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Turtles Move a Notch Up the Ladderby Donald L. Blanchard |
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This article was originally published in The Cold Blooded News, the newsletter of the Colorado Herpetological Society, Vol 26, #3, March, 1999. For centuries, turtles and tortoises have occupied the lowest rung of the reptilian ladder. Traditionally, reptiles and their descendants have been classified by the holes in their head. Snakes, lizards, the tuatara, and crocodilians, as well as dinosaurs and birds, have two holes on each side of the skull. These holes, called apses, allow the jaw muscles located on the inside of the skull to expand when the jaw is operated. Squamates, tuataras, crocodilians, dinosaurs, and birds have therefore been grouped as diapsids (di = two, + apses). Mammals and the early mammal-like reptiles have one hole in the head, and are called synapsids. Turtles, along with all primitive Paleozoic and Early Mesozoic stem reptiles have none, and have been called anapsids. Diapsid reptiles are further divided, with lizards, snakes, and tuataras comprising the 'lower' group, the Lepidosauria, while crocodilians, dinosaurs, and birds represent the 'higher' group, the Archosauria. Archosaurs differ from lepidosaurs in having a four chambered heart and a more erect posture, with the legs beneath (or more nearly beneath, in the case of crocodilians) the body. In lepidosaurs, the heart has only three chambers, two auricles, and only one ventricle which is only partially divided, allowing oxygen-rich blood from the lungs to partially mix with oxygen depleted blood from the rest of the body. Turtles also have only a three (or 3 1/2) chambered heart. While some recent cladistic analyses of various reptilian traits has suggested that turtles may in fact be more closely allied with lepidosaurs (see "Where Do Turtles Go?" in the September, 1998, Cold Blooded News), this work has not yet been widely accepted. Now, as reported in the February 12, 1999 issue of Science, a new study suggests that turtles be elevated to the very top of the reptilian family tree, along with birds and crocodilians. This study compared the DNA sequences of 24 nuclear genes, 1 ribosomal RNA gene, and 9 mitochondrial genes. In all cases but one, the closest match to the turtle genes was either crocodilian or avian genes, with crocodilian predominating. In only one case was a squamate gene found to be a closer match with the corresponding turtle gene.
This study clearly contradicts a sizeable body of morphological evidence, the only contradictory morphological trait shared uniquely by turtles and archosaurs being the presence of dorsal and ventral bony plates. There is no denying that turtles are highly evolved animals; they are the only vertebrates to have their pectoral and pelvic girdles (shoulders and hips) inside their rib cages, a feat of anatomical prestidigitation that is difficult to fathom. They are also the only living reptile to forego teeth in favor of a horny beak, a trait shared by many extinct dinosaurs and other archosaurs. The study also cast in doubt the relationship between the tuatara and squamates. While fewer gene sequences were available for the tuatara, six of eight comparisons showed closer affinities with archosaurs or turtles, while only two showed squamates as the closest relative. While the results of this study are not conclusive, it clearly demonstrates that we don't know all that we thought we knew about the phylogenetic relationships of living or fossil reptiles. Reference:
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Time estimates, based on 23 nuclear genes and 9 mitochondrial genes, suggests that squamates diverged from the other reptiles at 245 ± 12.2 million years ago (Ma), birds diverged from turtles and crocodilians at 228 ± 10.3 Ma, and that turtles diverged from crocodilians at 207 ± 20.5 Ma, thus supporting a Triassic (248 to 213 Ma) origin for all current reptilian lineages. According to the article, turtles first appeared in the fossil record at 223 to 208 Ma, crocodilians at 210 to 208 Ma, and the first known squamates at 157 to 155 Ma. However, the relationship of the first fossil identified as a (possible) turtle remains somewhat problematical, showing relatively few turtle-like traits. Birds are generally believed to be derived from dinosaurs, which first appeared in the Middle Triassic, but the first fossil identified as a bird is from the Late Jurassic, around 150 Ma. Estimated divergence times for the tuatara are not given, probably due to the paucity of DNA sequences.
