Palaeocastor

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Palaeocastor
Skull from Palaeocastor

Skull from Palaeocastor

Temporal occurrence
Upper Oligocene to Lower Miocene
29.7 to 18.5 million years
Locations
Systematics
Higher mammals (Eutheria)
Euarchontoglires
Rodents (Rodentia)
Beaver (Castoridae)
Palaeocastorinae
Palaeocastor
Scientific name
Palaeocastor
Leidy , 1869

Palaeocastor is an extinct genus of rodents within the beaver family. Fossil finds are from western North America and belong to the Upper Oligocene and Lower Miocene around 30 to 20 million years ago. The representatives of Palaeocastor were smaller than today's beavers and, unlike them, lived in open steppe landscapes where they fed on grasses. On the basis of distinctive skeletal features, an adaptation to a digging way of life can be concluded, whereby mainly the front teeth were used. Also characteristic are thecomplex underground passages leftby Palaeocastor , some of which occur quite frequently and in which a spiral-shaped, descending passage is particularly evident. The fossil traditional burrows are due to this peculiar form under the scientific name Daimonelix out ( "Devil's Corkscrew" or "demons screws"). The genus Palaeocastor was named in 1869, the first burial tunnels were observed in 1891. The fact that Palaeocastor is connected with Daimonelix could be proven at the beginning of the 20th century by joint finds.

features

Palaeocastor was a relatively small representative of extinct beavers . An almost complete skeleton of a medium-sized individual reached a total length of almost 40 cm. In proportion to today's species, he had a body weight of 3.6 to 3.8 kg. The skull was large, but relatively short and wide, analogous to today's beavers. It was between 5.9 and 8.2 cm long, on the zygomatic arches the width was 4.3 to 6.6 cm. The skull of the largest species was 9.2 cm long and 8.8 cm wide. Especially in the more subterranean species, the skull could sometimes have a very massive structure with strong, widened zygomatic arches and a high crest . The rostrum was narrow and comparatively longer than that of today's beavers. The bones that formed the rostrum ( nasal bone and upper jaw ) showed slight thickening, but were not as strong as in rodents living underground today . The central jawbone also had a strong structure that mainly supported the large incisors . The frontal bone was broad and very short, the more the animals followed a burrowing way of life, the clearer the mark. At the back of the parietal bones there were two transverse bone ridges, which served as additional muscle attachment points. In contrast to today's beavers or the similarly living stump-tailed squirrel , the occiput took up the entire back of the skull and was largely vertical.

The lower jaw was very solid and also short; it also increased in robustness the more the representatives were adapted to an underground way of life, which was mainly reflected in the depth and width of the horizontal bone body. The length varied from 5.9 to 6.4 cm. The massive symphysis reached about halfway through the distinctive diastema between the anterior and posterior teeth. On the underside there was an outgrowth of bone to which the digastric muscle attached and which is also found in today's beavers. There was a large mental foramen below the fourth premolar . The articular branches were extremely broad and took up around half of the horizontal bone. The crown process (coronoid process) was higher than the articular process (condylar process). The dentition showed a significant reduction in the number of teeth and dentition formula was: . Thus the dentition consisted of a total of 20 teeth. All teeth were extremely high crowned ( hypsodontic ). As in recent beavers, the incisor had an extremely strong and chisel-like shape, its length was up to 2.5 cm in the lower jaw, in the upper jaw it was slightly shorter. It was located in very strong, tube-like alveoli and protruded obliquely forwards ( procumbent ) , especially in the upper jaw . Overall, it was slightly curved, at the upper end it mostly had strong bevel facets. The posterior dentition had a diastema that was up to 3 cm in length in the upper jaw and only half of the length in the lower jaw. The last premolar, with a length of 4 to 4.5 mm and a rectangular outline, represented the largest tooth in each case, the molars became continuously smaller towards the rear and assumed a square shape, except for the rearmost one, which was round or triangular. The entire back row of teeth was 1.2 to 1.6 cm in length. The chewing surface pattern of the molars was particularly striking. This consisted of several depressions (fossettes) that were formed from dentin and were surrounded by tooth enamel . The shape and number varied greatly with the degree of wear and tear on the teeth.

The postcranial skeleton is well known. The spine was composed of an estimated 7 cervical, 13 thoracic, 6 lumbar, 5 sacrum and possibly 18 tail vertebrae. Special differences to today's beavers occurred on the caudal spine. The vertebral bodies had a round shape and hardly decreased in size over the entire tail. The transverse processes were clearly pronounced, the foremost vertebrae possibly had chevron bones on the underside, as some bony bulges suggest. Overall, the tail turned out to be strong and clearly round in cross section. The humerus was very short and compact at 3.5 to 4.6 cm. It had a wide lower joint end and a narrow upper joint end on which a large joint head was attached. On the side of the shaft there was a massive, partly plate-like bone crest (crista deltoidea) as an attachment point for the chest and shoulder muscles, which reached from the upper end to half of the diaphysis. The cubit measured 4.1 to 5.6 cm and was quite wide at the top, becoming thinner towards the bottom. Overall, it was laterally flattened, the upper articular process ( olecranon ) was around 1 cm long. The spoke was a little lighter than the ulna and had a round shaft and a large head. Both forearm bones were not fused together. The thigh bone was also relatively delicate and the shaft was not as flattened as in the recent beavers. The length was 4.8 to 5.4 cm, the joint ball sat on a distinct, albeit short, neck. In contrast to the more recent forms, there was no bone bridge connecting the large and small trochanter (rolling hillock). A third trochanter existed in the middle of the shaft but was not very pronounced. The lower end of the joint was slightly asymmetrical, but the two joint rollers were about the same length. The bones of the lower leg section did not yet have the noticeable elongation of today's beavers and were a little shorter or the same length as the upper leg section. The tibia was not fused with the fibula , the shaft was round in cross-section and was slightly twisted along the longitudinal axis. Arms and legs ended in five-pointed hands and feet. The individual bones, especially the metapodia and phalanges , were strongly built and short and wide. The respective end links of the fingers and toes ended pointedly, those of the hand exceeded those of the foot in size.

Fossil finds

Palaeocastor fossils
Palaeocastor fossils

Palaeocastor finds are mostly known from the western part of North America and come from the US states of Nebraska , Wyoming , South Dakota and Montana . They date to the Upper Oligocene and the Lower Miocene (local stratigraphic Arikareeum ) and are between 30 and 20 million years old. The earliest finds include several partial skeletons of specimens of different individual ages from the Fort Logan Formation in Montana, which died about 29 to 27 million years ago. The finds of the Sharps Formation , which are exposed on the banks of the White River near the village of Wounded Knee in South Dakota and from where the type material of the genus in the form of a largely complete skull and several other skull fragments originate, are similarly old . Finds, such as a partial skull, were also recovered from the overlying Monroe Creek formation . The Harrison Formation , which covers the Monroe Creek Formation and is distributed over large areas of western Nebraska and eastern Wyoming, is of outstanding importance . It consists of massive, gray-brown and fine-grained sandstones , some of which go back to a former river bed, apart from this on loess brought in by the wind . Especially in the sediment layers influenced by loess, countless remains of Palaeocastor came to light, some of which also include complete skeletons. The Harrison Formation belongs to the Lower Miocene and was determined to an age of 21.3 to 22.9 million years with the help of radiometric measurements on activated volcanic ash layers.

Daimonelix , a fossil tomb

From the loess formations of the Harrison Formation, trace fossils in the form of screw-like winding burial tunnels come under the Ichnotaxon Daimonelix (sometimes also Dæmonelix ; "devil's corkscrew", also "ghost" or "demon screws"). These are spirals, often winding around a vertical axis, which reach a diameter of 11 to 15 cm and penetrate into the subsurface to a depth of 210 to 275 cm, the average angle of inclination being 25 to 30 °. At the lower end there is an elongated passage, which mostly rises at an angle of 2 to 37 ° and is noticeably larger with a diameter of 15 to 22 cm. The length of this passage can reach 200 to more than 450 cm. Noticeable are the smaller bulges on the sides, which usually only occur in flatter, sloping corridors. In some cases, the number of tombs in the Harrison Formation is extremely high, while in the underlying Monroe Creek Formation only a few such trace fossils occur. The fossils were first observed in 1891 by Erwin Hinckly Barbour in outcrops of northwestern Nebraska and made public in the following year under the scientific name Daimonelix . In his publication, Barbour assumed that they were relics of huge freshwater sponges , with the lower extension representing a kind of rhizome . As early as 1893, however, Edward Drinker Cope and Theodor Fuchs independently recognized that these were rodent burial tunnels. The final proof was the discovery of a largely complete skeleton of an extinct beaver in 1904, which was found in the rootstock-like extension. Accordingly, it concerns the burrows of fossil beavers of the genera Palaeocastor and Pseudopalaeocastor . The rootstock-like extension can be interpreted as the actual living chamber of the building, which is built with individual side chambers, the more complex the lower the angle of ascent is. Their rise from the base of the corkscrew-like corridor is interpreted as protection against penetrating moisture. Further examinations also revealed the scratch marks the animals left with their claws and teeth.

Paleobiology

Live reconstruction of Palaeocastor

Palaeocastor belongs to a group of fossil beavers in North America that were adapted to an underground ( fossorial ) way of life. These are also the closely related forms Pseudopalaeocastor , Fossorcastor or Euhapsis , but also the more distant related Migmacastor . The appearance of these species of burrows in the subsurface occurred during a period of cooler climates in the Upper Oligocene and Lower Miocene . This went hand in hand with the formation of open landscapes up to steppes and consequently a stronger radiation of the grasses , which gave the animals the opportunity to develop new ecological niches . Within the genus Palaeocastor, however , the individual species show different degrees of adaptations to the burrowing way of life. It is most strongly developed in the phylogenetically younger, submiocene forms Palaeocastor fossor and Palaeocastor magnus , less prominent in the older, Upper Oligocene Palaeocastor peninsulatus . The unusual shape of the helical grave structures is possibly a result of the changing climatic conditions with more pronounced seasons. The extreme depth and spiral design of the downhill corridor reduced the air circulation between the surface and the living chamber and thus ensured even temperatures there.

Overall, Palaeocastor was an inhabitant of open steppes, fossil finds are usually not in connection with water points. It resembled today's prairie dogs in its way of life , but was slightly larger. Due to the high-crowned molars, a specialization in hard grass can be concluded, which in turn allows the assumption that the majority of the food was acquired above ground. On the skeleton there are numerous features that indicate a burrowing way of life, which are similar to those of the prairie dogs. The third and fourth rays of the hands and feet are slightly elongated, the claws rather short and wide, which is to be understood as a typical characteristic of fossorial mammals that use their limbs to dig ( scratch-digging ), comparable to today's flatland pocket rats within the Rodents. In contrast to today's beavers, the tail has a round cross-section and is not particularly long. The skull shows a broad and flattened structure. In addition, there are clear signs in the teeth that indicate digging activities. For example on the incisors, which also have a flat shape so that straight cutting edges are created. Particularly noteworthy is the inclined forward ( procumbente or proödonte ) position of the upper incisors, which indicates the use of the teeth for digging ( chisel-tooth digging ). In this characteristic, Palaeocastor is more similar to today's mountain pocket rat , which mainly uses its teeth for digging. The upper incisors, however, were not used for digging themselves, but rather anchored the skull in the ground through their forward position. The special position of the incisors also meant that they sat far in front of the lower jaw and thus prevented the penetration of earth into the mouth. The actual digging activity was carried out by the lower incisors, which were steeper in the mouth. The chewing muscles required for this are acceptable due to the crest and the wide zygomatic arches. A series of finds from the Fort Logan Formation of the Upper Oligocene in central Montana, which ranges from juvenile to adult individuals by an early representative of Palaeocastor , shows that obviously within the ontogeny of an animal there is a stronger adaptation from scratch-digging to chisel-tooth digging took place. This directly reflects the phylogenetic development, in which an increase in the use of teeth when digging becomes apparent, which can be seen, among other things, in the increasingly inclined alignment of the upper incisors, which increases from 90 ° in older forms (about the value of today's beavers) increases up to 114 ° in later species.

Studies on the Daimonelix -Grabgängen in Harrison formation revealed that palaeocastor docked the helical thread of its structure mainly with the teeth and wegschaufelte the resulting ground material with the front feet. It is unclear whether it was then carried to the surface with the hind feet or by turning the animal's body with the flat forehead of the head. It is noteworthy that when digging with its teeth, the position of the head of the animal was decisive for the orientation of the downward spiral, since right and left turning spirals could be detected in half of the documented cases; thus there is no preferred direction of rotation of the helical corridor and therefore no proven "handedness" of Palaeocastor . The living room, on the other hand, was laid out with both the incisors and the claws of the front feet. However, the proven claw marks are limited to the side walls and the floor, while tooth marks dominate the ceiling. It is also noticeable that the individual buildings rarely have escape routes. This may indicate that Palaeocastor was defending its burrows, for which the narrow, screw-like passages, the diameter of which a single animal hardly exceeded, were well suited. In individual cases the remains of Zodiolestes , an extinct representative of the martens, were discovered in some burrows . The daimonelix structures can occur locally in great frequency with several hundred specimens in a small space. In the respective regions, the upper exits are mostly in a layer-synchronous paleo-soil as a relic of the former surface, which is indicated by fossilized plant roots, which shows them to be largely simultaneous. It can therefore be assumed that the individual populations of Palaeocastor formed large colonies, comparable to today's prairie dogs. It is unclear whether the extinct beavers lived in a similar social system.

Systematics

Internal systematics of the beavers, mainly the Palaeocastorinae, according to Samuels et al. 2009
 Castoridae  

 Agnotocastorinae


   


 Migmacastorinae


  Palaeocastorinae  

 Palaeocastor


   

 Pseudopalaeocastor


   

 Fossorcastor


   

 Euhapsis






   

 Castorinae


   

 Castoroidinae





Template: Klade / Maintenance / Style
Joseph Leidy

Palaeocastor is a now extinct species from the family of beavers , which today only two species within the genus Castor contains. The beaver family has a rich fossil history that dates back to the Upper Eocene and thus has numerous members in several subfamilies. Today's beavers belong to the Castorinae subfamily . Together with their sister group , the Castoroidinae , which include the better-known genera Castoroides (giant beaver) and Trogontherium (old beaver), they form a group of semi-aquatic representatives. They also represent a younger member of the evolution of the beavers in terms of their tribal history. Palaeocastor , on the other hand, is classified in the subfamily of the Palaeocastorinae , whose members are limited to the Oligocene and Miocene . Its closest relatives are Fossorcastor and Pseudopalaeocastor . The sister group of the Palaeocastorinae in turn form the Migmacastorinae . The members of both subfamilies adapted to a predominantly terrestrial way of life in open landscapes and were sometimes specialized in digging.

Both the semi-aquatic group (Castorinae and Castoroidinae) and the terrestrial living underground group (Palaeocastorinae and Migmacastorinae) form phylogenetic studies show that in each case a monophyletic unit, the former genetic consists According to research, since at least 19.7 million years ago. Since today's beavers also create ditches and channels in the river banks and the steneofibers from the Lower Miocene of Europe and Nothodipoides of the Middle Miocene of Nebraska, which are considered to be semi-aquatic, also dug caves, it can be assumed that the common ancestor of the two groups already had adaptations to the ditch decreed. The beavers that primarily burrowed and inhabited open landscapes died out in the course of the Miocene in competition with other mammals living in this way, such as the Mylagaulidae from the relatives of the stumpy-tailed squirrels . The characteristic gnawing of trees evidently only developed twice within the semi-aquatic group, with Castor and Dipoides , the latter belonging to the relationship of the old and giant beaver. It may have arisen from the original use of the bush vegetation accompanying the shore. Since this characteristic thus developed within the semi-aquatic beaver group, it is to be understood as a derived characteristic.

Numerous species of Palaeocastor have been described in the course of research history, the following are widely recognized:

Initially, most of the species were assigned to other genera, mostly Castor or Steneofiber . The steneofiber barbouri , also introduced by Peterson in 1905, was partly considered to belong to Palaeocastor , but today forms the genus Pseudopalaeocastor . Likewise, the Steneofiber gradatus named by Cope in 1879 was temporarily carried within Palaeocastor , but later referred to Cpacikala .

The first scientific description of Palaeocastor was carried out by the American paleontologist Joseph Leidy in 1869. For this he had several fossil finds, including an almost complete skull of an individual, from outcrops of the White River in today's Badlands National Park in the southwest of the US state South Dakota ( Mauvaises Terres of White River ), which Ferdinand Vandeveer Hayden had discovered. He assigned the species P. nebrascensis to the genus , which he had established more than ten years earlier under the genus name Steneofiber . The holotype includes the skull used by Leidy, but has no designated specimen number. It is kept in the Academy of Natural Sciences in Philadelphia .

Individual evidence

  1. ↑ PS Reynolds: How big is a giant? The importance of method in estimating body size of extinct mammals. Journal of Mammalogy 83 (2), 2002, pp. 321-332
  2. ^ Alfred Sherwood Romer and JT McCormack: A large Palaeocastor from the Lower Miocene. The American Journal of Science, series 5, 15 (85), 1928, pp. 58-60
  3. ↑ a b c d e f g Jonathan Jean-Michel Calede: Skeletal Morphology of Palaeocastor peninsulatus (Rodentia, Castoridae) from the Fort Logan Formation of Montana (early Arikareean): Ontogenetic and Paleoecological Interpretations. Journal of Mammalian Evolution 21, 2014, pp. 223-241
  4. ↑ a b c d e Olof August Peterson: Description of new rodents and discussion of the origin of Daemonelix. Memoirs of the Carnegie Museum 2 (4), 1905, pp. 139-202
  5. ↑ a b c d e f g Joshua X. Samuels and Blaire Van Valkenburgh: Craniodental Adaptations for Digging in Extinct Burrowing Beavers. Journal of Vertebrate Paleontology 29 (1), 2009, pp. 254-268
  6. ^ Clara Stefen: Morphometric considerations of the teeth of the palaeocastorine beavers Capacikala, Palaeocastor and "Capatanka". Palaeontologia Electronica 13 (1), 2010, p. 2A ( online )
  7. ↑ a b Joseph Leidy: Extinct Mammalia of Dakota and Nebraska, including an account of some allied forms from other localities. Journal of the Academy of Natural Sciences Philadelphia, 7, 1869, pp. 23–472 ( [1] )
  8. ↑ a b James Reid Macdonald: The Miocene faunas from the Wounded Knee area of ​​western South Dakota. Bulletin of the American Museum of Natural History 125, 1969, pp. 143-238
  9. ^ J. Graham: Agate Fossil Beds National Monument Geologic Resources Inventory Report. Natural Resource Report NPS / NRPC / GRD / NRR — 2009/080. National Park Service, Denver, Colorado, 2009 ( [2] )
  10. ↑ a b c d Larry D. Martin and Debra K. Bennett: The burrows of the Miocene beaver Palaeocastor, Western Nebraska, USA Palaeogeography, Palaeoclimatology, Palacoecology 22, 1977, pp. 173-193
  11. ^ Erwin Hinckly Barbour: Notice of new gigantic fossils. Science 19, 1892, pp. 99-100
  12. ^ Edward Drinker Cope: A supposed new order of gigantic fossil from Nebraska. The American Naturalist 27, 1893, pp. 559-569
  13. ↑ Theodor Fuchs: About the nature of Daimonelix Barbour. Annals of the KK Naturhistorisches Hofmuseum - Notes 8, 1893, pp. 91–94 ( [3] )
  14. ↑ Olof August Peterson: Recent observations upon Daemonelix. Science 20, 1904, pp. 344-345
  15. ^ William W. Korth and Natalia Rybczynski: A new unusual castorid (Rodentia) from the Earliest Miocene of Nebraska. Journal of Vertebrate Paleontology 23 (3), 2003, pp. 667-675
  16. ↑ a b c d Natalia Rybczynski: Castorid Phylogenetics: Implications for the Evolution of Swimming and Tree Exploitation in Beavers. Journal of Mammalian Evolution 14, 2007, pp. 1-35
  17. ^ Robert C. Meyer: Helical burrows as a palaeoclimate response: Daimonelix by Palaeocastor. Palaeogeography, Palaeoclimatology, Palaeoecology 147, 1999, pp. 291-298
  18. ↑ Barbara R. Stein: Morphology of subterranean rodents. In: Eileen A. Lacey, James L. Patton, and Guy N. Cameron (Eds.): Life Underground: The Biology of Subterranean Rodents. University of Chicago Press, Chicago, 2000, pp. 19-61
  19. ↑ a b Clara Stefen: Cranial morphology of the Oligocene beaver Capacikala gradatus from the John Day Basin and comments on the genus. Palaeontologia Electronica 17 (2), 2014, p. 25A ( online )
  20. ^ William W. Korth: Comments on the Systematics and Classification of the Beavers (Rodentia, Castoridae). Journal of Mammalian Evolution 8 (4), 2001, pp. 279-296
  21. ↑ Georgios Xenikoudakis, Mayeesha Ahmed, Jacob Colt Harris, Rachel Wadleigh, Johanna LA Paijmans, Stefanie Hartmann, Axel Barlow, Heather Lerner and Michael Hofreiter: Ancient DNA reveals twenty million years of aquatic life in beavers. Current Biology 30, 2020, pp. R110 – R111, doi: 10.1016 / j.cub.2019.12.041
  22. ↑ Marguerite Hugueney and François Escuillié: Fossil Evidence for the Origin of Behavioral Strategies in Early Miocene Castoridae, and Their Role in the Evolution of the Family. Paleobiology 22 (4), 1996, pp. 507-513
  23. ^ William W. Korth: The skull of Nothodipoides (Castoridae, Rodentia) and the occurrence of fossorial adaptions in beavers. Journal of Paleontology 81 (6), 20078, pp. 1533-1537

Web links

Commons : Palaeocastor  - collection of images, videos and audio files