The figure shows a reconstruction of the external appearance of one of the most unusual crustaceans that ever lived in the seas of our planet - Dollocaris ingens from the Thylacocephala class.
Dollocaris lived in shallow seas in what is now Germany in the late Late Jurassic, but the history of the Thylacocephala class began much earlier. Their earliest representatives were found in sediments of the Silurian age, and some finds (though not all researchers are sure that they belong to the same class) are dated to the Cambrian.
Paleontologists got acquainted with these unusual crustaceans back in the 19th century. In 1888, the species Clausia lithographica was described from the famous lithographic schists of Solnhofen (see Solnhofen Limestone). The first skeleton of Archeopteryx was found earlier in the same shale - these animals were contemporaries and, moreover, became namesakes (the Latin name for Archeopteryx is Archeopteryx lithographica). Later, the Ashofen thylacocephalus had to be renamed from Clausia to Clausocaris, since the flowering plant Clausia was described under the name "Clausia" at the beginning of the 19th century. In addition to Clausocaris, two more genera of thylacocephals, Dollocaris and Mayrocaris, differing in the details of the structure of the limbs and shell, were also described from the Scholnhofen shales.
Clausocaris lithographica from lithographic schists near Solnhofen, Germany. Print length 3.5 cm (plate size 15 × 11 cm). Photo from the site fossilmall.com
Although specimens from Solnhofen were first found in the 19th century, they did not attract much attention for almost a century. Only in the 80s of the twentieth century, with the advent of scanning electron microscopes and tomographs, did specialists return to these findings. A detailed study of strange animals with large shells and long limbs has shown that they do not fit into any of the known classes of crustaceans. Therefore, in 1982, a new class was allocated - Thylacocephala: its name is derived from the Greek words θύλακος ("bag" or "bag") and κεφαλή ("head").
To date, it is known that representatives of the class Thylacocephala were rather large crustaceans, from 3-5 to 20 cm long. Their tail segments were reduced, and the whole body was placed under a bivalve, laterally flattened shell, which was smooth in some species, and others decorated with ridges and ledges. From below, three pairs of limbs emerged from under the carapace, of which at least one pair (rear) was adapted for capturing prey. They swam with the help of small paddle-like limbs located under the back of the shell.
Silurian Thylacares brandonensis (left) and Jurassic Clausocaris lithographica (right). Drawing from article C. Haug et al., 2014. The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group
Although the Silurian and Jurassic thylacocephals (they have been studied better than their relatives from other eras) have a fundamentally similar structural plan, significant differences are observed between them, primarily in the structure of the eyes and limbs. Silurian Thylacares brandonensis has small "crayfish" eyes and short, rather thick legs, barely protruding from under the shell. Its distant descendant Clausocaris lithographica, which was already discussed above, looks completely different. It has significantly longer and thinner limbs covered with thorns (somewhat resembling the grasping limbs of praying mantises) and huge hemispherical eyes that occupy the entire front of the body. The same huge eyes, unusual for crustaceans, were also found in other Mesozoic Thylacocephala (including Dollocaris, with which the story began). Obviouslyvision was of paramount importance to these animals.
Promotional video:
Artistic reconstruction of the Silurian Thylacares brandonensis (left) and the Jurassic Clausocaris lithographica with caught belemnite (right). Pictures & copy; Robert Johnson from K. Broda et al., 2015. Thylacocephalans
The lifestyle of thylacocephals is still a subject of discussion. Late Mesozoic representatives of this class were undoubtedly active predators, but their Paleozoic ancestors could have been scavengers. Most likely, the victims of the Mesozoic thylacocephals were fish and cephalopods with an inner shell - coleoidea, which did not have strong protective covers, since the thin thorny limbs of thylacocephals studded with thorns are well suited for holding soft-bodied prey, but not for cracking shells or shells.
The huge eyes of the Jurassic thylacocephals could have been an adaptation to life in low light conditions. Perhaps thylacocephals lived in deep layers of water, where sunlight did not penetrate, or they were nocturnal. The latter option seems to be quite probable, since the remains of thylacocephals are found in not very deep continental seas. There are known finds of fragments of Thylacocephala shells in shark coprolites, which means that thylacocephals periodically became their prey.
The relationship of thylacocephals with other crustaceans is very difficult to trace due to their extremely unusual structure: they are too different from their relatives, and many of the diagnostic signs by which crustaceans are classified are absent.
In the Mesozoic, thylacocephals were widespread and rather diverse animals, but they were never very numerous anywhere, and their findings are rare throughout the world. The last representatives of Thylacocephala are known from the deposits of the Campanian stage of the Upper Cretaceous; however, it cannot be ruled out that somewhere they could have survived to the great extinction at the Cretaceous-Paleogene boundary.
Illustration © Andrey Atuchin from eartharchives.org.
Author: Alexander Mironenko