Fruitafossor
| Fruitafossor | ||||||||||||
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Fruitafossor |
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| Temporal occurrence | ||||||||||||
| Kimmeridgian ( Jurassic ) | ||||||||||||
| 150 to 148 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Fruitafossor | ||||||||||||
| Luo & Wible , 2005 | ||||||||||||
Fruitafossor is an extinct genus early mammals from the Mesozoic . So far it has only been documented on the basis of a skeleton from the Morrison Formation in the US state of Colorado , whichdates backto the Upper Jurassic around 150 million years ago. It was a very small representative of the mammals, who probably lived a burrowing way of life based on the structure of the limbs. Particularly noteworthy are individual anatomical features in the area of the spine and the dentition, whichanticipatecertain developments in today's secondary articulated animals , without Fruitafossor being closely related to them. The closer systematic relationships are not known precisely because of the peculiarities of the skeleton structure. The genus was scientifically introduced in 2005. Their discovery and description broadened our knowledge that the Mesozoic mammals were already adapted to numerous different lifestyles.
features
Fruitafossor was a small representative of the original mammals . So far there is only a partial skeleton, which consists of the lower jaw along with some skull bones and parts of the body skeleton. The weight of the adult animal is estimated at only around 6 g, based on the almost complete lower jaw. This was a total of 12.5 mm long and had a convex course at the lower edge, the lower jaw body thereby continuously merged into the articular branch, which is reminiscent of the equally primitive eutriconodonta . In contrast to this, a small angled extension was formed at the rear end. The posterior opening of the mandibular canal on the inside near the articular process also formed the foramen mandibulae and was located in the very broad Meckel's furrow, a combination of features that is not found in other Mesozoic mammals. The Meckel's furrow contained the Meckel's cartilage , from which the ossicles of the middle ear were formed in the higher mammals . The most striking properties of Fruitafossor can be found in the dentition and tooth structure. The upper teeth row is not fully delivered, as opposed to the lower, the dental formula is: . The individual teeth had a nail to tubular shape with high crowns. Almost all of them only had singular roots that were open at the molars . The lack of tooth enamel was also remarkable . In principle, the teeth were similar to those of today's armadillos , but also showed certain similarities with those of the aardvark . The open roots meant that tooth material could be reproduced permanently and that the teeth would grow for a lifetime. Such teeth are now formed in gnawing animals or those with hard food components, such as grasses , whereby they are exposed to high abrasion. Overall, the teeth represent an extreme specialization for an early mammal. The teeth were relatively small overall, but increased continuously up to the last premolar , which also represented the largest tooth.
About 40% of the body skeleton has been handed down. It shows some ancient features, but also has special adaptations. On four of the five lumbar vertebrae, there were additional articular surfaces ( xenarthrals or secondary joints) on the transverse processes for greater entanglement of the spine, which is mainly known today from the secondary articulated animals of South America. In addition, attachment points for lumbar ribs were formed, as they still exist in today's monastery . The shoulder blade also resembled that of the monotonous and had a saddle-shaped joint pit, which was formed from the shoulder blade itself and the raven bone . The humerus was extremely massive, a spherical head, as it is typical for the higher mammals , was missing, the shaft showed a clear rotation. It was also characterized by a hypertrophied groin as the attachment point for the shoulder muscles (deltopectoral groin). The elbow joint was very protruding laterally and took up around 65% of the entire humerus length, which is significantly more than in all modern mammals. An original feature was also the separation of the articular surfaces for the ulna and the radius . The ulna had an extensive upper articular process ( olecranon ), which corresponded to 66% of the total bone length. The hand is complete and consisted of four rays (I to IV). All carpal bones showed significant longitudinal compression, as did the metacarpal bones and phalanges from the second to fourth rays, whereby the first phalanges of the two median rays (II and III) were the shortest. In contrast to these rather block-like bones, those of the first ray appeared long-limbed and graceful. The end links of the fingers were the greatest length at each ray, they ended narrow and pointed, but had no special notches.
Reference
Fruitafossor has so far only been occupied by a single partial skeleton from the Fruita Palaeontological Area of the Morrison Formation , a lithostratigraphic unit of the Upper Jurassic in the central area of the USA . The Morrison Formation stretches in a wide band from Montana to southern Arizona and New Mexico and covers an area of approximately 1.56 million square kilometers. It consists of predominantly terrestrial deposits that include limestone , clay / silt and sandstones as well as conglomerates between which individual volcanic layers are embedded. The best outcrop conditions can be found in the area of the Colorado Plateau , where several layers can be kept apart. The abundant fossil material consists of invertebrates and vertebrates , in the latter case dinosaurs and early mammals are of particular importance. The paleontological material in connection with the geological deposits suggests an open savannah landscape interspersed with alluvial forests , lakes and wetlands.
The Fruita Palaeontological Area is about 1 to 2 km southwest of Fruita along the Colorado River in the US state of Colorado. Here the Morrison Formation reaches a thickness of around 180 m and consists of three layers, of which the Brushy Basin Member is the youngest and most important, as a larger part of the finds come from here. It is mainly composed of gray-green clay / silt stones. Radiometric age measurements from the intermediate layers of ash showed values from 148.1 to 150.3 million years and thus place the layer member in the kimmeridgium . The Fruita Palaeontological Area has been known since 1891 for some fossil remains, it developed into one of the most important find regions of the Morrison Formation in the following period. Its importance lies in the great variety of finds on an area of only 1 km², which includes not only bony remains but also trace fossils , coprolites and traditional soft tissue . Remains of scale lizards such as Eilenodon or of dinosaurs such as Ceratosaurus and Fruitadens should be emphasized . The first evidence of early mammals was found in the late 1970s, and almost a dozen species have survived to this day, including priacodon from the group of eutriconodonta or glirodon from the group of multituberculata . In addition, representatives of the Symmetrodonta and Dryolestida occur, the latter being the most common mammals in the Fruita Palaeontological Area .
Paleobiology
Of all known mammals of the Morrison Formation, Fruitafossor is the smallest representative to date, which has a body weight comparable to that of today's narrow-footed pouch mice . In terms of their entire body, Fruitafossor resembles the recent ant urchins , some of which live underground. The construction of the forelegs, for example the humerus with a massive deltopectoral groin, the protruding elbow joint and the clearly elongated olecranon , which indicates excellent shoulder and arm muscles, speaks for such a digging way of life at Fruitafossor , whereby the olecranon in particular is very strong Triceps speaks. The broad and strong development of the hand bones also suggests digging activities; In contrast to many of today's burrowing mammals, the end links of the fingers are not split. Two feature complexes can be emphasized as remarkable, which anticipate some extreme specializations in the secondary articulated animals . These include, on the one hand, the secondary joints formed on the lumbar vertebrae, which stabilize the spine more strongly. These additional articulations help armadillos and anteaters , for example , some of which also pursue a burrowing way of life or break up hard termite burrows, to cushion the stronger rotational movements of the spine. The other specialization is found in the dentition with its homodontic teeth, which lack tooth enamel and whose roots are open. The armadillos and, to a certain extent, the aardvark , which mainly feed on insects which form colonies , have a comparable set of teeth . Perhaps that is why the main food at Fruitafossor was also insects.
Systematics
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Systematic position of Fruitafossor within mammals according to Luo et al. 2005
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Fruitafossor is a genus from the class of mammals whose relationship is not exactly known. The difficulties in classifying and determining the systematic position are primarily based on the deviating design of the teeth, which are usually used as the main feature for the classification of mammals. In its skeleton structure, Fruitafossor has a mixture of original ( plesiomorphic ) and derived ( autapomorphic ) characteristics. The design of the lower jaw with the pronounced Meckelian furrow and the resulting Meckelian cartilage suggests that the middle ear was not yet fully developed, but was still connected to the lower jaw. As a result, Fruitafossor must be placed in a very early radiation of mammals, since the separation of the middle ear from the lower jaw is a typical feature of the crown group of developed mammals ( marsupial and higher mammals ). The downward curve of the lower edge of the lower jaw with its seamless transition into the joint is reminiscent of the Eutriconodonta , an early mammalian group of the Mesozoic Era in the line of development to the modern representatives of the crown group. Fruitafossor differs from most other Mesozoic mammals and their immediate predecessors ( Mammaliaformes ) by the presence of a small crown process on the lower jaw. In the shoulder girdle and especially in the area of the shoulder blade with the developed raven bone , there are again similarities to the monotones and thus clear differences to the phylogenetically younger mammal forms. In contrast, the single-rooted molars, the pronounced front dentition and the design of the hand with the very slender first finger form noticeable deviations from the monotones, while the extremely short and wide metacarpal bones and phalanges have no equivalents in the eutriconodonta and multituberculata . Due to the conspicuous combinations of characteristics, Fruitafossor cannot currently be assigned to any of the previously known orders and families of mammals.
The genus Fruitafossor was first described scientifically in 2005 by Zhe-Xi Luo and John R. Wible . The basis was the only known skeleton from the Fruita Palaeontological Area in the US state of Colorado , which is recorded as a holotype with the specimen number LACM 150948 and is kept in Los Angeles . The name Fruitafossor refers on the one hand to the region of discovery, on the other hand to the burrowing way of life of the animals as it could be reconstructed on the basis of the skeletal features ( Latin fossor for "graves"). The only named species is Fruitafossor windscheffeli . The specific epithet windscheffeli refers to Wally Windscheffel, who found the fossil. The discovery of Fruitafossor shows that the early mammals and mammal-like forms of the Mesozoic Era already had a high degree of adaptation to different biotopes , as, among other things, at about the same time as Castorocauda there was also a beaver-like animal specializing in aquatic life, or with Haldanodon, a desmanes comparable Shape are known.
Individual evidence
- ^ A b John R. Foster: Preliminary body mass estimates for mammalian genera of the Morrison Formation (Upper Jurassic, North America). PaleoBios 28 (3), 2009, pp. 114-122
- ↑ a b Zhe-Xi Luo: Developmental patterns in Mesozoic evolution of mammal ears. Annual Review of Ecology, Evolution, and Systematics 42, 2011, pp. 355-380
- ↑ a b c d e f Zhe-Xi Luo and John R. Wible: A Late Jurassic Digging Mammal and Early Mammalian Diversification. Science 308, 2005, pp. 103-107
- ↑ Debra S. Jennings, David M. Lovelace and Steven G. Driese: Differentiating paleowetland subenvironments using a multi-disciplinary approach: An example from the Morrison formation, South Central Wyoming, USA. Sedimentary Geology 238, 2011, pp. 23-47
- ↑ Christine E. Turner and Fred Peterson: Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem — a synthesis. Sedimentary Geology 167, 2004, pp. 309-355
- ↑ Bart J. Kowallis, Eric H. Christiansen, Alan I. Deino, Fred Peterson, Christine F. Turner, Michael J. Kunk and John D. Obradovich: The age of the Morrison Formation. Modern Geology 22, 1998, pp. 235-260
- ^ Richard J. Butler, Peter M. Galton, Laura B. Porro, Luis M. Chiappe, Donald M. Henderson, and Gregory M. Erickson: Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America. Proceedkings of the Royal Society B 277, 2010, pp. 375-381
- ^ TE Rasmussen and George Callison: A new species of triconodont mammal from the Upper Jurassic of Colorado. Journal of Paleontology 55 (3), 1981, pp. 628-634
- ↑ James I. Kirkland: Fruita Paleontological Area (Upper Jurassic, Morrison Formation), Western Colorado: an example of terrestrial taphofacies analysis. New Mexico Museum of Natural History and Science Bulletin 36, 2006, pp. 67-96
- ↑ a b Thomas Martin: Early Mammalian Evolutionary Experiments. Science 311, 2006, pp. 1109-1110