Arandaspids

Most often, 4 genera are referred to arandaspids ^{[7] [8]} , 2 of which are presumably ^[2] , since they are known only by small fragments. In each of these genera, only one species is described ^[8] :

• Sacabambaspis janvieri Gagnier et al. , 1986 , known from Bolivia , Argentina , Australia, and Oman ^[8] . Named after the typical location - the Bolivian city of Sacabamba ^[11] . He lived at the end of the Lower Ordovician (the Llanwyrn and Karadok age, about 470 million years ago) ^{[5] [6]} . Known for the remains of much better preservation than the rest of the arandaspids ^[12] , including for practically whole exoskeletons ^{[7] [6]} .
• Andinaspis suarezorum Gagnier, 1991 from Ordovician or Devonian Bolivia (not far from the location of Sacabambaspis ). Known for a single fragment ^[11] ; belonging to arandaspids has not been proven ^[2] .
• Arandaspis prionotolepis Ritchie & Gilbert-Tomlinson, 1977 from central Australia ( Alice Springs ). It is named after the ethnic group living in these places ^[4] . He lived at the beginning of the Upper Ordovician ( Karadok age , about 450 million years ago) ^[5] . Known for imprints of the anterior part of the exoskeleton ^[9] in fine-grained sandstone ^[12] ;
• Porophoraspis crenulata Ritchie & Gilbert-Tomlinson, 1977 from there ^[9] . In addition, similar fossils are known from the Early Ordovician rocks (the Arenigian stage , about 480 million years ago), and these are the oldest finds of arandaspids) ^{[7] [13]} . He was named for the large pores on the cusps of the shell. Known only for small fragments ^{[9] [7]} ; belonging to arandaspids has not been proven ^[2] .

In addition, the genera Sacabambaspis , Arandaspis, and Porophoraspis probably have at least one undescribed species ^[8] . It is possible that another Bolivian species, Pirchanchaspis rinconensis Erdtmann et al., Belongs to arandaspids . , 2000 , but its systematic position is very unclear ^{[8] [13]} .

Arandaspids had an elongated teardrop-shaped body up to 35 cm long ^{[14] [3]} , covered with a well-developed exoskeleton . The ventral side was more convex, the dorsal one was flatter ^[15] . The only fin is the tail . It was well studied only in Sacabambaspis , which had a long but narrow central lobe (where the chord entered), a shorter dorsal and even smaller abdominal ^{[6] [8]} .

The eyes were at the very end of the head ^[2] . They had ossifications - sclerotic rings ^[15] . Nostrils are located between the eyes, and behind them are a pair of holes, which are usually interpreted as pineal ^{[3] [15]} .

Arandaspide mouth was on the underside of the head ^[4] . They had up to 10 pairs of gill sacs, which were opened outward with separate openings ^{[5] [2] [7] [8]} , and according to other data, into common passages that opened with one opening on each side ^{[3] [9] [12 ]} .

Skeleton

Arandaspids did not have an endoskeleton or almost none ^{[15] [9] [12]} , but the exoskeleton was well developed. The front part of the body was covered with two large bone plates (dorsal and ventral), and the back - strongly elongated vertical scales. The length of the front plates is about half the length of the animal ^[8] , and the thickness is very small (in Arandaspis <0.1 mm ^{[9] [12]} ). They are solid (without traces of merging of individual elements) and have no signs of growth. This means that they appeared already in an adult animal ^{[9] [12]} .

Between the dorsal and abdominal plates on each side was a row of 15–20 small polygonal plates, between which were gill openings ^{[8] [3] [15] [7]} . In addition, many rows of small bony plates were on the lower side of the mouth ^{[5] [15] [12]} . On the dorsal and abdominal scutes, as well as on small scales ^[12] , small but well developed ^[15] canals of the lateral line are visible ^[3] .

All elements of the exoskeleton are formed by aspidine - acellular bone ^[5] . They distinguish 3 layers: the lower (lamellar), middle (honeycomb or mesh) and upper (tuberous) ^{[3] [15]} .

Outside, the shell was ornamented with tubercles. For genera known only by fragments ( Andinaspis and Porophoraspis ), their shape is the main distinguishing feature. In Arandaspis, they are elongated, tapering to the ends and have a longitudinal crest, while in Sacabambaspis , Andinaspis and Porophoraspis they resemble oak leaves to a different extent, with the latter being penetrated by large pores ^{[7] [9] [3]} .

The shape and ornamentation of the shell of arandaspids resemble miscellaneous and astraspids , while the scales of the tail resemble anaspids ^[5] .

Pineal Holes

Behind the eye sockets arandaspid, in front of the dorsal shield, is a pair of small holes. Both Arandaspis and Sacabambaspis ^{[16] [12] have them} . They are usually interpreted as openings for the organs of the pineal complex, a photosensitive structure from which the parietal eye develops in many ancient and some modern vertebrates ^{[3] [2] [5]} . Thus, arandaspids had two such eyes: one hole contained the pineal organ , and the other parapineal ^{[14] [17] [18]} . This is a very rare situation among vertebrates (except for arandaspids, a paired or double pineal orifice is known only in some fossil fishes - a number of placoderms , paleoniscid ^[19] , porous-like ^{[20] [21]} and early bipedal ^{[22] [23]} ). In addition, the oculosity of both organs of the pineal complex was preserved in most lampreys (although they do not have a pineal opening) ^[24] .

According to another interpretation, these holes in arandaspids represent the exits of the endolymphatic ducts ^{[7] [15]} , which are on the carapace and some other ostracoderms. This is supported by the fact that they are located further back than usual for the pineal opening ^[16] .

Arandaspids lived in a shallow sea ^[4] . Since they did not have stabilizing fins, their movement was probably awkward and resembled the movement of a tadpole ^[4] . Nevertheless, they were the most progressive vertebrates of their time ^[12] .

As for other ostracoderms, for arandaspids, a benthic lifestyle and nutrition with detritus ^[3] and microorganisms are assumed. This is indicated by the position of the mouth from below the head and (as with other ostracoderms ) the absence of jaws ^{[9] [12] [4]} .

Arandaspids are the oldest vertebrates that are known for their rather complete skeletons (from older ones only small fragments of the shell or imprints of a skeletal body were found). The oldest fragments, probably belonging to arandaspids, are about 480 million years old (beginning of the Ordovician), and the youngest ones are 440 million years old (end of the Ordovician) ^{[7] [25]} . The cause of their extinction is unknown; they may have disappeared due to glaciation ^[13] .

Unlike other early ostracoderms (living mainly in the waters of Lavrentia ) ^[13] , arandaspids are known only from Gondwana (namely, from the coastal deposits of the seas surrounding it) ^{[8] [13]} . The fossils of these animals are found in South America ( Bolivia and Argentina ), Australia (the center of the continent) and the Arabian Peninsula ( Oman ). These places were far apart and in the Ordovician, although all were on the edge of Gondwana ^{[8] [26] [7]} .

Arandaspid fossils are rare, but almost all the Ordovician ostracoderms of Gondwana belong to this group ^[26] . In the waters of this continent, ostracotherms were rare in subsequent times. In addition to arandaspids, only telodonts are known from there (a very widespread group that appeared, apparently, in the waters of Lawrence and by the end of the Ordovician penetrated the Gondwana Sea) and pituriaspids (a small group endemic to Australia from the early to Middle Devonian ) ^[13] .

• Ivanov A. O., Cherepanov G. O. Fossil lower vertebrates: a training manual. - 2. - St. Petersburg: Publishing House of St. Petersburg University, 2007. - P. 58. - 228 p. - ISBN 978-5-288-04342-0 .
• Jawless and ancient fish / L.I. Novitskaya. - Moscow: GEOS, 2004. - S. 10-268. - 436 p. - ISBN 5-89118-368-7 .
• Eakin RM The Third Eye . - Berkeley: University of California Press, 1973.- 168 p. - ISBN 0-520-02413-3 .
• Janvier P. Skull of Jawless Fishes // The Skull / Hanken J., Hall BK. - University of Chicago Press, 1993. - Vol. 2: Patterns of Structural and Systematic Diversity. - P. 152-154. - ISBN 0-226-31568-1 .

• Sacabambaspasis is the oldest among vertebrates (neopr.) . nauka20vek.ru (February 6, 2013). Date of treatment April 18, 2013. Archived May 2, 2013.
• Janvier, Philippe. Arandaspida. Version 01 January 1997 (under construction ) . The Tree of Life Web Project. Date of treatment November 21, 2012. Archived January 23, 2013.
• Marco Casti. Gli agnati del Sud: gli arandaspidi (Italian) . paleostories.blogspot.com (10 dicembre 2011). Date of treatment April 20, 2013. Archived May 2, 2013.
• Monroe MH Arandaspis (English) . Australia: The Land Where Time Began . austhrutime.com (October 30, 2011). Date of treatment November 21, 2012. Archived on May 1, 2013.
• Pteraspidomorphi. Anatolepis , Arandaspids & Astraspids . Palaeos: Life Through Deep Time. Date of treatment November 27, 2012. Archived on May 1, 2013.
• Reconstruction of arandaspid (English) . deviantart.com. Date of treatment November 27, 2012. Archived on May 1, 2013.