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.Sweet WC. 1988. The Conodonta : Morphology, Taxonomy, Paleoecology, and Evolutionary History of a Long-Extinct Animal Phylum. New York: Clarendon Press Their body consisted entirely of soft tissues, except for an assortment of phosphatic elements believed to be their feeding apparatus.Sweet, W.C., Donoghue, P.C.J. 2001. Conodonts: Past, Present, Future. Journal of Paleontology (75)6: 1174-1184. 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.R. S. Nicoll, I. Metcalfe, W.C Yuan. 2002. New species of the conodont Genus Hindeodus and the conodont biostratigraphy of the Permian–Triassic boundary interval. Journal of Asian Earth Sciences (20)6: 609-631 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.Kozur, H.W., Ramovš, A., Zakharov, Y.D., & Wang, C. 1995. The importance of Hindeodus parvus (Conodonta) for the definition of the Permian-Triassic boundary and evaluation of the proposed sections for a global stratotype section and point (GSSP) for the base of the Triassic.Geologija (38)37: 173-213.
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.C.B. Rexroad, W.M. Furnish.1964.Conodonts from the Pella Formation (Mississippian), South-Central Iowa. Journal of Paleontology (38)4: 667-676
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.Zhang, M., Jiang, H., Purnell, M.A. and Lai, X. 2017. Testing hypotheses of element loss and instability in the apparatus composition of complex conodonts: articulated skeletons of Hindeodus. Palaeontology (60): 595-608. 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.Goudemand, N.; Orchard, M.J.; Urdy, S.; Bucher, H.; Tafforeau, P. 2011. Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates. PNAS (108)21: 8720–8724. 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.Agematsu, S., Sano, H. and Sashida, K. 2014. Natural assemblages of Hindeodus conodonts from a Permian–Triassic boundary sequence, Japan. Palaeontology, 57, 1277– 1289. 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).Huang, J. Y., Zhang, K. X., Zhang, Q. Y., Lu, T., Zhou, C. Y., Bai, J. K. and Hu, S. X. 2010. Discovery of Middle Triassic conodont clusters from Luoping Fauna, Yunnan Province. Earth Science—Journal of China University of Geosciences, 35, 512– 514. 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.
Classification Hindeodus is characterized by a P element with a large cusp, denticles that increase in width anteriorly (toward the head) except for the anterior-most denticle and generally decrease in height anteriorly, except for the posterior-most three denticles (the ones furthest back) which are at equal heights. Their cusps are much higher than denticles, and they possess S elements with a short lateral processes that are slightly upturned laterally with denticles of variable size. Hindeodus is differentiated from other conodonts by having P elements with large fixed cusps located at the anterior end of the blade and usually grow primarily by adding new denticles only to the posterior end of the element. Other conodonts vary in growth pattern and location of their cusps. For example, Ozarkidina have cusps located within the blade and growth can occur both anteriorly or posteriorly. In Hindeodus, the P element is crucial for identifying the genus, and had a stable morphology from the Carboniferous into the Triassic with only one minor morphological change. However, in the late Permian and the early Triassic there was rapid evolutionary change especially in the P element. The cause of rapid change in morphology is not certain, but may be related to environmental changes leading to different availability of food source thus leading to changes in feeding mechanism.
Species of Hindeodus are divided into two groups based on the morphology of the posterior portion of the elements. Species such as H.parvus and H.eurypyge grow posteriorly and look rectangular from a lateral perspective. Elements grow by the addition of new denticles to the posterior margin. After one denticle fully grows, a bulge begins to form on the lower posterior margin of the element, and gradually grows upward until the denticle fully develops. The cycle repeats with a new bulge. These elements tend to grow evenly thus yielding a rectangular shape. In contrast, other species such as H.typicalis of H. latidentatus have a sloped lateral profile because the posterior section slopes downward. New denticles form near the posterobasal corner and grow gradually upward but also to the side. These elements tend to preferentially grow on the posterior portion of the element leading to a more sloped shape.
Hindeodus is part of a large clade Prioniodontida (otherwise known as "complex conodonts") which has two major orders of conodonts, Prioniodinina and Ozarkodinina. Hindeodus is part of Ozarkodinina in the family Anchignathodontidae. The synapomorphies that define the clade Prioniodontida is the presence of a P elements with an inner lateral process and peg-like denticles. The synapomorphies of Ozarkodinida are not as clear, but may be the presence of inner and outer lateral process on the S elements.