| Endothiodon | |
|---|---|
 | |
| Reconstructed skeleton ofE. bathystoma specimen AMNH 5613 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Suborder: | †Anomodontia |
| Clade: | †Dicynodontia |
| Family: | †Endothiodontidae |
| Genus: | †Endothiodon Owen,1876 |
| Type species | |
| †Endothiodon bathystoma Owen, 1876 | |
| Other species | |
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| Synonyms[1] | |
Genus synonymy Species synonymy
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Endothiodon (/ɛndoʊθiːoʊdɔːn/ "inner tooth" from Greek endothi (ἔνδοθῐ), "within", and odon (ὀδών), "tooth", most likely named for the characteristic of the teeth being placed internally to the maxilla[2]) is anextinctgenus of medium to largedicynodont from the LatePermian. Like other dicynodonts,Endothiodon was anherbivore, but it typically lacked the two tusks that characterized most other dicynodonts and instead had long rows of teeth inset in the jaws that replaced in waves. The anterior portion of the upper and lower jaw are curved upward, creating a distinct beak that is thought to have allowed them to be specializedgrazers.[3]
Endothiodon was widespread and is found across the southern region of what was then a single large continent known asPangea. It was originally only found in southernAfrica but has now also been found in eastern Africa,India andBrazil, which were both close to Africa at the time. The finding in Brazil marks the first dicynodont to be reported for the Permian ofSouth America.[2] This finding also shows that part of theRio do Rasto Formation in Brazil can now be correlated with deposits in India,Malawi,Mozambique,South Africa,Tanzania,Zambia, andZimbabwe.[2]
Historically, nine namedspecies in total from South Africa have been attributed toEndothiodon during the late 19th and early 20th centuries (sometimes variably split into distinct genera), but these were reduced down to just three accepted species ofEndothiodon in 1964. In the 21st century, these three were further thought to likely represent only a single species, the originaltype speciesE. bathystoma, a conclusion upheld by a thorough revision of their taxonomy in 2024.[4][5][6][7][1] A second valid species,E. mahalanobisi, was discovered in India and named in 2000. Although smaller thanE. bathystoma,E. mahalanobisi is recognised as a distinct species rather than simply juveniles ofE. bathystoma. Apart from size,E. mahalanobisi also has a pointed snout with only a single, low longitudinal ridge (compared to three raised crests onE. bathystoma), a narrowpineal foramen lacking a bony boss or collar, and a flat prefrontal bone.[1] A third species was discovered in Tanzania and namedE. tolani in 2015. Unlike the other species,E. tolani has a pair of small tusks.[5] Although initially discovered in and thought to characterise separate regions, their ranges have since been found to overlap in eastern Africa, with potentially all three present in Mozambique.E. bathystoma appears to be particularly widespread, with a range from Brazil, through southern and eastern Africa and into India.[1]
The skull ofEndothiodon is most quickly recognized by the prominent upturned beak. The premaxilla and palate of the upper jaw are vaulted and allows for the upturned and pointed lower jaw to fit into this region.[4] On the lower jaw, lateral to the teeth, is a broad groove.Endothiodon lacks a lateral dentary shelf but has a bulbous swelling of thedentary. The function of this swelling is not yet known. The pineal foramen (opening for the "third eye") is situated on a boss, which is high in three of the species and low in one of them (E. mahalanobisi). There is also a boss situated on the ventral margin of thejugal.[2] The anteroventral process is an anteroposteriorly short triangular bone, while in most other dicynodonts it is long and pointed.[8]
The teeth in the upper and lower jaw differ both inmorphology as well as in tooth replacement.[3] The teeth of the upper jaw tend to be larger (5–9 millimetres (0.20–0.35 in)) than those of the lower jaw (<5 millimetres (0.20 in)) and areserrated on the anterior edge while the lower jaw has serrations on the posterior edge.[4] Although it was originally thought that E. bathystoma had several rows of teeth on the upper jaw, it was later discovered that the tips of the teeth from the lower jaw had been left behind in the upper jaw. Now it is known that the upper teeth are roughly positioned into a single row.[4] The entire row is moved posteriorly so that the anterior portion of the premaxilla contains no teeth but the most posterior portion still holds two teeth. The teeth are also situated internally to the edge of the maxilla.[4]
It was first thought that the dentary contained three parallel rows of teeth. Instead of arranging the teeth in longitudinal rows, they are now known to fall into obliquely arranged Zahnreihen.[3] In each Zahnreihe, the anteriormost tooth is the oldest and the posterior most tooth is the youngest. There is active ongoing replacement of these tooth rows. The distal portion of each tooth is compressed from side to side and is somewhat pear shaped in cross section.[4] Teeth migrated labially throughout ontogeny rather than being resorbed as rows of new teeth developed.[9]
The palate shows two distinct regions that are covered in minuteforamina. These areas probably had a horny covering in life. The broad groove running along the tooth row on the dentary was probably also covered by a horny layer.[3] It is possible that these regions allowed forocclusion where the upper teeth met the groove lateral to the lower teeth and the lower teeth met one of the regions of the palatine.[4] This is still under scrutiny as the palatine region is short in comparison to the lower tooth row and the second horn covered area on the palatine does not oppose any structure in the lower jaw.[4] Because the palatine region is shortened, effective occlusion for shearing would only be possible when the lower jaw was in a retracted position.[10]
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Endothiodon was first described by Richard Owen in 1876 from fossils discovered in the Karoo region ofBeaufort Group, South Africa based on a skull and mandible. Thegenus was described based on the anterior portion of a snout and the corresponding part of the dentary, creating an upturned beak.[11] Several more specimens have since been collected, many of them in the Beaufort Group in South Africa, it is here that the first partial skeleton was discovered by Broom in 1915.[4] In the 1970s a badly weathered dicynodont skull and lower jaw was discovered in Brazil and was preliminarly assigned toEndothiodon. Boos later reexamined the specimen and confirmed this assignment, referring the specimen toEndothiodon (later confirmed asE. bathystoma by Maharaj and colleagues in 2024)[1] and markingEndothiodon as the first Permian dicynodont to be found in South America.[2]
Four main endothiodont genera,Endothiodon,Esoterodon,Endogomphodon, andEmydochampsa, were variably utilised and separated under the subfamily Endothiodontinae. Often these genera were based on species originally assigned toEndothiodon, e.g.Endothiodon uniseries, originally named by Owen in 1879, was made the type species of the genusEsoterodon by Seeley in 1894.[12] In 1964, Cox comprehensively revised the taxonomy of endothiodonts and found that the characteristics used to separate the four genera were not valid. The four genera were thus synonymised underEndothiodon.[4] From the original nine South African species attributable toEndothiodon, Cox was also able to narrow down just three species based on skull size and robustness of the lower jaw. However, Cox stated this arrangement was provisional, and diagnoses for each species were still inadequate to rule out the three species representing a growth series instead.[4]
Since Cox's 1964 revision, another new species ofEndothiodon was discovered in India and namedE. mahalanobisi in 2000, the first species recognised outside of Africa. Compared to the other three species, it had a smaller inferred adult size, only a single low longitudinal ridge on the snout, a more elongated pineal foramen situated on a low boss located midway on the intertemporal bar in front of instead of surrounding the pineal foramen, and a slender dentary symphysis. Some of these characteristics such as the shape of the pineal foramen and the presence of three longitudinal ridges were thought to be distinguishing characteristics of the genera as a whole, but are now only valid at specific level.[10] Another new species was collected in Tanzania in 1963 and was described in 2015 asE. tolani. It is distinguished from otherEndothiodon based on the lack of a pineal boss and the presence of a pair of tusks lateral to the tooth row.[5] Cox's suggestion thatE. uniseries andE. whaitsi were likely synonymous withE. bathystoma was supported by researchers in the 21st century, and were provisionally treated as such.[5][6][7] In 2024, the taxonomy ofEndothiodon was thoroughly revised again by Iyra Maharaj, and formally argued for the three-species concept ofEndothiodon including onlyE. bathystoma,E. mahalanobisi, andE. tolani.[1]
Endothiodon as a genus has been included in manyphylogenetic analyses of dicynodonts, although often represented by only one or two species, typicallyE. bathystoma. Thecladogram below shows and simplifies the results of Angielczyket al. (2017) to highlight thephylogenetic relationships of the genusEndothiodon relative to other dicynodont species and clades:[13]
The intrarelationships ofEndothiodon were phylogenetically tested for the first time in 2024 by Maharaj and colleagues in 2024 using a specimen-level analysis of individual specimens assigned toE. bathystoma,E. mahalanobisi,E. tolani andE. uniseries. Their results found consistent clades corresponding to the first three species, whilst specimens assigned toE. uniseries were spread withinE. bathystoma, supporting their synonymy. A simplified cladogram from Maharajet al. (2024) following their proposed taxonomy is presented below, showing the relationships ofEndothiodon species:[1]
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In adultEndothiodons the lower jaw teeth are pear shaped incross section, compressed distolaterally, and has posterior serrated edges while the upper jaw teeth have anterior serrated edges. In the juveniles, the lower jaw is a lot smaller and more slender. The lower jaw contains one functional tooth row with 5-6 teeth. The teeth are small, conical, and pointed. The distal edge contains serrations that are just starting to appear. The juvenile teeth are much simpler and are more similar to that of acarnivore than an herbivore. It is possible that the different tooth morphology might be due to a change in diet from insectivorous oromnivorous as a juvenile to herbivorous as an adult. This would be achieved as size increases and it is more able to adapt to being herbivorous.[10]
Endothiodon was first discovered in theKaroo region ofBeaufort West, South Africa.[11] The Karoo region is characteristic ofsiltstones that are fine-to medium- or coarse-grained, dark or greenish grey, and very finely crossbedded.[15] Since then several more specimens have been found in African countries including theUsili,Ruhuhu and lower part of theKawinga Formations of Tanzania, the basal beds ofMadumabisa Mudstone of Zambia, andChiweta Beds, Malawi.[16]Endothiodon has been placed in the Endothiodon[17] and/orCistecephalus Assemblage Zones and dates back to a Late Permian (Tatarian) age.[16] In 1997 the first specimen ofE. mahalanobisi was found in theKundaram Formation in the north-western part of Pranhita-Godavari valley nearGolet inAdilabad district,Andhra Pradesh, India.[10] The Kundaram Formation is characterized bymudstone,sandstone, and ferruginousshale.[10] In addition to Africa and India,Endothiodon is also known from the Morro Pelado Member ofRio do Rasto Formation in theParaná Basin, Brazil.[2]
Ataphonomic reconstruction of the Late Permian showed that there were well established, dense,riverine vegetation.[18] It was originally thought thatEndothiodon would grub matter out of the ground using its beak.[4] This is now seen as implausible because of the position of the external nares on the snout being placed so far anteriorly. Instead, it is now thought thatEndothiodon inhabited the dense riverine vegetation and would cropfoliage with its beak before processing it with its specialized and extensiveoral cavity.[3]δ18O andδ13C values reveal thatE. bathystoma fed on riverine vegetation, as well as that juveniles of the species incorporated insects into their diet.[19]
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