Paleontologists have found fossilised fish with severed heads and intact guts

Paleontologists have discovered decapitated skulls of the predatory fish Aspidorhynchus with the gastrointestinal tract still attached in the shales of the Solnhofen archipelago (Bavaria). These rare fossils have helped us understand who ancient predators hunted - and how they became prey themselves. This is reported by Phys.org with reference to the journal Fossil Record.

Predator with a "spear" instead of a snout

Aspidorhynchus lived in the tropical lagoons of Solnhofen at the end of the Jurassic period and was one of the key predators of the local ichthyofauna - accounting for about 4 per cent of all fish finds. It grew up to 1 m in length and was characterised by a long, spear-shaped upper jaw, reminiscent of modern marlins or swordfish. Based on its body structure and fins, researchers believe it was an active stalking predator.

Previously, its stomach contents could only be studied in whole skeletons hidden under a thick armour of ganoid scales. In the new work, Martin Ebert and Martina Kölb-Ebert examined 343 specimens of Aspidorhynchus and found that in about 16 per cent of the individuals, the heads were completely separated from the body, with a whole "trail" of viscera stretching to the skull.

Such "separate heads with intestines" turned out to be a unique "window" into the diet of an ancient predator: the body does not prevent the remains of prey inside the stomach and intestines.

What was found in the stomachs of Aspidorhynchus

Analyses have shown that the main prey of Aspidorhynchus were small bony fish - juvenile corvids (family Orthogonicleithridae), which the predator, judging by their size, easily swallowed whole. There is evidence that these fish may have been kept in packs, and the long upper jaw of Aspidorhynchus allowed it to hunt them in much the same way as modern swordfish do.

However, individual specimens indicate larger prey. For example, an individual about 56 cm long (specimen GZG.RF.999) swallowed a 16 cm long Allothrissops fish - very large relative to the size of the predator.

There were also some quite unusual finds. In one case, Aspidorhynchus was found to have a small ganoid scaled fish stuck in its mouth (specimen GZG.RF.998). Such a position, the researchers said, could have prevented the fish from properly passing water through its gills and actually caused it to "suffocate" - a situation rare but known in modern fish when prey is too large.

In another sample, the crustacean Knebelia schuberti was found in the stomach of Aspidorhynchus. Crustaceans are not considered a typical part of this predator's diet, and scientists are not yet sure whether this was an accidental capture or an occasional menu change.

How hunters became prey themselves

Equally interesting is the fact of decapitation of these fish. The authors of the work suggest that Aspidorhynchus became victims of larger predators - so-called "grabber predators", which do not swallow the prey whole, but grab it by the tail, shake it, biting off the head, and then eat more nutritious and softer body.

This way of feeding well explains why it is the skull with attached intestines that is found in shales: the spine was torn at a weak point at the junction of the head and torso, and the viscera were yanked out together with the skull.

The vulnerability was reinforced by the peculiarities of Aspidorhynchus' structure: incompletely ossified vertebral centres created a "weak link" between the skull and the body, making it easier to separate the head during sharp grasping and shaking.

According to Martin Ebert, the Solnhofen ecosystem had predators large enough to easily handle a metre-long fish:

• ichthyosaurs 2-4 metres long,

• saltwater crocodiles,

• even larger plesiosaurs, pliosaurs pliosaurs.

Most likely, they were the "hunters of hunters", leaving behind decapitated skulls with preserved innards.

A unique "window" into the Jurassic ecosystem

The study not only clarifies the diet and behaviour of Aspidorhynchus, but also demonstrates the exceptional fossil preservation conditions in the Solnhofen Archipelago. Not only bones, but also soft tissues and sometimes rare combinations such as the skull of a predatory fish with a whole intestine and the remains of its last prey are recorded here.