Jawless armored fish from the Ordovician: the Arandaspids
|My reconstruction of Sacabambaspis janvieri.|
The fossil record of fish during the Ordovician, the period that follows the Cambrian around 485 MYA, is quite poor and consists of just a little more than a handful of named taxa. One of the prominent groups of that time appears to be the Arandaspids. They were jawless (a condition shared by all other vertebrates in these ancient seas) and characterized by a head covered with a bony shield consisting of a flattish dorsal plate, a rounded ventral plate, and a few other smaller plates. Arandaspids were quite primitive looking with two eyes and two nostrils in the front, a series of branchial openings, each protected by bony platelets, on the side between the dorsal and ventral plates. The back portion of the animal was protected by strips of bony armor arranged in chevrons. They had a caudal fin, but no paired fin, making them not particularly good swimmers. They probably lived on the seafloor feeding on microorganisms or organic detritus sucked in through their jawless mouth. All Arandaspids were marine.
|My reconstruction of Arandaspis prionotolepis.|
The type species of the group is Arandaspis prionotolepis from the shallow marine deposits of the Stairway Sandstone in the Northern Territory, Australia, and dating from the Earliest Middle Ordovician. This 10-15 cm long fish, originally described in 1977 (Ritchie & Gilbert-Tomlinson, 1977) is known from several specimens, some quite complete. The other relatively well known species is Sacabambaspis janvieri from the Anazaldo Formation of Bolivia, which was discovered among a fauna composed almost exclusively of lingulid brachiopods, an indication that it lived near the littoral in a well oxygenated area. Initially described from three bone fragments in 1986 (Gagnier & Blieck, 1986), new fossils were later found including a complete articulated specimen (Pradel et al., 2007) that preserved the uniquely shaped hypocercal tail (the end tip of the vertebral column bends downward supporting the bottom lobe of the tail). The Anzaldo Formation was originally believed to be of Early Upper Ordovician age, but it may actually have been older making Sacabambaspis quite contemporary with Arandaspis (Gagnier et al., 1996). Sacabambaspis was a bit larger than its Australian counterpart, reaching a length of 25 cm.
|Articulated fossil specimen of Sacabambaspis janvieri. From User:Ghedoghedo, Wikipedia commons|
Another fish, Andinaspis suarezorum from the Capinota Formation of Bolivia, known from a single poorly preserved fragment, was once thought to be Early Middle Ordovician and classified as a possible Arandaspid, but there are now doubts on its actual age, which turned out to be in all probability Devonian (Gagnier et al., 1996). Also from Bolivia, but from the Pircancha Formation of Early Ordovician age, comes what seems to a be ventral shield of a possible large Arandaspid christened Pircanchaspis rinconensis (Erdtmann et al., 2000) This is the earliest record of a fish from South America. From Australia, Porophoraspis crenulata from the same location and age than Arandaspis was described in the same paper than the latter, but is much less known as only a single external mould of a small plate has been recovered. This one is also a possible Arandaspid.
Arandaspids belong to one of the two major groups of armored jawless fish that would dominate the first part of the Paleozoic era: the Heterostraci or Heterostracomorphs, the other group being the Cephalaspids. Arandaspids apparently did not last longer than the Ordovician, being replaced by far more efficient forms in the Silurian.
Erdtmann, B., Weber, B., Schultze, H.-P., & Egenhoff, S. (2000). A possible agnathan plate from the Lower Arenig (Lower Ordovician) of South Bolivia. Journal of Vertebrate Paleontology, 20(2), 394–399.
Gagnier, P., Blieck, A., & G., R. S. (1986). First Ordovician vertebrate from South America. Geobios, 19(5), 629–634.
Gagnier, P., Blieck, A., Emig, C., Sempere, T., Vachard, D., & Vanguestaine, M. (1996). New paleontological and geological data on the Ordovician and Silurian of Bolivia. Journal of South American Earth Sciences, 9(5/6), 329–347.
Pradel, A., Sansom, I. J., Gagnier, P.-Y., Cespedes, R., & Janvier, P. (2007). The tail of the Ordovician fish Sacabambaspis. Biology Letters, 3(1), 73–76.
Ritchie, A., & Gilbert-Tomlinson, J. (1977). First Ordovician vertebrates from the southern hemisphere. Alcheringa, 1(4), 351–368.