Thuliadanta
| Thuliadanta | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Lower Eocene | ||||||||||||
| 56 to 50.7 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Thuliadanta | ||||||||||||
| Eberle , 2005 | ||||||||||||
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Thuliadanta is an extinct, early representative of the Tapiroidea , whichlivedin North America in the Lower Eocene from 55 to 50 million years agoand was a close relative of today's tapirs . So far, several remains of the skull are known, which were found on today's Ellesmere Island in the far north of Canada . The peculiarity of the finds lies in the fact that the layers carrying the find also lay beyond the Arctic Circle when they were deposited. Thus Thuliadanta is the northernmost tapir species ever found, although the animal must have been adapted to the special light conditions of regions near the polar during its lifetime.
features
So far only individual skull fragments and isolated teeth are available, but these give a good insight into the structure of the skull. It was at Thuliadanta a medium sized Tapiroiden comparable to Desmatotherium . The skull clearly resembled that of today's tapirs with a nasal bone lying far behind the median jawbone and only short and weakly developed . This resulted in a very large interior nasal space. The infraorbital foramen was level with the last premolar and first molar .
The teeth show some differences to today's tapirs. The incisors , especially the third of the upper jaw (I3), were spatulate and not as clearly conical and enlarged as in recent tapirs. The upper canine, however, was also rather small compared to today's forms. The posterior set of teeth, which had the full number of teeth of the early mammals in both the lower and upper jaw, already had a large diastema . The posterior premolars showed a partially molarized structure with two transverse enamel ridges on the chewing surface, as did the molars, although they were more rectangular and not as round as the premolars. The two anterior premolars were rather small and characterized only by an enamel cusp.
Paleobiology
The low-crowned molars speak for a nutritional specialization in soft vegetable food, similar to what is still the case with the tapirs today. Computed tomographic scans of the juvenile skull showed a clear reduction of the nasal bone and an expansion of the nasal cavity. These anatomical structures suggest that Thuliadanta already had a short trunk, which, analogous to its current relatives, served as a grasping organ when ingesting food.
Thuliadanta lived during the early Eocene , at that time the position of today's Ellesmere Island was also in the far north beyond the polar circle , estimated at 74 to 80 ° north latitude. The temperatures at that time are reconstructed on the basis of the accompanying fauna and flora to an annual average of around 4 ° C (today −19 ° C), whereby these fell to or just below freezing point in winter and could rise to over 20 ° C in summer. The climate was mild and humid. Thuliadanta lived in a swamp forest with tall trees, which was inhabited by alligators ( Allognathosuchus ), various species of turtles, but also by Brontotherien ( Eotitanops ) and hyaenodonts ( Prolimnocyon ). The high arctic distribution of Thuliadanta is remarkable , today's tapirs are more tropical adapted, nocturnal animals. The tapir-like species living in the far north, on the other hand, must have been adapted to a change between long, bright summers ( polar day ) and dark winters ( polar night ). At least the occurrence of not fully grown, juvenile individuals suggests a year-round presence of this early tapir species in this region. The best comparisons with tapirs living today can be found with the mountain tapirs , which are both cooler climates in high mountain areas and sometimes active during the day.
Finds and find history
The holotype (copy number CMP 30804) comprises a right half of the upper jaw with the row of teeth from the first premolar (P1) to the last molar (M3) and was found at Bay Fjord on the west coast of Ellesmere Island in the far north of Canada . At least two more upper jaw fragments, that of a young animal and individual teeth, are assigned to it. An additional tooth find comes from the Swinnerton Peninsula on the south coast of the same island. All fossil remains come from the upper deposits of the Eureka Sound Group , the Margaret Formation . The entire deposit unit of the Eureka Sound Group was created during the Eureka Mountain Formation , which affected the entire Arctic and began in the Paleocene . The Margaret Formation consists of cross-layered sand , silt and silt stones, interrupted by coal deposits and volcanic ashes . A radiometric date of 52.6 million years comes from one of the ash layers . Thus, according to both biostratigraphy and geophysics , all fossils can be assigned to the early Eocene (local stratigraphic Wasatchian ) and thus between 50 and 55 million years old.
Systematics
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Internal systematics of the Tapiroidea according to Bai et al. 2017
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Thuliadanta is a member of the superfamily of the Tapiroidea , very early relatives of today's tapirs , all of which are now extinct except for the latter. All members of this group have molars that are characterized on the one hand with a low crown ( brachyodont ) and on the other hand by two transversely positioned and clearly raised enamel folds. It is assigned a total of seven or eight families, including the tapirs (Tapiridae). No family assignment has yet been established for Thuliadanta . Based on the characteristics, Thuliadanta can be classified as a close relative of Colodon and Irdinolophus , forms that have already developed characteristics, such as a large nasal cavity or semimolarized posterior premolars. In a first phylogenetic study, the two forms were therefore classified as sister groups to Thuliadanta . However, there were still some similarities in the skull structure of Helaletes , which, however, has more primitive molars. Colodon is also regarded by some experts as an early form of the tapirs, but can be assigned to the late Eocene. According to this investigation, Thuliadanta would also be regarded as a basal form of the Tapiridae, but this would shift the origin of the family back by at least ten million years, with finds of forms of the Tapiridae from the Middle Eocene not being proven. Another analysis sees Thuliadanta as more original than Helaletes and places the form further basal in the family tree.
The Thuliadanta fossils were discovered during fieldwork at the Carnegie Museum of Natural History in the 1970s to 1980s. They are now in the Canadian Museum of Nature in Ottawa . In a preliminary investigation, the tapiroid finds were first regarded by Mary R. Dawson as belonging to Hyrachyus . The first description was in 2005 by Jaelyn J. Eberle . The genus is monotypical and contains the species T. mayri . The word Thuliadanta is made up of the Latin word thule ("extreme north") and the Spanish word danta , which is used in South America as a loan word for "tapir" ; it thus refers to the northernmost known point of discovery of a tapir-like species. The species addition mayri honors Ulrich Mayr, a member of the Geological Survey of Canada (GSC), who has led numerous expeditions to the arctic north of Canada. The species T. mayri possibly includes two subspecies.
Individual evidence
- ↑ a b c d e f Jaelyn J. Eberle: A new 'tapir' from Ellesmere Island, Arctic Canada - Implications for northern high latitude palaeobiogeography and tapir palaeobiology. Palaeogeography, Palaeoclimatology, Palaeoecology 227, 2005, pp. 311-322
- ^ A b c Matthew Colbert and JJaelyn Eberle, The skull of Thuliadanta, a surprisingly derived tapiroid from the Early Eocene (Wasatchian) of the Canadian high arctic, as revealed by high-resolution CT scanning. Journal of Vertebrate Paleontology 27 (3, Supplement), 2007, p. 60A
- ↑ a b c d Jaelyn J. Eberle and David R. Greenwood: Life at the top of the Eocene greenhouse world - A review of the Eocene fl ora and fauna vertebrate from Canada's High Arctic. Geological Society of America Bulletin; January / February 124 (1/2), 2012, pp. 3–23
- ↑ a b Bin Bai, Yuan-Qing Wang, Fang-Yuan Mao and Jin Meng:: New material of Eocene Helaletidae (Perissodactyla, Tapiroidea) from the Irdin Manha Formation of the Erlian Basin, Inner Mongolia, China and comments on Related Localities of the Huheboerhe Area. American Museum Novitates 3878, 2017, pp. 1-44
- ^ Robert M. Schoch: A review of the Tapiroids. In: Donald R. Prothero and RM Schoch (Eds.): The evolution of the Perissodactyls. New-York 1989, pp. 298-320