Hindeodus

Hindeodus was first described by Rexroad and Furnish in 1964 during the Illinois State Geological Survey's study of Mississippian stratigraphy. While faunal diversity during the end Permian extinction event (251 million years ago) drastically plummeted, Hindeodus survived into the early Triassic. A possible explanation for this is the versatility of certain Hindeodus species in terms of the environments they are able to survive in. Additionally, there is evidence that Hindeodus was able to migrate during the Permian-Triassic transitional period which lead to its wide distribution worldwide during this time. == Description and paleobiology ==

Hindeodus elements Hindeodus was primarily soft-bodied; the only mineralized tissue of Hindeodus (and all other conodonts) are their “elements” that are tooth like structures arranged in particular positions and are thought to have served as a feeding apparatus functioning to grasp and intake prey. There are several different hypotheses for the functions of the Hindeodus apparatus. One hypothesis is that the elements were used as support structures for filamentous soft tissue used for suspension feeding. However, upon further analysis it was determined that the S, M and P elements would not provide enough surface area to support ciliated tissue needed for suspension feeding. The more accepted hypothesis is that the conodont elements were used for predation. It is predicted that the S and M elements open allowing the prey to be captured in the oral cavity of the animal. The cusps of these elements aid in food intake by firmly gripping the prey while the blade-like P elements slice like a pair of scissors. This was not highly plausible because every other element was exceptionally preserved on the same bedding plane, so it was unlikely that apparatuses were preserved incompletely. An alternative hypothesis was that Hindeodus lost two S elements which implies changes in capture of prey (as the primary function of the S and M array is to trap prey in the animal's mouth). A final hypothesis is that Hindeodus lost two P elements which implies changes in food processing ability, which may be due to a change of diet to food that requires less slicing or crushing to ingest. Additionally, the genus Isarcicella likely evolved from Hindeodus (H.parvus) in the Early Triassic. }} }} }} }} }}|label1=Prioniodontida}} == Paleoenvironmental and geological information ==

The paleoecology of Hindeodus was frequently debated. Clark (1974) proposed that Hindeodus was most abundant in nutrient-deficient deep waters of normal salinity, but some may have been in shallow water as well. Behnken (1975) proposed that Hindeodus lived in abnormal salinities. Wardlaw and Collinson (1984) proposed that Hindeodus dominated in lagoonal facies. Orchard (1996) considered Hindeodus to be dominant in shallow, near shore and warm regions. The general consensus now is that Hindeodus lived in wide range of marine depositional environments: nearshore, shallower, and warmer environments as well as deep-water environments, and offshore environments. For example, in the Meishan, Gondelellids were dominant in deep warm-water environments before the ecological stress that occurred in the Late Permian (which was possibly short-lasting cooling in low latitudes due to presence of aerosoles). Gondelellids and many other Permian species in the area disappeared, but ecologically tolerant Hindeodus survived and dominated the area. A similar situation occurred in Iran where Gondelellids were abruptly replaced by the Hindeodus in the deep-water areas. There is evidence that Hindeodus was able to migrate during the Permian Triassic transitional period which lead to its wide distribution worldwide during this time. They were able to survive and evolve in warm-water or cold water and shallow water or deep-water environments despite widespread anoxia during the Permian-Triassic transitional period. This is one of the reasons Hindeodus is an ideal index fossil for the defining the Permian-Triassic boundary. However, not all species of Hindeodus were able to survive and thrive in a variety of different environments. Species such as H. julfenis, H.changxingensis, H.altudaensis among others are ecologically restricted to deeper, but warm water environments. They are never found in shallow water facies, or in deep water that was presumably home to cold water fauna. In contrast, more common species such as H. typicalis, and H. parvus were more ecologically tolerant and could live in environments not tolerated by other conodonts. H.parvus in particular is exceptionally versatile in regards to what environments it inhabited. H. parvus was found in both shallow water deposits as well as pelagic deposits. It is found in Japan, North America, the Boreal realm (Greenland), and the entire Tethys. Although Hindeodus is globally widespread, the Meishan section in Changxing County, Zejiang Province, South China is one of the more notable locations Hindeodus fossils were located. The Meishan section is used as the GSSP (global boundary stratotype section and point) for the Permian-Triassic boundary defined by the first appearance of H.parvus. It is a continuous, pelagic sedimentary record across the Permian-Triassic boundary without any stratigraphic gaps, and is essentially thermally unaltered (CAI=1-1.5). The section consists of 7 quarries at the southern slope of the Meishan hill, 70 to 400m away from each other. The beds of these quarries are nearly identical as they have the same thickness, facies, and fossil content. Quarry D is best studied because it exposes the entire Changxing Limestone whereas the other quarries only expose the middle and upper part of the Changxing Limestone. == Biostratigraphic significance ==

The species Hindeodus parvus is an index fossil whose first appearance in the fossil beds at Meishan, Changxing County, Zhejiang marks the base of the Triassic, and thus the boundary between the Triassic and Permian. It is hypothesized that the extinction was caused by dense aerosols from strong volcanic activity in areas such as the Siberian Trap. These dense aerosols then caused short-lived rapid cooling in low latitudes, similar to a nuclear winter. Widespread anoxic conditions in the lower Triassic prevented the fauna from recovering. Despite the rapid decrease in fauna, the exact Permian/Triassic boundary was still undetermined. It was initially defined by the first appearance of ammonoid (cephalopod) Otoceras. Then, the base of the Triassic was defined by the appearance of Isarcicella isarcica. The location of Isarcicella isarcica is nearly identical to the base of H. parvus, but there are several advantages to using H. parvus to define the biostratigraphic Permian-Triassic boundary. Firstly, Hindeodus is the first globally distributed species that appears immediately after (5 cm above) the minimum in fossil diversity indicated by the minimum in Carbon 13 at the Meishan section. Hindeodus is also not environmentally restricted and can be found in both shallow water deposits and deep-water deposits. It is also thermally tolerant and is found in cool-water environments, mild environments as well as tropical warm-water. Additionally, the derivation of H. parvus from its forerunner H. latidentatus is clear because they are found sandwiched between transition forms. Despite the close proximity with similar fossils, Hindeodus is easily determinable and readily separable by its large cusp. The wide distribution, clear derivation, and easy identifiability of Hindeodus makes it the ideal index fossil, which is why the International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of Hindeodus parvus as the defining biological marker for the start of the Induan, 252.2 ± 0.5 million years ago, the first stage of the Triassic. == Notes ==