Plagiosternum granulosum was a broad-headed plagiosaurid temnospondyl from the Middle Triassic of Germany. Its flat skull, upper-facing eyes and shoulder bones are documented by fossils from several localities, while a 2025 revision added the first detailed account of the humerus and atlas and tracked changes through growth. The genus is one of the specialized aquatic forms in the ancient amphibian catalogue.
Quick facts
| Species | Plagiosternum granulosum |
|---|---|
| Group | Temnospondyli; Plagiosauridae |
| Age | Ladinian, Middle Triassic |
| Region | Southern and central Germany |
| Type specimen | SMNS 11824, left clavicle |
| Known anatomy | Skulls, jaws, shoulder bones, vertebrae and limb bones |
| In the catalogue | Ancient amphibians |
What the fossils establish
Fraas named the species in 1889 and established the genus in 1896. Later skull and jaw material expanded its anatomy; the type clavicle remains the name-bearing specimen.
Cranial bones preserve the distinctive outline and surface sculpture. A flat head is evidence; exact soft-tissue contours are not.
The 2025 revision describes ontogenetic variation, including relatively larger orbits and more pronounced bosses in larger specimens. These are changes within the species, not by themselves evidence of multiple species.
Its distribution through lagoonal, estuarine and shallow marine settings supports tolerance of brackish conditions. Fossils have not been reported from unambiguously freshwater lake deposits in the reviewed record.
From a shoulder bone to a better-known animal
The history of Plagiosternum began with fragmentary Triassic material. Hermann von Meyer mentioned fossils from Lorraine in 1855. Eberhard Fraas named Mastodonsaurus granulosus in 1889 from material near Crailsheim, then recognized the distinctive shoulder girdle as different enough to establish Plagiosternum in 1896.
The holotype, SMNS 11824, is a left clavicle. It anchors the name but is not a skull or a complete individual. Early reconstructions combined bones recovered from bone beds, where elements from different animals can accumulate together. Later discoveries of skull and jaw pieces, including material from Ummenhofen and Felberg, made it possible to describe more of the anatomy independently.
The 2025 revision reviewed occurrences across 29 localities and ten stratigraphic horizons, and added detailed descriptions of the humerus and atlas. Those additions matter because the genus had been known especially through robust skull and shoulder elements. Counts of bones from a deposit cannot be treated as counts of complete animals: bones can be disarticulated, transported and redeposited.
A flattened skull and changing ornament
The skull was extremely broad and low, with large openings for the eyes on its upper surface. Its dermal bones carry a coarse, reticulate sculpture. In larger individuals the intersections of the ridges develop more prominent bosses, and the rear outline and relative proportions of some bones change. The 2025 sample therefore documents growth-related variation rather than a single fixed skull shape.
The clavicles and interclavicle formed a large part of the shoulder armour of the skeleton. The interclavicle's structure has been interpreted as compatible with ventral scales. There is no secure evidence for a covering of dorsal osteoderms, so a heavily armoured back would go beyond the fossil record. Small limb bones and vertebrae are less frequently represented than the broad, durable skull and shoulder bones.
These proportions support a low, wide-bodied animal adapted to move in water, but no associated skeleton preserves every joint in place. The atlas and humerus now add information about the neck and forelimb, yet they do not supply a complete measure of stride, swimming motion or total body length.
Coastal settings and cautious ecology
The revised occurrences range from the upper Muschelkalk into the Erfurt Formation. Sediments and associated animals indicate lagoons, estuaries and shallow marine basins. Repeated records in these settings support the interpretation that Plagiosternum tolerated brackish water better than many other temnospondyls. This is an inference from its geological distribution, not a direct measurement of the salinity of every place where an individual lived.
The broad mouth and low posture are consistent with an ambush predator taking fish or other small vertebrates. No gut contents establish a particular meal, and the fossils do not preserve a hunting event. The related Plagioscutum had a similarly flattened plagiosaurid body plan, while Cyclotosaurus belonged to a different temnospondyl branch with a longer skull.
What a reconstruction can and cannot show
A reconstruction can responsibly show the unusually wide skull, dorsal eye position and small limbs, placing the animal in a shallow coastal lagoon. Skin texture, colour, exact body outline and feeding behaviour are not preserved. The evidence supports a specialized aquatic temnospondyl and a notable association with brackish environments, while details of its daily life remain open.
Frequently asked questions
When did Plagiosternum live?
It lived during the Ladinian Age of the Middle Triassic, roughly 242–237 million years ago.
What is the type specimen?
The name is anchored by SMNS 11824, a left clavicle. It is not a skull or a complete skeleton.
Did it have a bony back armour?
No dorsal osteoderms are securely documented. The shoulder bones are broad, but that does not demonstrate a fully armoured back.
How do we know it tolerated brackish water?
Its repeated occurrences are in lagoonal, estuarine and shallow marine settings. That distribution supports salt tolerance, but does not measure the salinity of each site.