The Age of Reptiles (Mesozoic Era) kicked off in grand fashion with the Triassic Period, which began about 250,000,000 years ago and ended about 45,000,000 years later. During the Triassic, reptiles evolved to fill almost every corner of the globe. Plant-eating reptiles munched fronds on vast fern praries, stalked by reptiles who very much wanted to much on them. Reptiles could be found ambushing prey in lakes and rivers, chasing fish in the open ocean, and crushing clams on the sea floor. Reptiles spread membranous wings and flew through the skies. And for a few million years near the end of the Triassic, one strange group of reptiles could be found climbing through the treetops. This group was so well-adapted for an arboreal lifestyle that they have been named the Simiosauria, or Monkey-Lizards.
Megalancosaurus. Click to enlarge.
Megalancosaurus preonensis (Long-armed Lizard from the Preone Valley), pictured above, is one of the best known simiosaurs, and clearly shows some amazing adaptations. It had a bird-like head perched at the end of a flexible neck. A small hump over its shoulders may have served as an attachment point for muscles that could have allowed it to strike at insects and other small prey with lightning speed. Two of the fingers on its hands were in opposition to the other three, allowing it to easily grip branches. Some specimens of Megalancosaurus show a primate-like opposable toe on each rear foot. Other specimens lack this feature, which suggests that it was specific to either males or females (presumably whichever sex needed a stronger grip during mating). Its tail was long and prehensile, and tipped with a sharp claw that might have been useful in securing itself among the branches.
Skeleton of Megalancosaurus preonensis, modified from Renesto 1994b. Click to view/download a 1.3Mb PDF.
Living alongside Megalancosaurus, was its larger relative, Drepanosaurus unguicaudatus (Claw-tailed Sickle Lizard). Whereas Megalancosaurus was a svelte 25 centimeters long, Drepanosaurus was a stocky half-meter long bruiser. Drepanosaurus is known from only one headless skeleton, with a shoulder hump and clawed, prehensile tail like its smaller cousin. Presumably it would have also had a birdlike beak filled with needle-sharp teeth.
Skeleton of Drepanosaurus unguicaudatus, modified from Renesto 1994a. Neck and head are based on Megalancosaurus. Click to view/download a 1.5Mb PDF.
Sketch of Drepanosaurus marvelling at its unusual hand.
The head is completely speculative.
The most striking and unique feature of Drepanosaurus, however, was its forearm. Unlike the slender, grasping limb of Megalancosaurus, Drepanosaurus bore heavily muscled arms and a giant, blade-like claw on its index finger that was almost as large as the rest of its hand. If you look closely at its skeleton, it appears to have an extra bone in its elbow. According to work done by Dr. Silvio Renesto, this bone is the ulna, which in most vertebrates (including us) forms a sort of curved rod that lies alongside the radius down the length of the forearm. But in Drepanosaurus, it is crescent-shaped and curves away from the radius. One of the little bones of the wrist was co-opted into filling the typical role of the ulna, extending from the elbow to the hand. Presumably, this drastic modification of the forearm is related to its massive claw, likely serving as an attachment point for the muscles needed to control such a tool.
At one point, Drepanosaurus was envisioned as a sort of reptilian mole, using its giant claw to dig burrows beneath the ground. But moles and other burrowing animals have very differently shaped claws than Drepanosaurus. The only living animal with a remotely similar hand is the pygmy anteater, which uses one well-developed claw to tear through bark in search of insects. Drepanosaurus likely did the same.
Drepanosaurus was the first Monkey-Lizard to be described, and for this reason the group is frequently referred to as drepanosaurs or drepanosaurids. The term "Simiosauria" was coined by Phil Sentor in 2004, who reserved the term drepanosaur for the three most closely related members of the group (Drepanosaurus, Megalancosaurus, and Dolabrosaurus (below)).
Little "Vallesaurus." Click to open a larger version in a new window.
Drepanosaurus and Megalancosaurus are known from northern Italy, which during the Triassic was a tropical coast. The fine-grained sediments that collected there preserved many unusual reptiles including three different types of Monkey-Lizard: the two mentioned above and a much smaller relative known from a single, 15 cm long skeleton. This skeleton has been referred to as "Vallesaurus" but is not yet officially described.
Moving across the Atlantic Ocean, which was just a wide valley in the Late Triassic, we find a couple of different Monkey-Lizards on the modern continent of North America.
In sediments from the bottom of ancient New Jersey lakes, dozens of specimens of Hypuronector limnaios (Deep-tailed swimmer from the lake) have been found. Hypuronector was similar to the Italian simiosaurs in some details of its skeleton, but at first glance it would have looked quite different.
Skeleton of Hypuronector limnaios, modified from Colbert and Olson, 2001. Click to view/download a 780Kb PDF.
Leaf-tailed Hypuronector limnaios taking an early morning climb up a cycad trunk, well camoflaged among broad-leafed dipteridaceous ferns.
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Hypuronector was tiny—about 12 cm long. Unlike the other Monkey-Lizards, its tail was not prehensile, but tall and flat and shaped like a fin. Some researchers propose that Hypuronector was an aquatic animal, based on the shape of its tail and the fact it was found in lake-bottom sediments. But all simiosaurs had relatively deep tails, and all are found in sediments that were deposited by water. Scientists have proposed aquatic habits for almost all of the drepanosaurs at one time or another, but now agree that they were a primarily arboreal group. Some paleontologists believe that Hypuronector is similarly misinterpreted, and that it was really a climbing Monkey-Lizard like its more obviously-adapted cousins. Unfortunately scientists have yet to find the hands, feet, or skull of Hypuronector, which would likely offer more conclusive evidence as to its lifestyle.
Dolabrosaurus aquatilis on a stalk of the giant horsetail, Neocalamites.
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Finally, in the Chinle Formation of New Mexico, where the little dinosaur Coelophysis gathered in huge flocks, the remains of Dolabrosaurus aquatilis (Aquatic Pick-Axe Lizard) are found. By far the most incompletely known of the Monkey-Lizards, Dolabrosaurus seems to be similar to Megalancosaurus, slightly larger and with a somewhat deeper tail. As part of its name suggests, it was once thought to be an aquatic, deep-tailed swimmer. The other part of its name refers to unusual processes on top of its vertebrae, that were shaped like miniature pick-axes. A well-preserved shoulder girdle from Ghost Ranch may belong to this elusive animal.
Want to learn more about Monkey-Lizards? Here's a listing of additional Resources.
Artwork and text © Copyright 2005 Matt Celeskey. All rights reserved, but negotiable.