Named after a legendary Scandinavian serpent, Joermungandr bolti here was a recumbisrostran “microsaur” – part of a group of animals that were traditionally considered to be lepospondyl amphibians, but more recently have been proposed to in fact be a lineage of early reptiles.
Discovered in the Mazon Creek fossil beds in Illinois, USA, this species dates to the late Carboniferous period around 310 million years ago. A single near-complete specimen about 5cm long (~2″) preserves impressions of the body outline and numerous tiny scales, giving us a pretty good idea of what it looked like in life.
Joermungandr had a long streamlined tubular body with small limbs and a short tapering tail, and a stubby snout with fused bones heavily reinforcing its skull. Along with microscopic ridges on its body scales that resemble the dirt-repelling scales of some modern reptiles, this combination of features suggest it was a headfirst burrower that wriggled its way through soil with snakelike motions.
A group of early tetrapods known as diadectids were some of the first land animals to become specialized herbivores, developing adaptations for the bulk processing of tough fibrous vegetation. They had powerful jaws, chisel-like front teeth, and grinding cheek teeth, and they grew to relatively large sizes for their time with bulky bodies supporting voluminous plant-fermenting guts.
Although usually considered to be reptilomorphs – “amphibian-grade” animals more closely related to amniotes than to modern amphibians – some studies have instead placed these early plant-eaters as being true amniotes related to the synapsids. Fossil trackways show they may have had amniote-like claws on their feet, and that their highly flexible lizard-like ankle joints allowed them to walk much more efficiently than other early tetrapods, possibly using a semi-upright gait, but these may be convergently evolved features. Since we don’t know whether they laid amniote-like eggs or if they instead spawned amphibian-style in water, it’s currently hard to tell for certain just what they really were.
Diasparactus zenos (sometimes alternately known as Diadectes zenos) was a diadectid that lived during the early Permian in New Mexico, USA, about 296 million years ago. Around 1.3m long (4’3”), it was only about half the size of its largest relatives, but it’s notable for having unusually high neural spines on its vertebrae – not quite long enough to be considered a sail, but more of a “high back” that may have supported powerful musculature or fatty deposits.
Westlothiana lizziae from the Early Carboniferous of Scotland (~338 mya).
About 20cm in length (8″), this superficially lizard-like creature (nicknamed “Lizzie” by its discoverer) had a long slender body with relatively small legs, which may have been adaptations for burrowing similar to modern skinks.
Its anatomy shows a mixture of both amphibian and reptilian characteristics, suggesting it may have been a close relative of the first true amniotes. But exactly where it fits in that area of the evolutionary tree is still uncertain, with different paleontologists classifying it as either an early amniote, a reptilomorph, or a lepospondyl.