For around 50 years some very unusual dinosaur tracks have been found in ancient desert sediments in South America: strange footprints showing the impression of only a single toe, a walking style never before seen in any reptiles.
And recently a fossil of what might be the track maker has actually been found.
Named Vespersaurus paranaensis, this new species lived during the Late Cretaceous of Brazil (~90 mya) and was a member of the noasaurid family of theropods, closely related to the weird-jawed Masiakasaurus from Madagascar.
Measuring about 1.5m long (~5′), Vespersaurus was fairly lightly built with legs proportioned for running – and its feet were absolutely unique. Although it had the standard three main toes of a theropod, it bore its weight entirely on the middle toe and held the other digits off the ground. The two raised toes on each foot also had large knife-like claws which may have been used during hunting, vaguely similar to the sickle claws on the feet of dromaeosaurs. But unlike dromaeosaurs these claws weren’t highly curved or pointed, suggesting Vespersaurus used more of a scratching and slashing technique rather than the raptors’ puncture-and-restraint strategy.
Much like ancient horses, it may have developed its single-toed stance as an adaptation for more efficient fast running, possibly to avoid larger predators or to chase down small fast-moving prey like hopping desert mammals.
The known one-toed fossil footprints are actually slightly older than the Vespersaurus fossil, and similar tracks in Argentina have been found dating back to the Late Jurassic (~150mya), so there may have been a long lineage of “one-toed” desert-dwelling noasaurids in South America that haven’t been found yet.
Eudibamus cursoris, a bolosaurid from the Early Permian of Germany (~284-279 mya).
Although very lizard-like in appearance, this animal was actually part of a completely extinct group known as parareptiles – a diverse group of early sauropsids who were once thought to be the ancestors of turtles, but are now considered to instead be the evolutionary cousins to the true reptiles.
With a total length of about 25cm long (8-10″), the structure and proportions of its limbs suggest it could run fast on its hind legs, making it one of the earliest known examples of bipedal locomotion. Since its teeth were adapted for a herbivorous diet, it wasn’t using its speed to chase down prey but was instead probably sprinting away from predators.
But unlike the sprawling running of some modern lizards, Eudibamus may have been capable of holding its legs in a more upright position directly under its body, convergently evolving a more energy-efficient posture similar to that of later bipedal animals like dinosaurs.
Ergilornis rapidus, a 1.2-1.5m tall bird (4′-5′) from the Early Oligocene of Mongolia (~33-28 mya). Closely related to modern cranes, trumpeters, and limpkins, it was part of an extinct group called eogruids – flightless birds which existed across Eurasia for a large portion of the Cenozoic from roughly 40-3 million years ago.
Although the earliest known eogruids were smaller and less specialized, and may even have still been somewhat capable of flying, later forms like Ergilornis had highly reduced wings, long legs adapted for running, and convergently ostrich-like feet with only two toes each.
Boverisuchus magnifrons*, a crocodilian from the early Eocene of Germany (~50-40 mya). Reaching about 3m long (9′10″) it was much more heavily armored than its modern cousins, with an interlocking “exoskeleton” of bony osteoderms covering its body and limbs – leading to it being given the nickname “panzer croc”.
It was adapted for walking and running on land, with relatively long legs and surprisingly hoof-like claws. It may even have carried its weight directly on these hooves similar to mammalian ungulates.
And if that’s not unusual enough, its hind leg musculature suggests it also might have been capable of short bursts of bipedal sprinting.
[ * Originally known as Pristichampsus rollinatii before being reassigned in 2013.]