Megalancosaurus.


Hypuronector.


Megalancosaurus.


Drepanosaurus unguicaudatus.


Drepanosaurus.


Vallesaurus.

Monkey-Lizards of the Triassic

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 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.

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.

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)).

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.

Megalancosaurus.


Skeleton of Megalancosaurus preonensis, modified from Renesto 1994b.


Skeleton of Drepanosaurus unguicaudatus, modified from Renesto 1994a. Neck and head are based on Megalancosaurus.


Sketch of Drepanosaurus marvelling at its unusual hand.
The head is completely speculative.