Agnostus pisiformis

Agnostus pisiformis
Representation from art forms of nature ( Haeckel , 1904) , on plate 47
Temporal occurrence
Middle to Upper Cambrian
500 to 488.3 million years
Locations
Europe
Systematics
Trilobites (Trilobita) Agnostida Agnostina Agnostidae Agnostus Agnostus pisiformis
Scientific name
Agnostus pisiformis
( Linnaeus , 1768)

Agnostus pisiformis is a few millimeters long, eyeless trilobite from the Cambrian .

features

Agnostus pisiformis is one of the easily recognizable agnostids in the fossil record . It only reached a length of about six millimeters. The cephalon was smooth, round to oval with a broad hem and a hem furrow. The frontal lobe, on the other hand, was small and rounded. The first two furrows on the glabella were missing. The basal lobes to the left and right of the glabella were small and triangular. The tubercle (small, rounded elevation) was in front of the middle of the glabella. From the glabella to the hem ran a so-called median pregellar furrow, which separated the free cheeks ( librigenae ). The absence of eyes (as in all kinds of order) gave the animals their name (from the Greek Gnosis: knowledge).

The pygidium was also rounded and is extremely similar in size and shape to the head shield. The spindle (end of the axial lobe) on the pygidium was about 1.5 to twice as wide as the flanks on the right and left and had small, rounded tubercles. On the left and right of the pygidium there were small spicules (spines). The hem and the hem furrow were wide. The median postaxial furrow was absent. This was the furrow that separates the left and right flanks of some agnostids and runs from the spindle to the edge, similar to the median pregellar furrow.

The free thorax is narrow and has only two segments. The rachis can be both wide and narrow. It is believed that these different physical characteristics may be due to sexual dimorphism .

According to the larval finds with preservation of the uncalcified belly (ventral) side ("Orsten" fossils, see below), the animals had relatively short antennae with 15 limbs, every second of which had a thorn-like protrusion. Based on this structure, it is more likely that the antennae were used to ingest food than that, as with most other trilobites, it was primarily a sensory organ. Behind the antennae were two legs with a wide outer branch (exopodites) and a missing or very small inner branch (endopodites). These had long bristles and could have served as swimming organs. The fourth extremity of the head section was a typical, two-branched split bone and its structure corresponded completely to the thoracic extremities. There were five pairs of legs on the thorax, two of them on the two free segments and three under the pygidial shield. The basal hip sections (coxes) of the front legs were enlarged and thorny on the inside, probably forming the boundary of a food channel in which the scraped or tumbled food particles were processed. Similar formations can be found in numerous crustaceans. The front end of the food channel was formed by a downwardly protruding, arched flap, the hypostomium. The mouth was behind it. At the side of the hypostomium the antennae were turned.

According to the reconstruction, it is very unlikely that living animals of Agnostus could fully open the shell so that the legs would never have pointed down. Similar to other agnostids, it is assumed that the animals, when alive, sat with their shell folded in and only slightly open. This had only one functional joint, so it was folded shut and not rolled up like many other trilobites. The extremely similar “flaps” of the head shield and tail shield enclosed the animal, similar to a clam shell. In unfavorable external conditions, or attacks by enemies, the shell could be completely closed, in this form Agnostis pisiformis has been fossilized by the thousands. The way animals live is still controversial. Either the animals lived, similar to today's ostracode species, on the sea floor, over the surface of which they moved by swimming and gliding, or they were planktonic colonists of the free water (pelagic). The argument for a planktonic way of life is the widespread use. While other agostids are distributed worldwide, Agnostus pisiformis has been discovered in addition to Scandinavia in Poland, England, Siberia and Newfoundland. Finds from Germany are only known as Ice Age bed load fossils.

Fossil deposits

Agnostus pisiformis is considered to be the key fossil for the base of the Upper Cambrian, which in classical stratigraphy begins with the "Agnostus pisiformis zone". The findings from the Upper Cambrian limestone formations in Sweden, in which fossils of the species are so common that they can form rock formations, are known above all. When hit, the rock smells characteristic of bitumen or petroleum ("stink stone"). The famous Swedish naturalists Magnus von Bromell and Carl von Linné described the rock and fossils as early as the 18th century. Phosphated microfossils of trilobite larvae of this species can be extracted with acid from some formations of this stinking stone. This fossil class was named "Orsten" after the local Swedish name for the stink stone. Orsten fossils of Agnostus pisiformis are only a few millimeters long, but reveal the finest body structures of the uncalcified cuticle down to the micrometer range, including hair and bristles.

literature

• Klaus J. Müller, Dieter Walossek: Morphology, Ontogeny, and Life Habit of Agnostus Pisiformis from the Upper Cambrian of Sweden . Fossils and Strata 19. Universitetsforlaget, Oslo.

Individual evidence

1. ↑ ^{a b c} Frank Rudolph, Die Trilobiten der Mittelkambrischen Geschiebe , Verlag Frank Rudolph, Wankendorf, 1994, ISBN 3-929671-04-2 .
2. ^ David L. Bruton & Hans Arne Nakrem (2005): Enrollment in a Middle Ordovician agnostoid trilobite. Acta Palaeontologica Polonica 50 (3): 441-448. download (PDF file; 1.1 MB).
3. ↑ Gerd Geyer & John Shergold (2000): The quest for internationally recognized divisions of Cambrian time. Episodes 23 (3): 188-195.
4. ^ P. Ahlberg (2003): Trilobites and intercontinental tie points in the Upper Cambrian of Scandinavia. Geologica Acta 1 (1): 127-134.