrial and aquatic turtles found in just about every tropical and subtropical region.

But there are only 7 species of marine turtle, which I will name (because I can):
Kemp's Ridley (Lepidochelys kempii) (smallest)
Olive Ridley (Lepidochelys olivacea)
Flatback (Natator depressus)
Green (Chelonia mydas)
Hawksbill (Eretmochelys imbricata)
Loggerhead (Caretta caretta)
Leatherback (Dermochelys coriacea) (largest)

So my question is, why the lack of marine species when there is such great diversification of land and freshwater testudines?

[imatfaal]

Could it be that with the shell and body covering ie the unusual barrier between turtle and outside world it copes with osmotic pressure well in one direction but badly in the opposite.  in fresh water the gradient would tend to have water crossing into the body and in briny the opposite - and that the turtle (for some reason, possibly concerned with shelly nature) has been unable to adapt to the reversal in osmotic gradient. 

Alternatively could it be a buoyancy (too much) issue?

[LeeE]

A pure guess: It may be because, being marine animals, you don't get isolated sub-groups where evolution can diverge.

[Geezer]

Perhaps they were wiped out by a bunch of yobo sharks with frikin lasers attached to their heads, although I think Lee's explanation is probably more plausible.

[LeeE]

Just an added qualification: fishes that tend to stay close to shore e.g. reef fish could be regarded as isolated, so you could get a lot of divergence between such isolated groups but fish that tend to stay in the open ocean would be like the turtles and wouldn't be isolated.

I guess the proof of this would be comparing the numbers of different variations between these two types of fish i.e. relatively few different types of ocean going fish compared with relatively lots of different types for inshore/reef fish.

However, Wikipedia says there are 440 different types of shark and more than 48 different types of tuna, both of which are ocean going fish, so that argument doesn't seem to hold up very well after all.

[Don_1 (OP)]

The fact that marine turtles are ocean wanderers did cross my mind as a reason for their lack of diversity. But like you LeeE, I came to the conclusion that this might be a flawed answer due to the diversity of other ocean wandering species.

On the matter of being too buoyant, I don't think so. Being air breathers, buoyancy is critical to turtles, so I don't think, under such circumstances, you could be 'too buoyant'. Apart from that, turtles are known to reach depths of up to 1000m.

As for osmotic pressure, surely if this were a problem, the turtles would have abandoned the marine environment 250 million years ago. The same would apply to the mammals and many fish. Turtles deal with their high intake of salt, through their diet, by way of an excretory gland which excretes all unwanted sodium chloride.

Just to throw a joker in the pack, there are some who think the Green Turtles of the Eastern Pacific, colloquially known as the Black Turtle (Chelonia agassizii) is a separate species and not a sub-species, though nDNA does not appear to back up this claim. But given time, perhaps the two will become so far apart (biologically) that they will be.

[LeeE]

How old are turtles?  If they're relatively young then that might be a factor in their lack of diversity (I'm assuming that turtles split from tortoises and are therefore younger, so they may have simply had less time to diversify).

I wonder too, whether the fact that they have to come ashore to lay their eggs could be a factor? (not that I can see any clear causality due to this).

This question is quite a good puzzle  []

[Don_1 (OP)]

The modern (if you can call them that) testudines evolved from Clade Anapsids, survivors of the Permian Extinction. Scutosaurus, of the late Permian, would likely have been the testudines' earlier ancestor. It is generally accepted that the tortoise pre-dates the turtle.

A couple of years ago two sets of the fossilised remains of a turtle were found in China. This turtle, Odontochelys semitestacea, had teeth rather than the beak of modern testudines and a plastron (underside of shell), but no carapace. At 220 million years old, it pre-dated the previously oldest turtle fossils by 40 million years.

The necessity to nest on dry land could well have been the downfall of Archelon ischyros, a giant turtle from the Late Cretaceous period. The energy expended by the Leatherback to nest a safe distance from the tide, leaves the turtle exhausted and may take all night. At around 1.8 - 2.0m and up to 550 kgs, the Leatherback, today's largest turtle, is only half the size of its giant prehistoric cousin. At up to 4m long and (assuming a weight to size correlation) over one ton, this species may well have become too big and heavy to successfully nest.

But only the Leatherback and, occasionally, Loggerhead are in the giant class of turtle. The others are far smaller and lighter. The Green turtle reaches a maximum of 1.5m @ 200kgs and the Hawksbill around half a metre and just 50kgs.

There is no doubt that turtle nests are heavily predated. Raccoons and Monitors are amoung the many which will plunder the nest, and the hatchlings run the gauntlet as soon as they emerge, again from these predators and from gulls, foxes etc. Once in the sea they still face predation from sharks and other fish. But as with other animals, sufficient survive to maintain a healthy population. Or at least, this was the case until Man began to take them.

[LeeE]

So they've been around long enough to diversify into many different types, but just don't seem to have done so.  Hmm...

Is there anything in the fossil record to suggest that there were once many more types than there are now?

[Geezer]

So they've been around long enough to diversify into many different types, but just don't seem to have done so.  Hmm...

Maybe they didn't see the need  []

I suppose it's possible they were so well suited to their environment that there was no pressure to evolve, although I must admit that does sound a bit dodgy.

[LeeE]

Yeah - but if they've been around that long there should have been some sign of diversification.

Another possibility is a highly specialised disease, but that's almost impossible to prove...

However, such diseases must have occurred in the past, for there have been a few well-documented near-misses since then i.e. from the Black Death and Spanish Flu to BSE and HIV/AIDS, just to mention a few examples that have come pretty close to decimating, or have had the potential to decimate, the Earth's human population.