Janusiscus

Janusiscus
Temporal occurrence
Lower Devon
~ 415 million years
Locations
Siberia
Systematics
Neumünder (Deuterostomia) Chordates (chordata) Vertebrates (vertebrata) Jaw mouths (Gnathostomata) Janusiscus
Scientific name
Janusiscus
Giles , Friedman & Brazeau , 2015

Janusiscus schultzei is a small, extinct species of fish that lived in lower Devonian about 415 million years ago. A partly preserved, one centimeter long skull and scales of janusiscus schultzei were found in Siberia near the river Sida in 1972 and initially the bony fish species Dialipina assigned. Early 2015 the fossil was finally as a new, independent type and kind described . The generic name Janusiscus refers to the double- faced Roman god Janus and was given because Janusiscus schultzei showsfeatures of both bone and cartilaginous fish as a mosaic shape . The species epithet schultzei recalls the author of the first description of the fossil, Hans-Peter Schultze .

The cranial roof of Janusiscus schultzei is similar to that of the early bony fish and consists of large bone plates with long ridges and partially closed sensory channels . Examination of the skull using high-resolution computed tomography reveals features of both bony fish and cartilaginous fish, e.g. B. large openings to accommodate the nerves and carotid arteries. A ventral brain cleft, a derived feature of the crown group of the jaws , is missing.

Janusiscus schultzei is therefore assigned to the jaw mouths, but neither to the bone nor the cartilaginous fish. It is close to the common ancestor of both large groups of fish and shows that this probably had both a bony endoskeleton and a bony exoskeleton , and was therefore not at all shark-like, as previously suspected. The cartilaginous fish lost the bony endoskeleton in the course of their evolution and replaced it with a cartilaginous skeleton, while the bony fish and the terrestrial vertebrates developing from them retained the original feature , a bony endoskeleton.

The following cladogram shows the phylogenetic position of Janusiscus schultzei :

	Galeaspida †      Osteostraci †   Jaws    Antiarchi †      Petalichthyida †      Ptyctodontida †      Rhenanida †      Arthrodira †      Janusiscus schultzei †      Bony fish (Osteichthyes)      Acanthodii †      Cartilaginous fish (Chondrichthyes)

literature

• Sam Giles, Matt Friedman & Martin D. Brazeau: Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome , Nature , January 12, 2015, doi: 10.1038 / nature14065L3 [1]

Individual evidence

1. ↑ Schultze, H.-P .: Early Devonian actinopterygians (Osteichthyes, Pisces) from Siberia. In Fossil Fishes as Living Animals (ed. Mark-Kurik, E.) 233–242 (Academy of Sciences of Estonia, 1992)