Menoceras
| Menoceras | ||||||||||||
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Skeletal reconstruction of Menoceras |
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| Temporal occurrence | ||||||||||||
| Lower Miocene ( Burdigalium ) | ||||||||||||
| 23 to 18 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Menoceras | ||||||||||||
| Troxell , 1921 | ||||||||||||
Menoceras is an extinct genus of rhinos and livedin Europe and North America in the Lower Miocene 23 to 18 million years ago. The genus was very small compared to today's rhinoceros and reached the size of today's pigs , but was one of the earliest representatives of the more modern rhinos of the subfamily Rhinocerotinae . It was characterized by two horns placed in pairs on the tip of the nose. The increased occurrence, especially in the Great Plains in North America, suggests a sociable way of life in herds, as it is sometimespracticed by horses today.
features
Menoceras represents a small to medium-sized representative of the rhinos. It reached a head-trunk length of around 180 cm - plus a 50 cm long tail - and a shoulder height of 100 cm. The rhinoceros representative was therefore comparable in size and weight to today's pigs , with the weight for the largest animals being given as around 250 kg. Characteristic were the long and slender limbs and the generally quite graceful physique.
The skull was between 35 and 47 cm long and showed a clear saddle on the forehead, so that the frontal bone was concave. The cheekbones were often very bulging. The occiput was slightly extended and had a prominent bulge as a muscle attachment point. The nasal bone was above the median jawbone ; both were not connected. Two raised bumps on the tip of the nose that bulged clearly forward and had a roughened surface proved to be characteristic. These show the position of the horns, which, in contrast to the tandem horns of today's two-horned rhinos, were arranged in pairs next to each other. Furthermore, the front face area showed clear cuts.
The lower jaw was up to 38 cm long and was shaped like a wedge. The jawbone reached a height of more than 7 cm, but the symphysis was quite short and only reached as far as the second premolar . The teeth had reduced a few teeth, dental formula is: . The primary dentition, however, also had the first premolar. While the upper incisor was shaped more like a chisel, the lower outer tooth had a conical shape and was up to 5 cm long. The inner incisor was very small, sometimes not formed. There was a large diastema to the posterior teeth . The premolars and molars were clearly low-crowned ( brachyodont ). The tooth size increased continuously from front to back. Both tooth types were similar, which means that the premolars were characteristically molarized.
The postcranial skeleton is largely fully known due to numerous finds. The spine consisted of 7 cervical, 19 thoracic, 5 lumbar, 4 to 5 sacrum and 26 tail vertebrae. Adult animals had an atlas that was twice as long as it was. The lengths of the spinous processes of the vertebrae were very variable, especially the first, which in very old animals often reached extremely long and wide dimensions. The humerus was short and strong with an average of 24 cm, with the lower end of the joint having an asymmetrical shape in that the inner joint role was more pronounced than the outer. However, this was pronounced less than phylogenetically older forms as diceratherium or teleoceras . The radius was 25 cm long and, like the ulna, was relatively flat. The thigh bone reached 32 cm in length and the shin 29 cm. The rear legs were thus significantly longer than the front legs. All limbs ended in three toes, whereby the central ray (Metapodium III) was most strongly developed, as is typical for unpaired ungulates . The metacarpal bone was 13 cm long , the metatarsal bone 12 cm. The toes that set on the sides were shorter and slightly curved.
Locations
Menoceras was mainly found in North America , but has also been found in Europe . In North America it is one of the most common rhino representatives of its time and is found there in large numbers , especially in Nebraska , Wyoming , Texas and Florida . The richest areas are in the Great Plains . One of the most important sites is the Agate Fossil Beds National Monument in Nebraska, where Menoceras is the most common form of mammal with hundreds of bone fragments . The animals there with Miohippus , an early member of the horse family and Daeodon , a member of the gigantic, pig-like entelodons , lived in a savannah-like landscape and possibly died as a result of a drought . A sediment block of 1.7 by 2.4 m uncovered in 1920 contained 22 rhinoceros skulls and numerous other skeletal remains. Other finds come from the Marsland Formation , also located in western Nebraska. The southernmost finds on the American continent have been described from Mexico and Panama . Fossils from Europe, on the other hand, are rather rare, an important find is an almost complete skull from Wischberg near Langental in Switzerland. Highlights from Germany come from Pappenheim , among others .
Paleobiology
The low-crowned molars suggest a specialized diet that included soft plant food such as leaves , bark or fruit ( browsing ). This was confirmed with the help of isotope analyzes on the teeth. Bushes and low trees provided the basis for food. In addition, the long and slender legs suggest a rather faster movement.
The numerous finds, especially from the Agate Fossil Beds National Monument, allow some conclusions to be drawn about the sexual dimorphism in Menoceras . While there are hardly any differences in size in the body skeleton, male animals often have more massive skulls than females, which are often characterized by wider zygomatic arches with rough surfaces for stronger masticatory muscles. Strikingly, the nasal bones are also much wider and the horn attachment points are more massive and more rounded. This suggests that cows had smaller horns or were largely hornless. The lower, tusk-like incisors also show marked differences in their length dimensions, which statistically do not overlap. Those of the bulls are more massive and, at 3 to 5 cm in length, significantly larger than those of the cows, which are more delicate and only up to a maximum of just over 2 cm in length.
The high number of finds indicated a way of life in herds with unequal numbers of male and female animals. Today this view is shared by a majority, further arguments for such a way of life, which is rather unusual for rhinos, are the small body size of the animals and the stronger sexual dimorphism in the form of smaller or largely missing horns in female animals. They could hardly defend their young against large predators. At the same time living predators were Osbornodon and Daphoenodon but also the huge, is omnivorous nourishing Daeodon (also Dinohyus called). Therefore, most experts consider a sociable way of life, which is not known from today's rhinos, with their large body size and developed horns in both sexes, as a prerequisite for the survival of the rhino representative. It is possible that the herds were hierarchically structured with dominant bulls at the top, so that the offspring could be defended more effectively. It is also partly assumed that the horn only gained importance as a defensive weapon against enemies, when it was trained in both sexes. In previous rhinoceros groups, it may have had a greater function in reproduction and was more likely to be used in partner choice.
Systematics
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Internal systematics of the North American rhinos according to Prothero 2005
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Menoceras belongs within the family of rhinos to the subfamily of Rhinocerotinae and forms with the tribe Menoceratini the basal member of this modern group. Originally, due to the paired nasal horns, it was placed closer to the more primitive Diceratheriinae , which have a similar characteristic, but Menoceras has much more modern developments in skeletal construction. In the opinion of the majority of experts, the Menoceratini represent the sister group of the Elasmotheriini .
Several species have been described by Menoceras :
Other species described are synonymous with those that are valid today. M. cooki is identical to M. arikarense , while M. marslandensis and M. falkenbachi are now grouped with M. barbouri . M. barbouri is the larger of the two American representatives, M. arikarense the smaller and the most common form in the Agate Fossil Beds National Monument. M. zitteli is largely only known from Europe.
The first description of Menoceras was in 1921 by Edward L. Troxell , who separated the genus from Diceratherium , but located it in its taxonomic proximity. The name was given based on the numerous finds from the Agate Fossil Beds National Monument. The generic name is made up of the two Greek words μένος ( ménos "strength") and κέρας ( kéras "horn") and refers to the two paired horns on the tip of the nose.
Menoceras first appeared in the Lower Miocene 23 million years ago and is common in Europe and North America. It is believed that the North American representatives, first recorded in the latest Arikareean , immigrated from Europe. In Europe itself, the rhino genus disappeared 20 million years ago, in North America it may have lasted until 18 million years ago (Middle Hemingfordian ).
Individual evidence
- ^ OA Peterson: A mounted skeleton of Diceratherium cooki Peterson. Memoirs of the Carnegie Museum 7 (14), 1911, pp. 274-279
- ↑ a b Bruce J. MacFadden and Pennilyn Higgins: Ancient ecology of 15-million-year-old browsing mammals within C3 plant communities from Panama. Oecologia 140, 2004, pp. 169-182
- ^ A b Donald R. Prothero: Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial carnivores, ungulates, and ungulatelike mammals. Cambridge University Press, 1998.
- ↑ a b c d e f g Donald R. Prothero: The evolution of North American rhinoceroses. Cambridge University Press, 2005, pp. 1-219
- ↑ a b c d Kurt Heissig: Family Rhinocerotidae. In: Gertrud E. Rössner and Kurt Heissig: The Miocene land mammals of Europe. Munich, 1999, pp. 175-188
- ^ A b Lloyd G. Tanner: A New Species of Menoceras from the Marsland Formation of Nebraska. Bulletin of The University! Of Nebraska State Museum 9 (8), 1972, pp. 204-213
- ↑ a b c d e f O. A. Peterson: The American Diceratheres. Memoirs of the Carnegie Museum 7 (6), 1920, pp. 399-477
- ^ A b c Matthew C. Mihlbachler: Sexual Dimorphism and Mortality Bias in a Small Miocene North American Rhino, Menoceras arikarense: Insights into the Coevolution of Sexual Dimorphism and Sociality in Rhinos. Journal of Mammal Evolution 14. 2007, pp. 14: 217-238
- ^ John Graham: Agate Fossil Beds National Monument - Geologic Resources Inventory Report. US Department of the Interior ( PDF (2.41 MB))
- ^ Bruce MacFadden: North American Miocene land mammals from Panama. Journal of Vertebrate Paleontology, 26 (3), 2006, pp. 720-734
- ↑ Pierre-Olivier Antoine: Middle Miocene elasmotheriine Rhinocerotidae from China and Mongolia: taxonomic revision and phylogenetic relationships. The Norwegian Academy of Science and Letters Zoologica Scripta, 32, 2003, pp. 95-118
- ↑ Edward L. Troxell: A study of diceratherium and the diceratheres. American Journal of Science (5) 11 (10), 1921, pp. 197-208
