Sifrhippus

• North America

Sifrhippus is an extinct genus of the horse family ( Equidae). This simultaneously the most primitive members of this family and lived at the beginning of the Eocene, about 56 million years ago in western North America. The representatives reached the size of today's domestic cats and lived in open woodlands.

Features

Finds of Sifrhippus, but include the most highly fragmented fossil remains of a nearly complete skeleton, have so far 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 about 15 to 20% smaller on average than comparable units of the same living Hyracotherium; but later representatives were noticeably larger. The skull was about 15 cm long and had a right occiput and a convex forehead line. The lower jaw measured 12 cm and had a good long narrow symphysis. The teeth had complete lower Säugetierbezahnung, which was composed of three incisors, one canine, four premolars and three molars, which occurs following dental formula: . The canine incisors significantly surpassed the height. Between canine and premolar front there was a nearly 8 mm long diastema. Another, but much shorter was the second premolars. The premolars itself had hardly molarisierte features and differ sharply from the molars ( back molars ). This had two transversely Asked enamel strips ( bilophodont ), but referred to each end of which a raised hump on which makes them 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 tail vertebrae is unknown. Of the limbs nearly all bone elements have survived. The humerus had a length of up to 10 cm, the radius of 9 cm. The front legs ended as the other early horses in four toes, with the metacarpal III ( middle finger ) with 4 cm was the longest. In the hind limbs of the femur to the tibia 14 cm and 12 cm in length pointed at. Here are just three toes were designed here, the mean ( metatarsus III ) with 6 cm length of the largest. The long bones were similar to those of Hyracotherium, but were significantly more gracile build. In its overall size and in structure they are comparable with the corresponding bones of a recent Siamese cat.

Paleobiology

The good preservation of the bones do allow some statements to Paleobiology. The vigorous training of the spinous processes of the cervical vertebrae, especially on the second and seventh, speak for strong muscles, which generated a high vertical mobility of the neck and head, just such a good horizontal movement but at the same time restricted. This was supported further by the first thoracic vertebrae, which also had large spinous processes. Overall, the spine was bent slightly convex in the state. The numerous joint surfaces and muscle attachment sites on the hind legs resulted in a high mobility and enabled a powerful movement, the largest power development was probably produced in the region of the knee. The significantly strong femoral head speaks for a high maneuverability in enclosed area, as well as that of the humerus. The front legs were very mobile and thus enabled different motion sequences, however, was due to the elbow structure is not possible and outward rotation. Another limitation of motion was carried by a lower mobility of the hand and foot, which is a movement in densely wooded or very uneven terrain made ​​it difficult again. For the early Eocene a park- forest-like landscape is in the detection area of Sifrhippus based on the remnants of the flora reconstructed with dense Untergrundbewachsung.

Sifrhippus lived in the early Eocene, nearly 56 million years. Since no previous forms are known in North America, it is believed that it has migrated through northern distribution channels. During this geological period it came to the global Paleocene / Eocene Thermal Maximum, which is characterized by a decrease in the concentration of the 13C isotope relative to the 12C isotope of thousands due to the release gigatons of depleted in 13C ( light ) carbon into the ocean-atmosphere system is. The event lasted some 175,000 years, and it came within the first 60,000 years to a temperature rise of up to 10 ° C compared to the initial situation, coupled with an increasing during the height of aridization the climate.

Investigations of the corresponding carbon isotope to more than 40 stratifizierbaren each first molar of unique adult individuals of Sifrhippus that stem all the Bighorn Basin, and carried out simultaneously size measurements of the respective teeth showed striking body size changes during the Paleocene / Eocene temperature maximum. The earliest representatives of Sifrhippus had thereby an average body weight of 5.6 kg. This was reduced in subsequent years by 30% to 130,000 about 3.9 kg, which was associated with the reduction of the molars. These members of Sifrhippus are thus the smallest known horses of the earth's history. Towards the end of the climate event with an appreciable temperature decrease the size of the animals by 76 % increased again to an average of 7 kg. These fluctuating body size changes are largely associated with the Bergmann 's rule in conjunction, which explains that in warmer areas, on average smaller body shapes are observed in endothermic animals. The shrinking process is not due to lack of food, which showed on the basis of isotope studies identified climate conditions that wetter climate and thus favor a higher biomass production during the temperature rise. Only during the temperature maximum of the Paleocene / Eocene temperature maximum prevailed dry climate.

Further isotopic analyzes based on the same teeth were extremely high fluctuations for the 18O isotope. These results are explained by the lifestyle of Sifrhippus which inhabited open landscapes and nourished there from soft plant foods, mainly of leaves and the evaporation of water.

System

Sifrhippus is so far the most primitive representatives of the Equidae family, which also includes the modern day einhufigen horses. The sister taxon of the Equidae represent the extinct Palaeotheriidae, which among others the genera Palaeotherium and Hyracotherium are assigned. Both families form the superfamily Equoidea and the interim order Hippomorpha that the Ceratomorpha face within the Perissodactyla systematics with today's tapirs and rhinos.

The first description of Sifrhippus was launched in 2002 by David J. Froehlich. The holotype ( specimen number 83567 UM ) includes a right lower jaw fragment of only 3 cm length and 1.7 cm in height with the three surviving back molars and comes from the Willwood Formation in the Clark's Fork Basin in northwestern Wyoming. The genus name derives from the Arabic word for " zero " ( صفر, sIFR ) and the Greek word for " horse" ( ἵππος, hippos ) from. Here, the term " sifr " refers to the basal part of the früheozänen Wasatchium - Faunenkomplexes in western North America, which originate from the finds and is abbreviated wa0 fauna.

Two species are currently recognized:

• S. grangeri Kitts, 1956
• Sandrae S. Gingerich, 1989

Both species have been assigned to their original descriptions each Hyracotherium, with S. grangeri originally as H. angustidens grangeri only possessed subspecies status and was lifted in 1989 by Philip D. Gingerich at the species level. David. J. Froehlich turn ordered 2002 H. sandrae due to the paraphyletic origin of the genus Hyracotherium the newly created taxon Sifrhippus to while annexing H. grangeri in the equally new genus Arenahippus. In 2012, ultimately Arenahippus was equated due to little separating features with Sifrhippus.