Anatolepis

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Anatolepis
Temporal occurrence
Darriwilium to Katium
461 to 450 million years
Locations
Systematics
Trunk : Chordates (chordata)
Class : Jawless (agnatha)
Subclass : Pteraspidomorphi
Order :
Family :
Genre : Anatolepis
Scientific name
Anatolepis
Bockelie and Fortey , 1976

As Anatolepis phosphatic shell and bones are called, probably very early pteraspidomorphi belonged. The finds of this problematic taxon incertae sedis come mainly from the Middle Ordovician , but can go back to the Upper Cambrian . They thus represent a very important link in the history of the development of the early vertebrates .

Initial description

The fragments, only a few square millimeters in size, were discovered in 1976 by Bockelie and Fortey on Svalbard and were the first to be scientifically described. At that time, the authors assigned their find to a heterostrotic jawless (Agnatha).

Taxonomy

Carroll (1988) lists Anatolepis as a genus of the pteraspidoform family Arandaspididae , together with Arandaspis and Porophoraspis as sister taxa. The only subtaxon of Anatolepis is Anatolepis heintzi Bockelie & Fortey.

features

The shell remains of Anatolepis were found together with benthic trilobites , conodonts and brachiopods . Its thickness is only 0.1 millimeters. The surface is ornamented like scales. Unfortunately, the remains do not allow any conclusions to be drawn about the shape of their wearer. Chemically, they consist of hydroxyapatite , the mineral component of bones. Their histology , however, is fundamentally different from later appearing, but still primitive vertebrates.

In cross-section, the shell remnants emerging from a dermis show a three-layer structure. The bony underside (with uncertain allocation of the bone material) is built up in layers, above it an intermediate, sponge-like, cavernous layer of aspidin (bone material without recognizable bone cells) and an equally layered, bony surface layer of dentin (is secreted by odontoblasts and is characteristic of vertebrates ). The cavernous middle layer is traversed by small porous canals, which probably represent small blood vessels or nerve tracts. The surface layer shows tubercular to scaly outgrowths, which look roughly similar to the "tesserae" (mosaic elements) of the back shield of Astraspis .

These "tubercles" or "odontodes" are variable in size and design and change from round to elongated oval shapes. Details are often difficult to see because the scales are usually heavily rubbed off. The tubercles are made up of tubular, mostly steeply standing dentine, rings of growth can be seen. At its base there is a cave-like opening ( English pulp cavity ), from which small dentine tubes rise to the surface, sometimes branch out, but ultimately do not reach the surface itself.

The phosphatic fabric lying in between can also be either flat or strongly curved. On its surface there are small holes that represent the end of pore channels that penetrate the entire layer of tissue. The contact points to the tubercles are usually slightly bent upwards.

In addition, other bony plates were found, the surface scales of which are not formed as tubercles, but follow one another like roof tiles.

The ornamentation of the remains of Anatolepis is somewhat reminiscent of the oldest known, around 465 million years old Heterostraci, Arandaspis and Porophoraspis , but is around 30 million years old.

Scientific controversy

The interpretation of Anatolepis as a vertebrate fossil has not remained undisputed among experts. Skeptics pointed to deviations in structure and thickness from the osteoderm of the other known early skullless (the Arandaspida or the Heterostraci ). According to their interpretation, the fossil could just as well have been a fossil cuticle of an arthropod , possibly belonging to the Aglaspidida . Among the arguments in favor of the vertebrate nature of the fossil is the evidence of relics that can be interpreted as dentin canals. However, the structure of the finds does not allow an interpretation as ganoid scales , since a scale structure is not pronounced. However, the dentin findings were not sufficient to convince skeptics, so that it is possible that only more recent, anatomically related findings will allow a final clarification.

Fenhsiangia ( Upper Tremadocium to Lower Floium ) has been described as another possible vertebrate microfossil from the same period as Anatolepis , and found together with them in the Australian Gola Beds , but the nature of which is just as controversial. Strangely enough, no meaningful macrofossils from this period exist to this day.

Conclusion

The taxonomic affiliation of Anatolepis remains controversial, as no reliable assignment could be made so far. It is very likely that Anatolepis is a primitive fish that appears for the first time around the same time as the first conodonts in the Upper Cambrian. This fact shows that at the beginning of the Ordovician, the development of the vertebrates was already well advanced.

Occurrence

Shell remains from Anatolepis occur at the following sites:

As the occurrences show, Anatolepis was already to be found in the Upper Cambrian and then widespread in the Lower Ordovician. The strictly marine occurrences mainly cluster at latitude 20 ° north to 25 ° south around the North American paleocontinent ( Laurentia ) and document subtropical to tropical conditions.

Individual evidence

  1. ↑ JE Repetski: A fish from the Upper Cambrian of North America . In: Science . tape 200 , 1978, pp. 529-531 .
  2. ↑ Tove G. Bockelie and Richard A. Fortey: An early Ordovician vertebrate . In: Nature . tape 260 , 1976, pp. 36-38 .
  3. ^ Robert Lynn Carroll : Vertebrate Paleontology and Evolution . WH Freeman and Company, New York 1988, ISBN 0-7167-1822-7 .
  4. ^ A b M. P. Smith, IJ Sansom and JE Repetski: Histology of the first fish . In: Nature . tape 380 , 1996, pp. 702-704 .
  5. ↑ Briggs, DEG and Fortey, RA: The cuticle of the aglaspidid arthropods, a red-herring in the early history of the vertebrates . In: Lethaia . tape 15 , 1982, pp. 25-29 .
  6. ^ Philippe Janvier: Facts and fancies about early fossil chordates and vertebrates . In: Nature . tape 520 , 2015, p. 483-489 , doi : 10.1038 / nature14437 .
  7. ↑ Civil Zigaite and Alain Blieck: Palaeobiogeography of Early Palaeozoic vertebrates. Chapter 23 . In: DAT Harper and T. Servais, Early Palaeozoic Biogeography and Palaeogeography (Eds.): Geological Society, London, Memoirs . tape 38 , 2013, p. 449-460 , doi : 10.1144 / M38.28 .
  8. ^ NJ Morris and RA Fortey: The significance of Tironucula gen. Nov. to the study of bivalve evolution . In: Journal of Paleontology . tape 50 (4) , 1976, pp. 701-709 .
  9. ↑ DL Clark, JK Sorenson and AN Ladd: Probable Microvertebrates, Vertebrate-like Fossils, and Weird Things from the Wisconsin Ordovician . In: Journal of Paleontology . tape 73 (6) , 1999, pp. 1201-1209 .