Masillamys

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Masillamys
Skeleton of Masillamys

Skeleton of Masillamys

Temporal occurrence
Lower to Middle Eocene
55.8 to 43.4 million years
Locations
Systematics
Higher mammals (Eutheria)
Euarchontoglires
Glires
Rodents (Rodentia)
Ischyromyidae
Masillamys
Scientific name
Masillamys
Tobien , 1954

Masillamys is an extinct genus from the rodent group . It waswidespreadfrom the Lower to the Middle Eocene 56 to 43 million years ago in what is now Europe . Particularly well-preserved skeletal finds have been preserved from the Messel pit , other fossil remains are from the Geiseltal and various locations in France . It is a small representative of rodents the size of today's rats. The animals had a relatively robust build with a long tail and comparatively short limbs. According to the skeleton structure, they climbed trees. The genus was scientifically introduced in 1954. From a systematic point of view, Masillamys is one of the earliest representatives of rodents. The exact position within the group is under discussion. Two types are currently known.

features

Masillamys is a small representative of the rodents. There are several skeletons, some of them complete, from the Messel pit . The animals reached a head-trunk length of around 20 cm and had a comparatively long tail. In terms of size, they roughly corresponded to today's rats , based on the size of the molars, a body weight of 75 to 130 g is assumed. The Massilamys fossil metric is largely undescribed, so far only the teeth have received more attention. In general, they were robustly built animals with short limbs, in which the front compared to the rear were proportioned shorter. The skull is mostly referred to as sciuromorphic , that is, like a squirrel , which results in a narrow infraorbital foramen and a masseter muscle that extends to the eye region . The problem in this context is that Masillamys has a large pre-eye hole and thus more closely resembles the hystricomorphic type known from porcupines .

The incisors corresponded to the incisor teeth of today's rodents and were hypertrophied and rootless. The rear dentition consisted of two premolars and three molars on each half of the jaw in the upper row of teeth , the anterior premolar was missing in the lower row . The top, last premolar and the three following molars each had three roots, the foremost premolar was small, like a pin, with only one root. The chewing surface pattern of the upper molars was rather humped ( bunodont ), the typical transverse ridges or yokes of today's rodents did not yet exist. There were a total of four main cusps (metaconus and paraconus on the cheek side and hypoconus and protoconus on the tongue side). The hypoconus was well developed and almost as large as the protoconus. The metaconus was usually shifted further inward than the paraconus. On the diagonal between the meta- and protoconus, a small minor hump rose with the metaconulus. This was hardly or not connected to the protoconus or the hypoconus. It was particularly pronounced on the last molar. The lower molars each had two roots, the molars were long and rectangular in outline. They also had a four-humped chewing surface pattern (protoconid and hypoconid on the cheek side and metaconid and entoconid on the tongue side). The paraconid, on the other hand, was missing, it had completely absorbed into the anterior edge bulge. On the anterior and posterior edge of the tooth, the anterolophid (“pre-yoke”) and the posterolophid (“post-yoke”) each ran a crossbar, the anterolophid connecting the protoconid and the metaconid, the posterolophid the hypoconid and the entoconid. The latter ridge weakened significantly from the first to the last molar, and its attachment shifted from the entoconid to the mesoconid, a smaller additional cusp. In Ailuravus and other very early rodents, the posterolophid was often not developed at all. A massive cingulum (a low bulge of enamel) settled on the posterior edge of the tooth. The length of the upper molar row (excluding the anterior premolars) was 9.2 to 9.9 mm, the lower one was 10.7 to 10.8 mm long. The lower second molar measured around 2.1 mm in length in smaller individuals, and up to 2.9 mm in larger individuals.

Fossil finds

Finds of Masillamys have so far been reported from Central and Western Europe . The most important and extensive material came to light in the Messel mine near Darmstadt. At least 20 skeletons, some of them complete, were found there, the good preservation of which can still be seen in part from the body outlines and fur. The finds belong to the Middle Eocene and are absolutely dated to an age of around 48 million years. In turn, a few lower jaw fragments are known from the almost equally old find complex in the Geiseltal near Halle (Saale). They all come from the Cecilie IV site in the upper middle coal, which makes them one of the most recent finds from Masillamys from a stratigraphic point of view . According to the molars, it is a larger representative of the genus. Other fossil remains are from France , but consist largely of individual tooth remnants. Mention can be made here of the fossils of the cleft filling of Vielase, which belongs to the complex of finds of the phosphorite deposits of Quercy in southern France. Mailhac in the Minervois and Saint-Martin-de-Londres , both also located in the south of the country, produced further remains . In the Paris Basin was Masillamys detected at several sites. The “Messelmaus” was found relatively frequently in Grauves, while it only rarely occurs in the nearby Prémontré . Investigations in Saint-Agnan uncovered more than 80 individual teeth from a total of eleven different individuals. All of the French sites mentioned are possibly somewhat older than the skeletons of Messel, with Prémontré being one of the oldest records belonging to the beginning of the Lower Eocene.

Paleobiology

Skeleton of Masillamys from the Messel Pit with clearly visible outline drawing of the body

Overall, the physique of Masillamys seems rather clumsy, the limbs are short in relation to the trunk and equipped with broad hands and feet. In general, these features would speak for an underground, burrowing ( fossorial ) way of life, comparable to today's voles , which have similar body proportions. However, the narrow and high claws and the rather long tail of Masillamys contradict this . On the other hand, the wide hands and feet could also indicate a swimming movement, as they allow a high water displacement and could therefore be used as a paddle. However, swimmers like swimming rats usually have longer lower legs than their thighs - this increases the leverage when moving in the water - and generally longer hind limbs, which is not the case with Masillamys . On the other hand, narrow, high-arched claws are typical signs of tree-climbing ( arboreal ) animals, which pry them as barbs into the tree bark. A long tail also helps, as it could be used both for balancing while climbing and for steering while jumping. According to some finds from Messel with bacterial tracing of the body outline (bacteriography), the tail of Masillamys was probably only sparsely hairy. As a result, a function as a balance organ comes into consideration, as is the case with today's climbing rats or tree rats . The relatively short rear and front legs, however, did not allow the nimble running and jumping of today's squirrels . As a result, Masillamys should have been climbing in the branches. As a result, the animals' habitus differs a little from the other rodents handed down from Messel such as Ailuravus and Hartenbergeromys . The former is significantly larger, the latter smaller, and both have in common the considerably longer hind legs compared to the forelimbs.

Systematics

Masillamys is an extinct genus from the order of rodents (Rodentia). The position of the genus is not clear within the order. Numerous authors assign them to the Ischyromyidae family , which is also extinct. The Ischyromyidae represent one of the most pristine rodent groups known to date. The predominant fossil record of the family amounts to North America , some forms also occurred in Eurasia , their temporal range encompasses the Lower and Middle Eocene . The animals had a body structure that was largely reminiscent of croissants , special characteristics are their often protrogomorphic skulls with a small infraorbital foramen , a crest , a non-ossified tympanic bladder , a croissant-like structure of the lower jaw ( sciurognath ) and an ancient set of rodents, in which the penultimate premolar is small and pin-like. The molars generally have low crowns and still have a simple chewing surface design. The Ischyromyidae are sometimes classified as the basal group of the Sciuromorpha , while other authors list them as the main group representatives of rodents in general. Sometimes Masillamys is part of the Ischyromyidae in the subfamily of Microparamyinae together with the Hartenbergeromys, which is also found in Messel . This initially took place as a tribe of the Microparamyini in the subfamily of the Reithroparamyinae, but later the tribe was raised to subfamily level. Problematic in this context is the position of the Paramyidae family, which on the one hand is considered to be independent, on the other hand is also split up into the Ischyromyidae. The two groups differ largely in the topography of the molar chewing surfaces, whereby the Ischyromyidae are estimated to be somewhat more modern than the Paramyidae due to the pronounced shear ridges. Further differences can be found in the design of various foramina on the skull. In the case of the independence of both families, Masillamys and Hartenbergeromys would have to be assigned to the Paramyidae, since the Microparamyinae are within them. In contrast, some scientists see Masillamys in turn embedded in the Theridomyidae family , which was largely restricted to Europe and existed from the Eocene to the Oligocene . It may have its origin within the Ischyromyidae. The representatives also had low tooth crowns, which were initially formed by four main humps, but in the further course of their tribal history received increasingly more shear bars. Masillamys was originally considered to be part of the Pseudosciuridae family, which sometimes includes original forms of theridomyid rodents. There was also an assumption of a closer relationship with the porcupine relatives .

Two types of masillamys are recognized:

A third, as yet undescribed species is possibly from the Geiseltal , which is characterized by its particular size and individual abnormalities in the tooth structure.

The first scientific description of Masillamys was presented by Heinz Tobien in 1954. It is based on some poorly preserved skeleton finds from the Messel mine . One of them with the excised left row of teeth represents the holotype (copy number HLMD -Me-1). Together with the genus Tobien introduced a total of three species. M. beegeri represents the nominate form , and he named another one with M. krugi . He gave the third one the scientific name M. parvus . Tobien distinguished the three types on the basis of special tooth features and the size of the individuals. His overall description is largely limited to the representation of the teeth. Tobien derived the generic name Masillamys from the original name for Messel ("Masilla") used in the Lorsch Codex around 800 AD and from the Greek word μῦς ( mŷs ) for "mouse", translated it means something like "Messel mouse" ". In a short article published fourteen years later, Jean-Louis Hartenberger suspected that M. beegeri and M. krugi are identical. He moved M. parvus to the genus Microparamys due to the different characteristics of the posterior teeth . Hartenberger did not consider both genera to be closely related to each other, which he justified with the respective structure of the infraorbital foramen . In 1999 synonymisierte then Gilles Escarguel in its general handling of under- and mitteleozänen Rodents Western Europe M. krugi with M. beegeri . For the former Masillamys species M. parvus , he also introduced the new genus Hartenbergeromys . The second, now recognized species of Masillamys , M. mattaueri , was established by Hartenberger in 1975. The type material consists of individual teeth from the Mas de Gimel in the south of France in the Languedoc region . As a further representative, Louis Thaler named M. cartieri in 1966 on the basis of remains of teeth from Egerkingen in Switzerland (the form had originally been used by Hans Georg Stehlin and Samuel Schaub in 1951 under the name Adelomys cartieri ). In 1968, however, Hartenberger moved it to the newly created genus Protadelomys within the Pseudosciuridae family.

literature

  • Gilles Escarguel: Les rongeurs de l'Eocène inférieur et moyen d'Europe occidentale. Systématique, Phylogénie, Biochronologie et Paléobiogéographie des niveaux-repères MP7 à MP14. Palaeovertebrata 28 (2-4), 1999, pp. 89-351
  • Irina Ruf and Thomas Lehmann: Rodents: Success story with a bite. In: Stephan SK Schaal, Krister T. Smith and Jörg Habersetzer (eds.): Messel - A fossil tropical ecosystem. Senckenberg-Buch 79, Stuttgart, 2018, pp. 263–269
  • Heinz Tobien: Remnants of rodents from the Middle Eocene from Messel near Darmstadt. Notes from the Hessian State Office for Soil Research 82, 1954, pp. 13-29

Remarks

  1. ↑ The hypoconus at Masillamys was originally referred to by Heinz Tobien in 1954 as "Pseudypoconus" ("Pseudhypoconus"), as it was believed to have arisen from the division of the protoconus in front of it. In contrast, the corresponding hump in Ailuravus , an early rodent that also appeared in the Messel Pit, was rated as a real hypoconus, as it had developed from the tongue-side cingulum (see Tobien 1954). The differentiation of the origin of the hypoconus goes back to Hans Georg Stehlin in the early 1910s (see Hans Georg Stehlin: Die Säugetiere des Schweizerischen Eocaens. Critischer Catalog der Material. Abhandlungen der Schweizerische Paläontologische Gesellschaft 56, 1916, pp. 1209–1556, herein p . 1534). The early rodents, however, show themselves to be very variable with regard to the hypoconus and its position, partly also within a genus, so that later authors largely dispensed with this distinction and only spoke of the hypoconus (see Wood 1962, Escarguel 1999).

Individual evidence

  1. ↑ a b c d e f g h i j Gilles Escarguel: Les rongeurs de l'Eocène inférieur et moyen d'Europe occidentale. Systématique, Phylogénie, Biochronologie et Paléobiogéographie des niveaux-repères MP7 à MP14. Palaeovertebrata 28 (2-4), 1999, pp. 89-351
  2. ↑ a b c d Heinz Tobien: Remnants of rodents from the Middle Eocene from Messel near Darmstadt. Notes from the Hessian State Office for Soil Research 82, 1954, pp. 13-29
  3. ↑ a b c d Irina Ruf and Thomas Lehmann: Rodents: Success story with bite. In: Stephan SK Schaal, Krister T. Smith and Jörg Habersetzer (eds.): Messel - A fossil tropical ecosystem. Senckenberg-Buch 79, Stuttgart, 2018, pp. 263–269
  4. ^ A b Wighart von Koenigswald, Gerhard Storch and Gotthard Richter: Rodents - At the beginning of a great career. In: Stephan Schaal and Willi Ziegler (eds.): Messel - A shop window into the history of the earth and life. Senckenberrg-Buch 64, Frankfurt am Main, 1988, pp. 219-222
  5. ↑ Hartmut Haubold: The reference fauna of the Geiseltalium, MP levels 11 to 13 (Middle Eocene, Lutetian). Palaeovertebrata 19 (3), 1989, pp. 81-93
  6. ↑ Meinolf Hellmund: Excursion: Former Geiseltalrevier, southwest of Halle (Saale). From the Vita of the Eocene Geiseltal. In: Jörg Erfurt, Lutz Christian Maul (Hrsg.): 34th meeting of the working group for vertebrate paleontology of the paleontological society March 16-18, 2007 in Freyburg / Unstrut. (Hallesches Jahrbuch für Geoswissenschaften BH 23). (2007), pp. 1-16
  7. ↑ S. Legendre, B. Marandat, B. Sigé, J.-Y. Crochet, M. Godinot, J.-L. Hartenberger, J. Sudre, M. Vianey-Liaud, B. Muratet and J.-G. Astruc: La faune de mammifères de Vielase (phosphorites du Qrercy, Sud de la France): Preuve paléontologique d'une karstification du Quercy dès l'Eocène inférieur. New yearbook for geology and palaeontology, monthly 7, 1992, pp. 414–428
  8. ^ A b Mary R. Dawson: Early Eocene rodents (Mammalia) from the Eureka Sound Group of Ellesmere Island, Canada. Canadian Journal of Earth Sciences 38, 2001, pp. 1107-1116
  9. ↑ a b Kenneth D. Rose: The beginning of the age of mammals. Johns Hopkins University Press, Baltimore, 2006, pp. 1–431 (pp. 316–334)
  10. ↑ Helder Gomes Rodrigues, Laurent Marivaux and Monique Vianey ‐ Liaud: Phylogeny and systematic revision of Eocene Cricetidae (Rodentia, Mammalia) from Central and East Asia: on the origin of cricetid rodents. Journal of Zoological Systematics and Evolutionary Research 48 (3), 2010, pp. 259-268
  11. ^ Mary R. Dawson: Paleogene rodents of Eurasia. Deinsea 10, 2003, pp. 97-126
  12. ^ Albert E. Wood: The Early Tertiary Rodents of the Family Paramyidae. Transactions of the American Philosophical Society 52 (1), 1962, pp. 3-261
  13. ↑ John H. Wahlert: The cranial foramina of protrogomorphous rodents; An anatomical and phylogenetic study. Bulletin of the Museum of Comparative Zoology at Harvard College 146, 1974, pp. 363-410 ( [1] )
  14. ^ Siv Hamre Paus: Reconstruction of the skull of Ailuravus macrurus (Rodentia) from the Eocene of Messel, Germany. Kaupia 11, 2003, pp. 123-152
  15. ↑ a b c Jean-Louis Hartenberger: Les Pseudosciuridae (Rodentia) de l'Eocène moyen et le genre Masillamys Tobien. Comptes rendus hebdomadaires des séances de l'Académie des sciences. Série D 267, 1968, pp. 1817-1820 ( [2] )
  16. ↑ Jean-Louis Hartenberger: Les Pseudosciuridae (Mammalia, Rodentia) de l'Eocène moyen de Bouxwiller, Egerkingen et Lissieu. Palaeovertebrata 3, 1969, pp. 27-61

Web links

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