Sacabambaspis

Bolivian specimens of Sacabambaspis fossils, displayed at Musée d'Histoire Naturelle, Paris.|left Sacabambaspis is named after the village of Sacabamba, Cochabamba Department, Bolivia, where the first fossils of the genus were found. S. janvieri, the type species of the genus, is known from the Anzaldo Formation of Bolivia. Later studies in the mid-2000s cleared up a few remaining points of ambiguity regarding its armor structure and tail anatomy. Over 30 specimens have been found in Bolivia, all crammed into a very confined area. Their close arrangement is believed to be the result of a fish kill, probably due a sudden inflow of fresh water and sediments from a large storm. They were found associated with a large number of lingulid brachiopods, also killed at the same time. Other specimens Indeterminate specimens (described as "Sacabambaspis sp.") have been found in many countries corresponding to the margin of Gondwana. Young (1997) described fossils of the genus from the Stokes Siltstone and Carmichael Sandstone of Central Australia. Specimens have also been reported from Argentina. The layers in Oman containing Sacabambaspis may be as old as the late Dapingian or early Darriwilian stages of the Middle Ordovician (roughly 470 million years ago). By comparison, the South American and Australian fossils are equivalent to the historical Llanvirn–Caradoc series (late Darriwilian to early Sandbian stages), persisting into the Late Ordovician (roughly 458 million years ago). The prehistoric fish models on display at the Natural History Museum of Helsinki are the work of Estonian paleontologist Elga Mark-Kurik (1928–2016), who the museum asked to design their 1995 fossil fish exhibit. Mark-Kurik was not an artist by trade, and her models were entirely made from scratch. The exhibited models are made of silicone (copied from a sculpted foam template), with added doll eyes purchased at a nearby craft store. == Description ==

Sacabambaspis averages around in total length, with the head shield about long and wide. The shields probably evolved from a mosaic of many tiny platelets, fused together at their lowest layer. A few early heterostracans (such as Tesseraspis and Lepidaspis) have tubercles of a similar shape, though in an unfused format. The side of the head has a curved row of diamond-shaped branchial plates, which separate the dorsal and ventral plates. The branchial plates most likely covered a series of gill openings. The eyes are reinforced by scleral rings and are positioned far forward, at the very front of the head. The jawless mouth is a narrow slit, with its lower "lip" a broad bundle of thin scaly strips. The sensory system of Sacabambaspis was not limited to eyes and nostrils: a pair of small pits near the front of the dorsal shield may represent pineal and parapineal organs. Arandaspids appear to be the only vertebrates with paired pineal-parapineal organs. Shallow lateral lines (sensory grooves) cover the headshield. The dorsal shield has a pair of intermittent grooves along its length, plus a complete loop behind each eye. The ventral shield has a pair of lateral lines extending down from the edge of the mouth. Body and tail . The tail fin of Sacabambaspis, though unique in its details, can broadly be classified as hypocercal. This means that the spinal cord bends down in the tail, with most of the fin webbing above it, like many other extinct and living jawless fish groups. Heterostracans appear to have a more symmetrical tail, though without a clear axis for the spinal cord. == Classification ==

, a group of armored jawless fish endemic to the Ordovician of Gondwana. Arandaspids are all very similar to each other, only differing in subtle aspects of their armor texture. They were closely related to two other groups: the astraspids (from Ordovician Laurentia, the core of modern North America) and heterostracans (Silurian-Devonian, worldwide). All three groups had several traits in common: large dorsal and ventral head shields (or a mosaic of armor in their place), a characteristic set of internal layers in the armor, and no fins apart from the tail fin. Arandaspids, astraspids, and heterostracans are collectively known as pteraspidomorphs. The most fundamental physical difference between the rare Ordovician species and the extremely diverse heterostracans is that heterostracans condense their gill openings into only two holes, one on each side of the head. Pteraspidomorphs are probably a clade, meaning that they are most closely related to each other and not ancestral to any non-pteraspidomorph fish. The most comprehensive study on pteraspidomorph relationships is by Randle et al. (2025). The result of their maximum parsimony analysis is shown below: ==Paleobiology==

Feeding Although it was jawless, the mouth of Sacabambaspis janvieri was lined with nearly 60 rows of small bony oral plates which were probably movable in order to provide more efficient suction feeding through expansion and contraction of the oral cavity and pharynx. Sensory system The fossils of Sacabambaspis show clear evidence of a sensory structure (lateral line system). This is a line of canal pores, within each of which are open nerve endings that can detect slight movements in the water, produced for example by predators. The arrangement of these organs in regular lines allows the fish to detect the direction and distance from which a disturbance in the water is coming. ==See also==