Sifrhippus

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Sifrhippus
Live reconstruction of Sifrhippus in the Natural History Museum in Stockholm.

Live reconstruction of Sifrhippus in the Natural History Museum in Stockholm.

Temporal occurrence
Lower Eocene
56 million years
Locations
  • North America
Systematics
Higher mammals (Eutheria)
Laurasiatheria
Unpaired ungulate (Perissodactyla)
Equoidea
Horses (Equidae)
Sifrhippus
Scientific name
Sifrhippus
Froehlich , 2002

Sifrhippus is an extinct genus from the horse family ( Equidae ). It is the most original member of this family and livedin western North America at the beginning of the Eocene around 56 million years ago. The representatives reached the size of today's domestic cats and lived in open forest landscapes.

features

Finds of Sifrhippus , which predominantly include strongly fragmented fossil remains but also an almost complete skeleton, are mostly only known from the Clark's Fork Basin and the Bighorn Basin in northwestern Wyoming . It was a very small representative of the early horses, which was on average about 15 to 20% smaller than comparable specimens of the Hyracotherium living at the same time ; later representatives were noticeably larger. The skull was about 15 cm long and had a right-angled occiput and a convex forehead line. The lower jaw measured a good 12 cm and had a narrow and long symphysis . The teeth had the full lower Säugetierbezahnung resulting from three incisors , one canine , four premolars and three molars composed, produced thereby following dental formula . The canine clearly exceeded the incisors in height. There was a diastema almost 8 mm long between the canine and anterior premolar . Another, but much shorter, existed for the second premolar. The premolars themselves showed hardly any molarized features and thus clearly differed from the molars (rear molars). These had two transverse enamel ridges ( bilophodont ), but each had a raised cusp at each end and thus appear more bunodont . Overall, the molars had low crowns ( brachyodont ).

The postcranial skeleton is almost completely preserved. The spine consisted of seven cervical, 17 thoracic, seven lumbar and five sacral vertebrae, the number of caudal vertebrae is unknown. Almost all bone elements have been handed down from the limbs. The humerus had a length of up to 10 cm, the radius of 9 cm. As in other early horses, the front legs ended in four toes (rays II to V), with the metacarpus III (middle finger) being the longest at 4 cm. In the hind limbs, the femur was up to 14 cm and the shin was 12 cm in length. Only three toes were formed here, with the middle one ( metatarsus III) also having the greatest length at 6 cm. However, the outermost toe ray (ray V) was still present as a stunted, crescent-shaped bone, which is altogether atypical for early odd ungulates. The long bones resembled those of Hyracotherium , but were built much more gracefully. In their overall size and structure they are comparable to the corresponding bones of a recent Siamese cat .

Paleobiology

The good preservation of the bones allows some statements about the paleobiology . The strong development of the spinous processes on the cervical vertebrae, especially on the second and seventh vertebrae, speak for strong muscles, which produced a high vertical mobility of the neck and head, but at the same time limited just as good horizontal movement. This was further supported by the first thoracic vertebrae, which also had large spinous processes. Overall, the spine was slightly convex while standing. The numerous joint surfaces and muscle attachment points on the hind legs resulted in a high degree of mobility and enabled powerful movement, with the greatest development of strength probably being generated in the knee area . The clearly strong femoral head speaks for high maneuverability in closed terrain, as does that of the humerus. The front legs were also very flexible and thus enabled different sequences of movements, but due to the elbow structure, turning in and out was not possible. Another limitation of mobility was the lower mobility of the hands and feet, which in turn made it difficult to move in densely wooded or very uneven terrain. For the early Eocene , a park forest-like landscape with dense subsurface vegetation is being reconstructed in the Sifrhippus detection area based on the remains of the flora .

Sifrhippus lived almost 56 million years ago in the early Eocene . Since no predecessor forms are known in North America, it is believed that it immigrated via northern distribution routes. During this geological epoch occurred to the global Paleocene / Eocene maximum temperature that with a decrease in the concentration of 13 C - isotope for relative 12 C isotope peaks due to the release Gt of at 13 C depleted (light) carbon in the ocean-atmosphere System is characterized. The event lasted for about 175,000 years, with a temperature increase of up to 10 ° C compared to the initial situation within the first 60,000 years, combined with an increasing aridization of the climate during the peak .

Investigations of the corresponding carbon isotope on more than 40 stratifiable first molars of clearly adult individuals of Sifrhippus , all of which originate from the Bighorn basin, and measurements of the size of the respective teeth carried out in parallel, revealed marked changes in body size during the Paleocene / Eocene temperature maximum. The earliest representatives of Sifrhippus had an average body weight of 5.6 kg. This decreased in the following 130,000 years by 30% to about 3.9 kg, which went hand in hand with the reduction in size of the molars. These members of Sifrhippus are the smallest known horses in the history of the earth. Towards the end of the climatic event with a noticeable drop in temperature, the size of the animals increased again by 76% to an average of 7 kg. These fluctuating changes in body size are largely associated with Bergmann's rule , which explains that, on average, smaller body shapes can be observed in endothermic animals in warmer regions . The fact that the shrinking process is not due to a lack of food is shown by the climatic conditions determined by the isotope studies, which advocate a more humid climate and thus a higher biomass production during the temperature rise. Only during the peak of the Paleocene / Eocene temperature maximum was a dry climate prevailing. Similar changes in body size were also found for later climatic fluctuations during the Eocene thermal maximum 53 million years ago at Arenahippus .

Further isotope analyzes using the same teeth showed extremely high fluctuations for the 18 O isotope. These results are explained with the way of life of Sifrhippus , who lived in open landscapes and lived there on soft vegetable food, mainly from leaves and their evaporation water.

Systematics

Sifrhippus is one of the most primitive members of the Equidae family , which also includes today's modern single-hoofed horses . The sister taxon of the Equidae are the extinct Palaeotheriidae , to which the genera Palaeotherium and Hyracotherium are assigned. Both families form the superfamily Equoidea and the intermediate order Hippomorpha, which are compared to the Ceratomorpha with today's tapirs and rhinos within the odd- toed ungulate system .

The first description of Sifrhippus was in 2002 by David J. Froehlich . The holotype (specimen number UM 83567) comprises a right mandibular fragment only 3 cm long and 1.7 cm high with the three preserved rear molars and comes from the Willwood formation in the Clark's Fork Basin in northwestern Wyoming . The generic name is derived from the Arabic word for "zero" ( صفر , ṣifr ) and the Greek word for "horse" ( ἵππος , hippos ). The term "Sifr" refers to the basal part of the early Eocene Wasatchian fauna complex in western North America, from which the finds originate and which is abbreviated as Wa 0 fauna.

Two types are recognized today:

Both species were each assigned to Hyracotherium in their original descriptions , although S. grangeri originally only had subspecies status as H. angustidens grangeri and was raised to species level in 1989 by Philip D. Gingerich. David. J. Froehlich in turn assigned H. sandrae to the newly created taxon Sifrhippus due to the paraphyletic origin of the genus Hyracotherium , while he incorporated H. grangeri into the likewise new genus Arenahippus . In 2012 Arenahippus was finally equated with Sifrhippus due to insufficient separating characteristics . Another study of the teeth of Sifrhippus with those of Minihippus , also made by Froehlich in 2002, doubts the independence of the latter genus. The authors of the study refer to the sometimes strong degree of chewing and weathering of the teeth and combined both genera. They suggested the species name S. index for the entire Sifrhippus - Minihippus complex , but urged further studies of the Eocene North American horses.

Individual evidence

  1. ^ A b c Philip D. Gingerich: New earliest Wasatchian mammalian fauna from the Eocene of northwestern Wyoming: composition and diversity in a rarely sampled high-floodplain assemblage. University of Michigan Papers on Paleontology 28, 1989, pp. 1-97
  2. ↑ a b c d David J. Froehlich: Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla). Zoological Journal of the Linnean Society, 134, 2002, pp. 141-256
  3. ↑ a b c Aaron R. Wood, Ryan M. Bebej, Carly L. Manz, Dana L. Begun and Philip D. Gingerich: Postcranial Functional Morphology of Hyracotherium (Equidae, Perissodactyla) and Locomotion in the Earliest Horses. Journal of Mammal Evolution 18, 2011, pp. 1-32
  4. ↑ Appy Sluijs, Henk Brinkhuis, Stefan Schouten, Steven M. Bohaty, Cédric M. John, James C. Zachos , Gert-Jan Reichart, Jaap S. Sinninghe Damsté, Erica M. Crouch and Gerald R. Dickens: Environmental precursors to rapid light carbon injection at the Palaeocene / Eocene boundary. Nature 450 (7173), 2007, pp. 1218-1221 doi: 10.1038 / nature06400
  5. ↑ a b c d Ross Secord, Jonathan I. Bloch, Stephen GB Chester, Doug M. Boyer, Aaron R. Wood, Scott L. Wing, Mary J. Kraus, Francesca A. McInerney and John Krigbaum: Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum. Science 335, 2012, pp. 959-962
  6. ↑ Ross Secord, Jonathan I. Bloch, Stephen GB Chester, Doug M. Boyer, Aaron R. Wood, Scott L. Wing, Mary J. Kraus, Francesca A. McInerney and John Krigbaum: Supporting Online Material for Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum. Science 335, 2012, pp. 1-32
  7. ↑ Felisa A. Smith: Some Like It Hot. Science 335, 2012, pp. 924-925
  8. ^ Abigail R. D'Ambrosia, William C. Clyde, Henry C. Fricke, Philip D. Gingerich and Hemmo A. Abels: Repetitive mammalian dwarfing during ancient greenhouse warming events. Science Advances 3 (3), 2017, p. E1601430 doi: 10.1126 / sciadv.1601430
  9. ^ Luke T. Holbrook: Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132, 2001, pp. 1-54
  10. ↑ Julie E. Rej and Spencer G. Lucas: Morphological comparison of two Early Eocene horse taxa: Minihippus of New Mexico and Sifrhippus of Wyoming. New Mexico Museum of Natural History and Science Bulletin 74, 2016, pp. 223-230