It seems like I have a soft spot in my heart for weirdo relicts. As I recall, one of the highlights of my previous semester was finding the book Primitive Fishes which discussed why fish like bowfin, gar, sturgeon and "fish" like lamprey and hagfish still linger on. It never ceases to fascinate me how an entire radiation can be reduced to a few species. I'll have to wait for those guys when I feel like talking about fish, but today I will discuss a group related to the softshell turtles, the Carettochelyidae.
This family of turtle has a single living species Carettochelys insculpta, the pig-nosed turtle of New Guinea and Northern Australia. It was described from the late 19th century as a New Guinea species (hence the alternate name Fly River turtle) and it wasn't known to have a breeding population in Australia until the 1970's (Georges & Kennett 1989). The species does not have epidermal scutes like most turtles; coupled with the fleshy snout and leather carapace and plastron it resembles members of Trionychidae, the softshell turtles. The limbs are paddle-like with two claws, and the species has been called the species of freshwater turtle most suited to an aquatic environment. I have not seen studies on the shell/plastron of this species, the reason why it evolved, and what advantages having such a shell would be; Trionychid shells offer many apparent advantages (lightweight, bendable, strong, able to absorb oxygen) although the pig-nosed turtle and extinct relatives do not have the same integumentary adaptations (Scheyer et al., 2007). It takes water in through the mouth and up through the nostrils, which may serve a respiratory function (Georges & Kennett 1989); I haven't seen any figures for how frequently it needs to surface, so the efficiency of this isn't clear. Carettochelys is apparently capable of living in salt water as well as rivers (Friar, 1985), although I have not seen any information if specimens do indeed move from New Guinea to Australia or if it posses adaptations to live long term in such an environment.
The distribution itself is unusual, since Carettochelys is the only freshwater non-pleurodire (side-necked turtle) in Australia (G. & K.), although one early authority considered it to be a pleurodire that convergently evolved upon Trionychidae (Friar, 1985). This apparently wasn't based upon biogeography and I'm baffled why the neck anatomy wasn't a dead giveaway. Due to the presence of flippers and a form of locomotion in between a freshwater and marine turtle (Friar, 1985) as well as the superficial similarity to Trionychidae, pig-nosed turtles were once considered to be transitional between marine and softshell turtles (Friar, 1983). Superficiality aside, Carettochelys is now strongly linked to the family Trionychidae forming the clade Trionychia, grouping with extinct forms in Trionychoidae, which in turn groups with the families Dermatemydidae and Kinosternidae to form Trionychoidea (Meylan et al., 2000). In a predictably unpredictable sort of way recent genetic evidence suggests that Trionychoidea is an invalid group, and that Trionychoidae is a distinctive lineage that split off very early from other turtle (Krenz et al 2005). Previous molecular clocks calculated pleurodires and cryptodires to have diverged 176 and 175 million years ago respectively with Trionychoidae branching off 155 million years ago, supporting one model of them being basal cryptodires. However, another possible model in Krenz et al 2005 suggests that Trionychoidae is a paraphyletic group encompassing the first two major divisions in the turtle lineage! If Carettochelys is the most basal turtle known that would be astounding (and the early worker was right about a pleurodire connection), though I think the previous model was much more probable. Genetic evidence seems to be undergoing a lot of flux, so I think it will be safe to wait before making any bold conclusions (which the authors didn't).
A photo marked "public" from Flickr. Note that the flippers are wider than those of sea turtles and have two prominent claws.
Regardless of whether or not this species occupies a very interesting position in the turtle family tree, its fossil relatives should be of species interest, well, to me at least. How does a group of animals wind up having only one species left and what caused it to survive? The group has been known since the Cretaceous in Asia, where it spread to Africa, Europe and North America post-KT only to wind up going extinct (Joyce et al., 2004). As of 1988 there appear to have been 6 genera of the family (18 were reported, many were unrelated or synonymous) (Meylan, 1988) in two subfamilies: Anosteirinae and Carettochelyinae. Anosteirine Carettochelyids are distinguished by the presence of marginal scutes which Carettochelyine Carettochelyids lost, apparently independently from Trionychidae (Joyce et al., 2004); other distinguishing characteristics include a small size (25-38 cm carapace length), one or more spines on the dorsal keel, and modified vertebral scutes and xiphiplastra (fourth lateral plate on the plastron) (Meylan, 1988). Large size is apparently a plesiomorphy in trionychoids which Carettochelyines retained - Carettochelys has a 55 cm+ carapace (Georges & Kennett, 1989), and Burmemys had one over 1 m (Hutchinson et al, 2004). Relict fish also tend to be rather large, although I can't be sure if size offers the same advantages to Carettochelys, if any. It is noted that this group is highly adapted to riverine environments (Joyce et al., 2004), and it is possible this is why (perhaps coupled with omnivory and large size) they survived the KT extinction and continued to straggle on like choristoderes.
Actually, it is somewhat puzzling now why this species has such a limited range. Georges & Kennett 1989 noted that it is a very broad omnivore, leaning towards herbivory, and it has been known to consume fruits, leaves, aquatic insects, fungi, and mammals such as flying foxes (presumably scavenged). It can of course survive a wide range of habitats include saline ones and its relatives have been known to inhabit temperate zones, so the authors suspecting it had to do with seasonal movements or the requirements of young individuals. Regardless of some areas with high densities it is considered a rare species (that term being hard to define), and it occupies home ranges much larger than those of other aquatic turtles: 36 hectares or 7.6 km linearly, much more than turtles of comparable sizes (Doody et al., 2002).
So here we have it, a remarkable relict species which may have survived due to its size and freshwater habitat and now appears to be somewhat constrained by it. Despite the late discovery and apparent rarity, this species does seem to be very well-studied; I've covered about half of the articles I've come across, many of which were from this decade. There certainly is a lot more to learn about this vulnerable species for those interested.
Is this what most American kids do on their 21st birthday anyways? Sigh...
Doody, J. S. et al. 2002. Sex Differences in Activity and Movements in the Pig-Nosed Turtle, Carettochelys insculpta, in the Wet-Dry Tropics of Australia. Copeia, pp. 93–103
Friar, Wayne. 1983. Serological Survey of Softshells with Other Turtles. Journal of Herpetology, Vol. 17, No. 1., pp. 75-79.
Friar, Wayne. 1985. The Enigmatic Plateless River Turtle, Carettochelys, in Serological Survey. Journal of Herpetology, Vol. 19, No. 4., pp. 515-523.
Georges, Arthur and Kennett, Rodney. 1989. Dry-season Distribution and Ecology of Carettochelys insculpta (Chelonia : Carettochelydidae) in Kakadu National Park, Northern Australia. Aust. Wildl. Res., 16, 323-35.
Joyce, Walter G. et al. 2004. Carettochelyine Turtle from the Neogene of Europe. Copeia, Vol. 2004, No. 2., pp. 406-411.
Krenz, James G. et al. 2005. Molecular phylogenetics and evolution of turtles. Molecular Phylogenetics and Evolution 37, 178–191.
Meylan, Peter A. 1988. Peltochelys dollo and the relationships among the genera of the Carettochelyidae (Testudines: Reptilia). Herpetologica, 44(4), 440-450
Meylan, Peter A. et al. 2000. Sandownia harrisi, a highly derived Trionychoid turtle (Testudines: Cryptodira) from the early Cretaceous of the Isle of Wight, England. Journal of Vertebrate Paleontology 20(3):522–532.
Scheyer, Torsten M. et al. A plywood structure in the shell of fossil and living soft-shelled turtles (Trionychidae) and its evolutionary implications. Organisms, Diversity & Evolution 7, 36–144