Sacabambaspis

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Sacabambaspis
Reconstruction of Sacabambaspis

Reconstruction of Sacabambaspis

Temporal occurrence
Dapingium to Katium
466 to 450 million years
Locations
Systematics
Sub-stem : Vertebrates (vertebrata)
Class : Pteraspidomorphi
Subclass : Arandaspida
Order : Arandaspidiformes
Family : Arandaspididae
Genre : Sacabambaspis
Scientific name
Sacabambaspis
Gagnier , Blieck & Rodrigo , 1986

Sacabambaspis is an extinct genus of the Pteraspidomorphi . This primitive jawless fish ( Agnatha ) lived in the Middle Ordovician on the continental margin of Gondwana . Among the Arandaspidida it is the best known genus.

etymology

The name Sacabambaspis is a combination of the type locality Sacabamba in Bolivia and the Greek word ἀσπίς aspis (shield).

Initial description

Sacabambaspis was discovered in 1986 and described scientifically for the first time in the same year by Pierre-Yves Gagnier, Alain Blieck and G. Rodrigo.

Systematics

Slab of stone with several Sacabambaspis janvieri , Musée d'Histoire Naturelle in Paris.

Sacabambaspis was placed by Gavin C. Young (1997) within the order Arandaspidida to the family of the Arandaspididae. However, many processors classify the genre directly in the order or superordinate order and leave the assignment to a family open. Subtaxa are the species Sacabambaspis janvieri , to which the type fossil was assigned, and the fragmentary finds of Sacabambaspis sp. Young 1997, Sacabambaspis sp. Albanesi & Astini 2002 and Sacabambaspis sp. Sansom et al. 2009.

features

Sacabambaspis was 25 to 35 centimeters long, with the head shield reached about a third of this length with a width of 8 centimeters. Its appearance resembled a tadpole with an oversized head, a flattened trunk and a movable caudal fin . Otherwise he had no other fins. His eyes were attached to the front of the head, something like car headlights.

Sacabambaspis wore a head shield, which was composed of a large back plate (dorsal plate) and a belly plate (ventral plate). Both plates were made up of about 2 millimeters large hexagonal small plates, the so-called tesserae , which were fused together over a basal layer. Viewed from the outside, they formed perforated tubercles, the edges of which were frayed like oak leaves - a pattern that is also found in other heterostraci and in the scales of thelodonti . The dorsal plate converged in the middle in a ridge-like elevation, the ventral plate was convex and strongly bent downwards.

The anterior 2.5 centimeters of the ventral plate were built up from square rows of platelets that surrounded the mouth opening. Even though Sacabambaspis did not have a jaw, its mouth was surrounded by nearly 60 rows of these very small bone plates. These platelets were probably mobile and could contribute to a stronger suction effect through contractions of the oral cavity and throat.

Viewed from below, the ventral shield had an oval shape. It was very strongly curved in front, but flattened out noticeably towards the rear. Both shield plates were connected via narrow branchial plates attached to the side. There were 20 of these diamond-shaped branchial plates, which had a hump in the middle at their rear end and obviously covered the gills . They reached roughly from the position of the pineal apparatus to the end of the ventral shield.

Between the very front eyes was a T-shaped bone, which separated depressions from each other, which could possibly be interpreted as nostrils - an otherwise characteristic feature of the Arandaspidida. 2.5 centimeters behind the front edge of the spinal shield was a paired depression that can be interpreted as a pineal or parapineal apparatus. Behind it, the tubercles of the tesserae became smaller and thus gave the impression of an independent plate.

Behind the head shield, the rest of the trunk, about 25 centimeters long, was covered by elongated, strip-shaped, relatively thin scales . They too had oak leaf tubercles, but they were arranged in parallel rows. Four larger rows of scales with a total of over 100 scales ran parallel to the animal's flank, with one row each forming the central ridge on the back and stomach. The four rows of flanks were arranged in zigzag pairs. The remaining scales pulled backwards like a herringbone, either to the dorsal or to the ventral median ridge. The dorsal median ridge ended just before the anus.

The approximately 10 cm long tail consisted of a relatively large back and abdominal lobe and in the middle of the lobe of the chorda dorsalis , which reached far back and was closed by a small fin and gave the impression of a rat tail. This structure differs very clearly from the heterostraci, which are currently grouped as Arandaspidida and Astraspidida within the clade Pteraspidomorphi (Gagnier 1993, 1995; Donoghue and Smith 2001; Sansom and colleagues 2005). In them, the caudal fin appears diphycerc (ie symmetrical) and, according to Janvier (1995), is stiffened by a few radials.

With the sideline organ , Sacabambaspis fossils clearly demonstrated the presence of organized sense organs. This was a series of pores with an open confluence of nerves, which registered the slightest water movements, for example triggered by potential predators. The straight line arrangement enabled the animal to pinpoint the exact direction and distance of the source of the interference. Line-shaped sense organs were not only located on the side of the animal, but also ran across the back and abdomen. On the ventral side, for example, a pair of line organs ran from the ventral shield over two-thirds of the length of the trunk, with the surface of the affected trunk scales being continuously and straight notched. A series of transverse commissures - connecting ligaments of nerve tissue - were attached to the ventral edge of 18 to 20 side plates . Lines also ran on the back shield, for example the paired back line, which branched out to the side at the level of the pineal apparatus, but continued behind it into the back. Infra- and supraorbital linear sense organs were also present.

Way of life

Petrification of Sacabambaspis

Sacabambaspis probably lived as a filter feeder on the continental margins of Gondwana in shallow marine coastal waters, among other things, the genus was also fossilized on submarine alluvial cones.

Occurrence

Artistic reconstruction of three sacabambaspis

In addition to the type locality in Bolivia, where the type fossil of Sacabambaspis janvieri occurs together with 30 other conspecifics, the following sites are known for Sacabambaspis :

The increased occurrence of Sacabambaspis janvieri at the type locality in Bolivia is attributed to a mass extinction, which is explained by the rapid influx of fresh water during a storm. With the primitive fish, a large number of lingulids ( brachiopods ) were killed at the same time . Isolated shell remains from Australia have a very similar ornamentation compared to the Bolivian finds. The finds from Oman finally prove that the fish not only stayed on the southern edge of Gondwana (as the finds from Australia and South America originally suggested), but were spread over the entire continental edge of Gondwana.

See also

Individual evidence

  1. ↑ Gagnier, P.-Y., Blieck, A. and Rodrigo, G .: First Ordovician vertebrate from South America . In: Geobios . tape 19, 5 , 1986, pp. 629-634 .
  2. ↑ Pierre-Yves Gagnier and Alain Blieck: On Sacambaspis janvieri and the vertebrate diversity in Ordovician seas . In: Mark-Kurik, E., Fossil fishes as living animals (ed.): Academia . tape 1 . Talin 1992, p. 9-20 .
  3. ^ Benton, Michael J .: Vertebrate Palaeontology 3rd Edition . Blackwell Publishing, 2005, ISBN 978-1-4051-4449-0 , pp. 47 .
  4. ^ Gagnier, P.-Y .: The oldest vertebrate: a 470-million-year-old jawless fish, Sacabambaspis janvieri , from the Ordovician of Bolivia . In: Nation. Geogr. Res. Band 5 (3) , 1989, pp. 250-253 .
  5. ↑ GL Albanesi, JL Benedetto and P.-Y. Gagnier: Sacabambaspis janvieri (Vertebrata) y conodontes del Llandeiliano temprano en la Formacion La Cantera, Precordillera de San Juan, Argentina . In: Boletin de la Academia Nacional de Ciencas . tape 60 (3-4) , 1995, pp. 51 .
  6. ^ Gavin C. Young: Ordovician Microvertebrate Remains from the Amadeus Basin, Central Australia . In: Journal of Vertebrate Paleontology . tape 17 (1) , 1997, pp. 1-25 .
  7. ↑ A. Pradel, IJ Sansom, P.-Y. Gagnier, R. Cespedes and P. Janvier: The tail of the Ordovician fish Scabambaspis . In: Biology Letters . tape 3 , 2007, p. 72-75 .
  8. ^ IJ Sansom, CG Miller, A. Heward, NS Davies, GA Booth, RA Fortey and F. Paris: Ordovician fish from the Arabian Peninsula . In: Palaeontology . tape 52 (2) , 2009, pp. 337-342 .
  9. ^ Gagnier, P.-Y .: Sacabambaspis janvieri , un heterostraceo del Ordovicico Superior de Bolivia . In: Acta del IV Congresso Latinoamericano de Paleontologia . tape II , 1987, pp. 665-677 .