Hindeodus is an extinct genus of conodonts in the family Anchignathodontidae. The generic name Hindeodus is a tribute to George Jennings Hinde, a British geologist and paleontologist from the 1800s and early 1900s. The suffix -odus typically describes the animal's teeth, essentially making Hindeodus mean Hinde-teeth. Conodonts such as Hindeodus are typically small, elongate, marine animals that look similar to eels today. Hindeodus existed from the early Carboniferous through the early Triassic during which they inhabited a wide variety of different environments in the Paleozoic and Triassic seas. Their body consisted entirely of soft tissues, except for an assortment of phosphatic elements believed to be their feeding apparatus. Despite years of controversy regarding their phylogenetic position, conodonts such as Hindeodus are now considered to be vertebrates. They are slightly more derived than the early vertebrates called Cyclostomata, and are part of a large clade of "complex conodonts" called Prioniodontida in the order Ozarkodinina. Hindeodus fossils are distributed worldwide due to the diversity of environments they inhabited. Species of Hindeodus are differentiated by slight variation of the elements of their feeding apparatus. A species of Hindeodus called Hindeodus parvus is particularly well studied because it is used as an index fossil defining the Permian-Triassic boundary.
History and discovery Hindeodus was first described by Rexroad and Furnish in 1964 during the Illinois State Geological Survey's study of Mississippian stratigraphy. The specimen was found in the Pella Formation of South-central Iowa which is known for excellent preservation of conodonts. However, species of Hindeodus were among the least abundant fossils (less than 0.25 specimens per kilogram of sample). They initially believed that Hindeodus may be a species of Trichondella or Elsonella but determined that Hindeodus is not morphologically and phylogenetically comparable to either and thus, must be a new genus. 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. Because conodont elements are essentially the only basis for conodont taxonomy, elements are extensively studied and debated. Therefore, there is specific categorization of elements based on their shape and position. The elements are divided into S, M and P elements. In Hindeodus, S elements are ramiform (branch-like), M elements are makellate (pick-shaped) and P elements are pectiniform (cone-shaped). The H.parvus apparatus in particular consists of six kinds of elements arranged in 13 different positions: nine S elements (unpair S0, paired S1, S2, S3, S4), two M elements, and one pair of P elements (P1). The S0 element is unpaired and has a long sharp cusp but lacks a posterior process. S1 and S2 elements are differentiated by being laterally compressed and having a long sharp cusp with two lateral processes. S3 and S4 elements have a long sharp cusp and an anterior process that is shorter than the posterior process. The M element is the typical makellate (pick-shaped) structure and the P1 element is pectiniform. 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 hypothesis is supported by the presence of lingual cartilage found in conodonts that resembles those found in extant cyclostomes (Hagfish and Lamprey) which are also predators. Morphology of conodont teeth varies widely, but the 15-element dentition of conodonts and their relative position is stable from the Ordovician to the Triassic. The typical 15-element apparatus consisted of 4P elements, 9S elements, and 2M elements. However, Triassic conodonts (such as Hindeodus parvus) only had a 13-element apparatus (9S elements, 2M elements and 2P elements). It was previously believed that the 2 missing elements were due to failure to preserve S1 elements. 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. Upon analysis of conodont history, it is evident that while P elements vary among conodonts, the S-M array is essentially conserved for over 250 million years. This suggests that evolutionary constraints on the number of S and M elements are stronger than those for the P elements, and thus are less likely to change. The loss of P elements is likely linked to the Permian-Triassic extinction event during which many environmental changes occurred that may have impacted the availability of Hindeodus prey, resulting in a change of diet and a new food processing mechanism.
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