Friday, November 11, 2016

A re-evaluation of the four-legged snake Tetrapodophis amplectus

Reconstruction of Tetrapodophis.
In 2015, Martill et al. described Tetrapodophis amplectus, a fossil snake with four legs. Tetrapodophis was found in the B├╝rgermeister-M├╝ller-Museum, a natural history museum in Solnhofen, Germany, while students were on a field trip to the museum.  The Brazilian fossil was part of an exhibit on the Cretaceous and estimated to be 110 million years old. The fossil was part of a larger exhibition on Cretaceous fossils.
The snake, was 20 cm from head to toe, although it may have grown much larger. The head is the size of an adult fingernail, and the smallest tail bone is only a quarter of a millimeter long. But the most remarkable thing about it is the presence of four limbs each ending in digits. The front legs are about 1cm long. The back legs are slightly longer and the feet are larger than the hands. The authors hypothesized that they may have been used to grasp prey or mates. The fossil Tetrapodophis apparently had food in its guts when it was preserved, the remains appeared to be from a salamander.
The authors considered Tetraphodophis a snake, not a lizard because of the elongated body; the tooth implantation, the direction of the teeth, and the pattern of the teeth and the bones of the lower jaw are all snake-like. The fossil also suggests a single row of ventral scales.
In the same issue of Science, Evans (2015) notes that snakelike bodies evolved at least 26 times in squamates and that body elongation is always correlated with limb reduction and that the forelimbs are usually lost first. She also observed that the threshold body length at which limb reduction begins is about 70 body vertebrae (or precaudal vertebrae). Tetrapodophis is remarkable in having about 160 precaudal vertebrae and retaining its anterior limbs. Evans also notes Tetrapodophis is like lizards in having distinct vertebral regions of the vertebral column.  It has 10 or 11 short-ribbed neck vertebrae adjacent to the tiny forelimbs. Some generalized terrestrial lizards and a neck of about this same length. Thus, as in long-bodied lizards, elongation of the snake skeleton occurred in the trunk region and not the neck. If Tetrapodophis is indeed a stem-snake, then body elongation preceded loss of the forelimbs.
In second look at the fossil by Lee et al. (2016) suggests Tetrapodophis may not, in fact be a snake at all. Instead they suggest it may be a dolichosaurid, a Cretaceous four-legged marine lizard with an elongated, snake-like body.
Tetrapodophis lacks characters that would be expected in a snake, including re-curved teeth. Lee and colleagues reevaluated the ecomorphology of this fossil using a multivariate morphometric analysis and reexamination of the limb anatomy. Their analysis suggests that the body proportions are unusual and similar to both burrowing and surface-active squamates. They also show it exhibits enlarged first metapodials and reduced tarsal-carpal ossification. These traits imply Tetrapodophis was aquatic.
Unfortunately, the fossil is privately owned and after Lee’s team took photos and measurements, the specimen was removed from the museum so that it can no longer be studied.  

Citations
Evans S. 2016. Four legs too many? Science. 349(6246):374-5.

Lee MS, Palci A, Jones ME, Caldwell MW, Holmes JD, Reisz RR. 2016. Aquatic adaptations in the four limbs of the snake-like reptile Tetrapodophis from the Lower Cretaceous of Brazil. Cretaceous Research. 30, 66:194-199.

Martill DM, Tischlinger H. Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science, 349(6246): 416-419.


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