Meridiolophus
| Meridiolophus | ||||||||||||
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Holotype - lower jaw and teeth of Meridiolophus (lower jaw: A - from above, B - from outside, C - from inside; teeth: D - from above, E - from outside, F - from inside) |
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| Temporal occurrence | ||||||||||||
| Lower Eocene | ||||||||||||
| 56 to 47.8 million years | ||||||||||||
| Locations | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Meridiolophus | ||||||||||||
| Bai , Wang , Meng , Li & Jin , 2014 | ||||||||||||
Meridiolophus is an early, now extinct representative of the odd ungulate . It lived in the Lower Eocene and 50 million years ago, but is only known from a fragmented lower jaw. This was discovered in the southern Chinese province of Guangdong . The genus was assigned with reservation to the Isectolophidae family, which occupies a very basic position within the odd ungulate and belongs to the lineage from which today's tapirs and rhinos emerged on the one hand, and whichalso led to the extinct Chalicotheriidae on the other. Due to the formation of the teeth, a closer position to the tapirs and rhinos, whose common group is called Ceratomorpha , islikely.
features
Meridiolophus has so far only been handed down on the basis of a damaged lower jaw in which the teeth from the second premolar to the third molar have been preserved. The lower jaw belonged to a small animal from the family history of the unpaired ungulates, which are closer to today's tapirs and rhinos than they are to horses . The posterior part is missing, but the symphysis is completely preserved. It widened slightly towards the front, resulting in a tighter tooth position in the middle section of the lower jaw, while the canine and incisors were clearly separated. The posterior end of the symphysis was between the second and third premolars. The lower jaw body became more massive towards the molars, the rostrum must have been rather low due to the sloping upper edge and the rising lower edge in the area in front of the second premolar. Below the molars, however, the lower jaw has broken off and is therefore not completely preserved. A mental foramen was formed at the level of the first and third premolars, the posterior lying lower and the anterior slightly larger in size. The number and appearance of the incisors are not known; the canine and the first premolar are marked by their respective alveoli . An extensive diastema about 9 mm long lay between the two teeth . The upper edge of the lower jaw was slightly dented here. A second diastema, significantly shorter with a length of 1.4 mm, extended between the first and second premolars. The premolars themselves were noticeably smaller than the molars, oval shaped and had only a single main cusp on the chewing surface ( protoconid ). Only the posterior premolar showed a tendency towards molarization and thus resembled the posterior molars. These had a rectangular shape and were significantly wider than other early tapir relatives. On the chewing surface there were two paired cusps between which small ridges were formed ( bilophodont ). The length of the premolars was 4.2 to 6.1 mm, the two anterior molars were 7.4 and 8.3 mm long and 5.6 and 6.3 mm wide, respectively.
Fossil finds
So far, Meridiolophus is only known from a left, incompletely preserved lower jaw branch. This was discovered in the Huayong Formation in the Sanshui Basin in the southern Chinese province of Guangdong. The paleogenic deposits in the Sanshui Basin are rich in microfossils and fish remains, land-living vertebrates are indicated by birds , among other things . On the basis of ostracodes , an age can be assumed for the Huayong Formation in the Lower Eocene , which roughly corresponds to the time before 56 to 47 million years ago (local stratigraphic Bumbanium ). Meridiolophus is the only second fossil mammal find of this time in the Sanshui Basin. The other represents a representative of the now extinct Pantodonta .
Systematics
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Possible family relationship of Meridiolophus within the early odd ungulates according to Bai et al. 2014
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Meridiolophus is a genus of the family of Isectolophidae within the order of odd-toed ungulates . The Isectolophidae include small, mostly fast-moving ( cursorial ) representatives, according to their skeletal structure , which are considered a very basal group within the intermediate order Tapiromorpha . The tapiromorpha, a morphologically diverse group, in turn include today's tapirs and rhinos and their extinct relatives (Tapiroidea and Rhinocerotoidea; both are grouped together to form the subordination Ceratomorpha ), but also the purely fossil-based chalicotheria (Chalicotherioidea) and their relatives (summarized as Ancylopoda ) . As a sister group , the Isectolophidae are opposite to all other members of this Ceratomorpha-Ancylopoda kinship group and are to be understood as the oldest members of the Tapiromorpha in phylogeny . The Isectolophidae were introduced by OA Peterson in 1919 and at that time included forms such as Homogalax or the Isectolophus, which gives the family its name . Numerous scientists accepted Peterson's classification, including Leonard Burton Radinsky , who, however, viewed the group as paraphyletic in 1963 , since he classified Homogalax as a basal representative of the tapir line with a closer relationship to the original tapir-like such as Heptodon , while he considered Isectolophus as a rather primeval sideline. Later investigations also came to different results, in that Homogalax would be a rather primitive member, but Isectolophus would be closer to the tapir-like. The discovery and inclusion of other forms such as Cardiolophus or Gandheralophus , which was first described from Pakistan in 2012, could not clearly clarify the relationships within the Isectolophidae.
A conspicuous development trend within the Isectolophidae is the increase in the lophodontia of the posterior molars, that is, the stronger expression of the transverse enamel ridges between the cusps on the occlusal surfaces of the teeth in the course of the tribal history. Compared to numerous other representatives of the Isectolohidae, the enamel ridges of the molars in Meridiolophus are less well developed than, for example, in Isectolophus , Homogalax or Gandheralophus , so it is close to Karagalax , which is known from a skull from the Lower Eocene of Pakistan. Furthermore, within the Isectolophidae there is a tendency to shorten the diastemata in the course of the tribal history. So has Meridiolophus a long diastema between the canine and the first premolars and a short between the first and second premolars. Other forms such as Isectolophus or Homogalax only have short gaps between the teeth that vary between the three teeth. Gandheralophus, on the other hand, shows a long diastema behind the canine, but the two anterior premolars are closed. The so far only one known lower jaw of Meridiolophus is phylogenetically difficult to classify, as studies have shown that several possible, closer relationships could exist. Most likely, due to the age of the find and to avoid long ghost lines, a sister group relationship between Meridiolophus and the Ceratomorpha, the community group of tapirs and rhinos. This is supported by the long postcanine diastema, which is a characteristic of numerous developed representatives of this group, and the smaller premolars compared to the molars. Karagalax is also in a very close relationship , both genera would then form the “parent group” of the Ceratomorpha. The more lophodontic dentition in other, mostly later members of the Isectolophidae, on the other hand, could speak for a closer relationship with the Ancylopoda and thus the Chalicotheria. In this case, the Isectolophidae as a family would actually have to be viewed as paraphyletic.
The first scientific description of Meridiolophus was in 2014 by Bin Bai and fellow researchers. The basis was the only known lower jaw from the Sanshui Basin in southern China, which represents the holotype (specimen number IVPP V 20125). With Meridiolophus expansus only one species is recognized. The generic name Meridiolophus is made up of the Latin word meridies ("south") and the Greek word λόφος ( lóphos , Latinized lophus "bar", "hill"). "South" refers to the discovery area in southern China, lophus is a frequently used addition to the name for odd- toed ungulates and refers to the enamel ridges on the chewing surfaces of the molars. The additional species expansus (Latin for "extended") reflects the characteristic of the more extended mandibular symphysis.
Individual evidence
- ↑ a b c d e Bin Bai, Yuanqing Wang, Jin Meng, Qian Li and Xun Jin: New Early Eocene Basal tapiromorph from Southern China and Its Phylogenetic Implications. PlosONE 9 (10), 2014, p. E110806
- ^ Luke T. Holbrook and Joshua Lapergola: A new genus of Perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal Perissodactyl phylogeny. Journal of Vertebrate Paleontology 31 (4), 2011, pp. 895-901
- ^ A b Luke T. Holbrook: Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132, 2001, pp. 1-54
- ^ Leonard B. Radinsky: Origin and Early Evolution of North American Tapiroidea. Peabody Museum of Natural History Yale University Bulletin 17, 1963, pp. 1-106
- ^ Robert M. Schoch: A review of the Tapiroids. In: Donald R. Prothero and Robert M. Schoch (Eds.): The evolution of Perissodactyls. New York and Oxford, 1989, pp. 298-320
- ^ Pieter Missiaen and Philip D. Gingerich: New Early Eocene Tapiromorph Perissodactyls from the Ghazij Formation of Pakistan, with Implications for Mammalian Biochronology in Asia. Acta Palaeontologica Polonica 57 (1), 2012, pp. 21-34
- ^ Luke T. Holbrook, Spencer G. Lucas, and Robert J. Emry: Skulls of the Eocene Perissodactyls (Mammalia) Homogalax and Isectolophus. Journal of Vertebrate Paleontology 24 (4), 2004, pp. 951-956
- ↑ MC Maas, ST Hussain, JJM Leiders and JGM Thewissen: A new isectolophid tapiromorph (Perissodactyla, Mammalia) from the Early Eocene of Pakistan. Journal of Paleontology 75 (2), 2001, pp. 407-417
