Microbrachis: a small Carboniferous tetrapod from Bohemia

Microbrachis pelikani was a small, long-bodied tetrapod from the Late Carboniferous coal-shales of Nýřany in the Czech Republic. The unusually large fossil sample includes about one hundred individuals and has allowed detailed study of skeletal growth. Most specimens have 38 presacral vertebrae, and traces of a lateral-line system persist across growth stages. Other fossils preserve evidence consistent with tail regeneration. It is a distinctive member of the ancient amphibian catalogue.

Quick facts

A rich sample from Nýřany

Microbrachis pelikani is a lepospondyl tetrapod from the Upper Carboniferous “Gaskohle” deposits of Nýřany in the Pilsen Basin, Czech Republic. The Plattelkohle is a thin, finely laminated fossil bed within the coal-shale succession. Its preservation has produced a rare sample of small tetrapods that can be compared across a wide range of skeletal sizes.

In a detailed study, Jason Anderson examined 100 specimens of M. pelikani, nearly all the material then known. This large comparative series allowed him to revise skeletal descriptions and distinguish changes during growth from differences between species. The count refers to specimens studied in collections, not to a head count of animals living at one moment.

Antonín Frič named the species in the nineteenth century. Later research has reassessed its position among early tetrapods. “Microsaur” and “lepospondyl” are used for the group in much of the literature, but the relationships of these Paleozoic forms remain an active phylogenetic subject. Their label should not be mistaken for a direct link to modern salamanders.

A long trunk and persistent water-sensing canals

Nearly all complete specimens examined have 38 presacral vertebrae, the vertebrae before the sacrum. Four specimens may differ, with 39 counted, although the relevant parts are difficult to assess. The long trunk and comparatively small limbs fit the animal’s distinctive body proportions. As with any fossil count, a damaged or overlapping skeleton can make an individual vertebra hard to identify.

Small pits and shallow grooves on the skull preserve traces of lateral-line canals. Anderson found them across the sampled sizes, suggesting that the sensory system persisted through growth rather than disappearing in larger individuals. Lateral lines detect water movements in living aquatic vertebrates. Their presence is direct anatomical evidence for aquatic sensory capacity, though not a record of a precise habitat or hunting tactic.

Some specimens have structures near the skull that have been interpreted as branchial elements. They are uncommon and poorly ordered, so external gills remain a cautious interpretation. The long tail and its many vertebrae are better represented. Some specimens contain up to about 45 or 46 caudal vertebrae, though preservation affects the count.

Growth without a simple salamander-style metamorphosis

The growth series shows that skull and limb bones became more fully formed as animals increased in size. Even small individuals already had ossified skulls, vertebrae, long bones and digits. Anderson argued that the pattern does not show the major skeletal remodelling expected in metamorphosis. Growth was comparatively gradual and largely isometric, rather than a dramatic transformation from a larval to an adult body plan.

Some pelvic contacts remained incompletely fused in larger specimens. That condition does not by itself prove that the animal was a juvenile. Different skeletal elements can ossify on different schedules, and fossil maturity must be evaluated from several features together. This is why a large sample is more informative than a single small skeleton.

Tail injury, autotomy and regeneration

At least two specimens preserve an abrupt change along the tail: well-formed vertebrae give way to much smaller, irregular bone blocks. Anderson interpreted this morphology as evidence that a tail had regenerated. The fossil does not preserve the injury itself, and it cannot identify the predator or event that caused it.

Later work compared the anatomy with living salamanders and squamates. The arrangement has been interpreted as an intravertebral autotomy plane, in which separation occurred within a vertebra, rather than between vertebrae as in many salamanders. Regrowth and the mechanism of tail separation are related but distinct questions; evidence for one does not automatically establish every detail of the other.

What the fossils support about habitat

The persistent lateral-line evidence, possible branchial structures and deep tail all support a water-associated animal. The local coal-shale deposits and aquatic preservation are consistent with that interpretation. They do not prove the exact depth, diet or reproductive mode of Microbrachis. Small invertebrates are plausible prey, but no stomach contents establish a menu.

A reconstruction can show the elongate body and swimming tail while marking gills, skin and colour as uncertain. The strongest record is unusually specific: a large growth sample, a fairly stable presacral count, persistent water-sensory traces and rare fossils that may document tail regrowth. Those details make Microbrachis much more informative than a generic small tetrapod from a coal swamp.