Zimbakubwa

Zimbakubwa
Temporal occurrence
Lower Miocene
25.6 to 23.1 million years
Locations
East Africa (Kenya)
Systematics
Laurasiatheria Ferae Hyaenodonta Hyainailouridae Hyainailourinae Zimbakubwa
Scientific name
Zimbakubwa
Borths & Stevens , 2019

Simbakubwa is a genus of the order Hyaenodonta , extinct carnivorous mammals that may be close to predators. Fossil remains of the genus were found in western Kenya and consist of individualremains ofteeth, including an almost complete lower jaw, plus a few elements of the musculoskeletal system. The lower jaw is characterized by its extraordinary size, it was correspondingly larger than that of most of today's predators. Size reconstructions indicate a body weight of 280 to 1550 kgfor Simbakubwa . In addition to the enormous size of the lower jaw, the dentition is characterized by its graceful construction and the special design of the rear teeth, which advocates a predominantly cutting use. The age of the finds corresponds to the Lower Miocene with 26 to 23 million years. The Simbakubwa fossil remainswere discovered in scientific field studies in the late 1970s, but the genus was only introduced in 2019.

description

Simbakubwa is one of the largest known representatives of the hyaenodonts. The previous fossil material mainly consists of a lower jaw, an upper jaw fragment and individual isolated teeth. The lower jaw is almost completely preserved, but has been crushed by sediment . It still bears the canine , the last premolars and the last molars , and the alveoli of the third premolar and the first two molars are also preserved. The canine as well as the last premolar and the first two molars are still in the upper jaw; the penultimate premolar is indicated by the corresponding alveolus. The horizontal bone of the lower jaw was built low, its height was only about twice the height of the last molar. The lower edge was drawn out slightly convex in the rear section. Behind the last molar the ascending branch with the crown process rose at an angle of a good 150 °, the process itself was rounded. The masseteric fossa as the attachment point of the masseter muscle on the outside of the ascending branch was indicated above by a strong and below by a less prominent bone ripple.

The upper canine had a teardrop-like cross-section and a sharp edge ran along the cutting surface. The last premolar had a T-shaped outline when viewed from above. In side view, it was characterized by a towering paraconus that overhanged slightly towards the rear. Here the metastyle connected to the posterior edge of the tooth as a flat cone, which at the base took up about two thirds of the length of the paraconus. It had a sharp shear edge which, together with the sharp edge of the rear flank of the Paraconus (Postmetacrista), formed the front part of the crushing scissors . The tooth section was functionally identical to the corresponding formations of the two maxillary molars. As a difference, the respective metastyle showed a relatively greater length here. They were also longer and narrower than the Hyainailouros . The chewing surfaces of the molars were characterized by three main cusps (Para-, Meta- and Protoconus). The paraconus formed the dominant cusp. At the same time it was fused into one unit with the metaconus and protruded slightly above its tip, a typical characteristic of the hyainailouriden hyaenodonts. The tips of the two cusps were only clearly separated by a transverse furrow and thus distinguishable on the first molar; this furrow is missing on the second. The protoconus was small and isolated. The parastyle on the front edge of the tooth was rather low and equipped with individual small peaks, which in turn deviates from Hyainailouros .

The lower canine was laterally narrowed and provided with a deep pit on the inside that ran from the base to the tip. The last premolar had two roots. Its chewing surface was dominated by the tall and pointed protoconid, the rear flank of which formed a sharp edge. As on the maxillary molars, three main domes shaped the chewing surfaces of the lower molars (Para-, Proto- and Metaconid). The trigonid, a raised area of ​​the chewing surface with the three main cusps, took up a large part of the length of the tooth, the talonid, a deeper section, was very short. The effect was reinforced by the extremely shortened talonid on the rearmost molar. The paraconid was lower than the protoconid and, in relation to this, slightly shifted to the tongue side, which was even more evident on the posterior molars than on the anterior. The cusps were laterally narrowed so that their cross-section appeared lenticular or teardrop-shaped. Between the posterior edge of the paraconid (postparacrista) and the anterior edge of the protoconid (preprotocrista) was a deep notch, which took up part of the breaking scissors on the posterior molars. The third main cusp, the metaconid, was, as with almost all hyainailouriden hyaenodonts, greatly reduced in size, which gave the teeth of the Simbakubwa a hypercarnivorous character. The hypoconid, a small hump on the talonid, was only a third of the height of the protoconid. There were no additional bumps here, with the exception of the third molar. Here, however, the talonaid was greatly shortened, as in Hyainailouros . The molars increased in size from front to back, which is to be understood as a characteristic of hyaenodonts. In Simbakubwa , however, the first molar was relatively larger than in the equally huge Megistotherium , whose front molar was extremely shortened. Overall, Simbakubwa had a relatively delicate dentition. In the upper jaw, the first molar measured 4.6 cm in length and 3.0 cm in width, while the values ​​for the second molar were 5.2 and 4.1 cm. The three molars of the lower jaw were 2.9 cm, 4.6 cm and 5.6 cm long, respectively.

In addition to the remains of the skull and teeth, there are also individual elements of the postcranial skeletal system. Below that is a heel bone , which in those with its wedge-shaped hock (tuber calcaneus), the rounded sustentaculum-facet and the position of other joint surfaces of Hyainailouros recalls. Furthermore, individual end links of the toes and fingers were discovered. These had a deeply notched end, as is typical of the hyaenodonts.

Fossil finds

The previously known find material from Simbakubwa comes from the Meswa Bridge site in southwestern Kenya . The important fossil deposit is the oldest known of numerous ancient sites in western Kenya, including Songhor and Rusinga Island . Meswa Bridge lies in the Muhoroni agglomerates at the base of the Koru Formation . The Muhoroni agglomerates are fine to coarse-grained pyroclastic sediments into which gneiss pebbles are occasionally incorporated. The volcanic deposits are crossed by channels that can be traced back to short-term river courses. These reach up to 10 m in thickness, are weakly stratified and contain both floristic and faunistic fossil material. Known since the 1920s and highlighted by the descriptions of fossil primate forms such as Proconsul or Xenopithecus in the 1930s, it was not until the intensive investigations by Martin Pickford in 1978 that a direct focus of science was directed towards the Muhoroni agglomerates. The Meswa Bridge site north of Muhoroni on the north bank of the Meswa River was discovered and extensively excavated between 1979 and 1980. During the scientific field research, significant remains of primates came to light, among them again from Proconsul . Meswa Bridge gained further fame through the evidence of fossil trunk animals such as eozygodons . Radiometric age dating gives the Muhoroni agglomerates an age of 25.6 to 23.2 million years, which corresponds to the Lower Miocene .

The fossil remains of Simbakubwa were also recovered during the excavations in the late 1970s . They consist of a lower jaw, an upper jaw and several isolated teeth of the rear dentition, as well as individual foot and toe bones. All remains are assigned to an individual, which can be deduced, among other things, from the similar signs of wear on the teeth and the lack of double skeletal elements in the postcranium . The original excavators also assigned other limb elements such as a humerus or an ulna and a spoke to the same individual. However, they cannot be clearly assigned to the hyaenodonts and, due to their structure, could rather belong to a representative of the Anthracotheriidae , which in turn are closely related to the hippos .

Paleobiology

What is particularly striking about Simbakubwa is the enormous size of the finds, which far surpass the corresponding skeletal elements of today's predators . Various methods can be used to determine the body weight of extinct forms, some of which are based on the dimensions of the teeth. Taking into account the average size of the teeth of the crushing shears , a body weight of 1308 kg can be assumed for Simbakubwa , which is greater than that of a recent polar bear . Using the dimensions of the third lower molar in hyaenodonts compared to the first lower molar in modern cats with a comparable hypercarnivorous dentition results in a body weight of about 1554 kg. If all predators with a body size of over 100 kg, but partly different dentition, were included, the body weight of Simbakubwa would be 280 kg, which roughly corresponds to that of the largest lions . The predators are only closely related to the hyaenodonta to a limited extent, so the body weight estimates made do not take into account any deviations in the general proportions of the body. In addition, the teeth of the hyaenodonts differ from those of the predators and basically include more teeth in the crushing scissors, which are also relatively shifted further back. The estimates of body size can therefore only be viewed as an approximation of the real weight of extinct forms. Further finds are therefore required to narrow down the information on body weight. Despite the existing inaccuracies, Simbakubwa is one of the largest known hyaenodonts and was only surpassed by a few forms of Hyainailouros and Megistotherium .

The finds from Simbakubwa date back to the Lower Miocene during a period in which the closure of the Tethys resulted in a land bridge to Eurasia and as a result the predators found their way into Africa. The increasing body size of the hyaenodonts could in turn reflect the spread of open landscapes in the course of climatic cooling during this time, which also resulted in a change in the size of some herbivorous mammals. The structure of the teeth in Simbakubwa is typically hypercarnivorous, that is, the molars show a strong specialization in a cutting function, while a perforating or breaking function is regressed. This is indicated, among other things, by the reduced metaconid (perforating) or the shortened talonid and the smaller protoconus (breaking). In today's predators with hypercarnivorous teeth, the meat content in the diet is at least 70%. Further studies on predators showed that from a body weight of 21 kg, more and more prey animals of their own weight class or more are provided. The possibly enormous weight of Simbakubwa puts the shape in the order of magnitude of the extinct Anthracotheriidae (relatives of the hippopotamus ) or of smaller rhinos and proboscis . Insofar as the hyaenodontic form specialized in such large mammals, it occupied an ecological niche that is largely unoccupied today, since predators ignore herbivores of this size with few exceptions. Something similar is assumed for the comparably large megistotherium , whose approximately 66 cm long skull from the Middle Miocene from Jebel Zelten in Libya is one of the largest of a hyaenodont and whose open mouth was probably wide enough to grasp the leg bones of a proboscis. Its robust dentition, like that of other large hyaenodonts at the same time, such as Hyainailouros , differs from the slightly more delicate teeth of Simbakubwa , which is expressed, among other things, in the wider metastyle of the upper jaw molars in the former two compared to the narrow ones in the latter. With their higher proportion of tooth enamel, wider metastyle counteract increased abrasion with permanent use of the teeth and thus keep the sharp edges of the crushing scissors in function. The changes in the dentition morphology from the lower Miocene Simbakubwa to the Middle Miocene hyaenodonts, which can be recognized as a result , in turn indicate the constant adaptations of the representatives of this group to the environmental conditions during this time (in some cases it is also assumed that the delicate dentition and the low lower jaw of Simbakubwa compared to Robust dentition with a high lower jaw in Megistotherium possibly reflects a sexual dimorphism , but so far there is too little evidence to support corresponding statements). However, specializing in hunting particularly large mammals has direct consequences for the ecology of early Miocene predators in Africa. Since the reproductive rate of large mammals is very slow and the animals in turn are sometimes sensitive to environmental changes, the resulting restructuring of the fauna can sometimes have a serious impact on the large predators.

Further statements on the paleobiology of Simbakubwa are difficult to make due to the small amount of fossil material. The heel bone is largely identical to that of Hyainailouros match, but differs somewhat from that of Kerberos , a large Hyaenodonten from the Middle Eocene of Europe. A sole gait is being reconstructed for the latter , while the completely traditional foot of Hyainailouros refers to a partially developed toe gait , which in some cases also supported powerful jumps. The habitual change from soleplate to toe gait in today's predators is a reaction to the adaptation to open landscapes in the course of climatic changes. Life in savannah-like areas largely requires greater mobility through greater distances traveled. In this case, a higher foot position such as in the toe gait combined with the reduction of the foot area in direct contact with the ground supports such a way of life and is more energy-saving in open spaces.

Systematics

Internal systematics of the Hyainailouridae according to Borths and Stevens 2019   Hyainailouridae    Apterodontinae   Hyainailourinae    Orienspterodon      Kerberos      Pterodon      Hemipsalodon      Akhnatenavus      Paroxyaena      " Pterodon "      Sectisodon      Megistotherium      Leakitherium      Mlanyama      Metapterodon      Exiguodon      Metapterodon      Falcatodon      Zimbakubwa      Hyainailouros      Sivapterodon

Simbakubwa is a genus from the extinct family of Hyainailouridae within the extinct also order the Hyaenodonta . The Hyaenodonta were originally part of the Creodonta group , which in German sometimes have the somewhat misleading designation "primal predators " and which were regarded as the sister group of today's carnivores (Carnivora) within the superordinate group of the Ferae . According to phylogenetic studies, the Creodonta do not form a closed group. Therefore they were split into the Hyaenodonta and the Oxyaenodonta . The crushing shears , which are shifted further back in the teeth compared to the predators, are typical for both groups . In the case of hyaenodonts, the second upper and third lower molars are usually involved. The tribal history of the hyaenodonts lasted for a long period from about the Middle Paleocene around 60 million years ago to the Middle Miocene a good 9 to 10 million years ago. As a typical feature of the Hyainailouridae, the para- and metaconus united to the amphiconus can be considered, whereby the former dominates the latter (in the related Hyaenodontidae the relationship is reversed). Within the Hyainailouridae belongs Simbakubwa to the evolutionarily younger forms within the subfamily of Hyainailourinae . In these, the degree of fusion of the para and metaconus is very advanced. Other giant forms such as Hyainailouros come into consideration as closely related representatives .

The first scientific description of Simbakubwa was in 2019 by Matthew R. Borths and Nancy J. Stevens . It is based on the fossil material from the sub-Miocene Muhoroni agglomerates at the Meswa Bridge site in western Kenya . A left branch of the lower jaw was selected as the holotype , on which the canine , the last premolar and the third molar are still formed and the alveoli of the third premolar and the first two molars are preserved (copy number KNM -ME 20A). The other finds from Meswa Bridge are classified as paratypes . The finds probably all belong to a single individual, but the authors of the first description only draw this conclusion with reservations, as the find material had been excavated several decades before the first description and there is no direct documentation associated with the find objects. The generic name Simbakubwa is taken from Swahili and is made up of the words simba for "lion" and kubwa for "large". S. kutokaafrika is the only species so far . Here, too, the specific epithet comes from Swahili and means something like "from Africa". The binomial would therefore have to be translated as “big lion from Africa”.

literature

• Matthew R. Borths and Nancy J. Stevens: Simbakubwa kutokaafrika, gen. Et sp. nov. (Hyainailourinae, Hyaenodonta, 'Creodonta,' Mammalia), a gigantic carnivore from the earliest Miocene of Kenya. Journal of Vertebrate Paleontology, 2019, p. E1570222, doi: 10.1080 / 02724634.2019.1570222

Individual evidence

1. ↑ ^{a b c d e f g h i j k l} Matthew R. Borths and Nancy J. Stevens: Simbakubwa kutokaafrika, gen. Et sp. nov. (Hyainailourinae, Hyaenodonta, 'Creodonta,' Mammalia), a gigantic carnivore from the earliest Miocene of Kenya. Journal of Vertebrate Paleontology, 2019, p. E1570222, doi: 10.1080 / 02724634.2019.1570222
2. ↑ D. Tab Rasmussen, Christopher D. Tilden and Elwyn L. Simons: New Specimens of the Giant Creodont Megistotherium (Hyaenodontidae) from Moghara, Egypt. Journal of Mammalogy 70 (2), 1989, pp. 442-447
3. ↑ Peter Andrews, Terry Harrison, L. Martin and Martin Pickford: Hominoid Primates from a New Miocene Locality Named MeswaBridge in Kenya. Journal of Human Evolution 10, 1981, pp. 123-128
4. ^ ^{A b} Terry Harrison and Peter Andrews: The anatomy and systematic position of the early Miocene proconsulid from Meswa Bridge, Kenya. Journal of Human Evolution 56, 2009, pp. 479-496
5. ↑ Martin Pickford and Pascal Tassy: A new species of Zygolophodon (Mammalia, Proboscidea) from the Miocene hominoid localities of Meswa Bridge and Moroto (East Africa). New yearbook for geology and palaeontology, monthly booklet 4, 1980, pp. 235-251
6. ↑ Pascal Tassy and Martin Pickford: Un nouveau mastodonte zygolophodonte (Proboscidea, Mammalia) dans le Miocène inférieur d'Afrique orientale: Systématique et paleoenvironnement. Geobios 16 (1), 1983, pp. 53-77
7. ^ Lars Werdelin: Chronology of Neogene mammal localities. In: Lars Werdelin and William Joseph Sanders (eds.): Cenozoic Mammals of Africa. University of California Press, Berkeley, 2010, pp. 27-43
8. ↑ Michael Morlo and Jörg Habersetzer: The Hyaenodontidae (Crerodonta, Mammalia) from the lower Eocene (MP 11) of Messel (Germany) with special remarks on new x-ray methods. Courier Forschungsinstitut Senckenberg 216, 1999, pp. 31–73
9. ↑ Anthony R. Friscia and Blaire Van Valkenburgh: ecomorphology of North American Eocene carnivores: evidence for competition between carnivorans and Creodonts. In: A. Goswami and A. Friscia (Eds.): Carnivoran Evolution: New Views on Phylogeny, Form, and Function. Cambridge University Press, 2010, pp. 311-341
10. ^ Blaire Van Valkenburgh: Déjà vu: the evolution of feeding morphologies in the Carnivora. Integrative and Comparative Biology 47, 2007, pp. 147-161
11. ↑ Floréal Solé and Sandrine Ladevèze: Evolution of the hypercarnivorous dentition in mammals (Metatheria, Eutheria) and its bearing on the development of tribosphenic molars. Evolution & Development 19 (2), 2017, pp. 56-68
12. ↑ Chris Carbone, Amber Teacher and J. Marcus Rowcliffe: The Costs of Carnivory. PLoS Biology 5 (2), 2007, p. E22, doi: 10.1371 / journal.pbio.0050022
13. ^ Robert Joseph Gay Savage: Megistotherium, gigantic hyaenodont from Miocene of Gebel Zelten, Libya. Bulletin of the British Museum of Natural History (Geology) 22, 1973, pp. 483-511 ( biodiversitylibrary.org )
14. ↑ Floréal Solé, Eli Amson, Matthew Borths, Dominique Vidalenc, Michael Morlo and Katharina Bastl: A New Large Hyainailourine from the Bartonian of Europe and Its Bearings on the Evolution and Ecology of Massive Hyaenodonts (Mammalia). PLoS ONE 10 (9), 2015, p. E0135698 doi: 10.1371 / journal.pone.0135698
15. ↑ Michael Morlo, Gregg Gunnell, and P. David Polly: What, if not nothing, is a creodont? Phylogeny and classification of Hyaenodontida and other former creodonts. Journal of Vertebrate Paleontology 29 (3 suppl), 2009, p. 152A
16. ↑ Kenneth D. Rose: The beginning of the age of mammals. Johns Hopkins University Press, Baltimore, 2006, pp. 1–431 (pp. 122–126)
17. ↑ Floréal Solé, Eli Amson, Matthew Borths, Dominique Vidalenc, Michael Morlo and Katharina Bastl: A New Large Hyainailourine from the Bartonian of Europe and Its Bearings on the Evolution and Ecology of Massive Hyaenodonts (Mammalia). PLoS ONE 10 (9), 2015, p. E0135698, doi: 10.1371 / journal.pone.0135698