Tuesday, July 28, 2015

Hyracotherium, the First Horse

Hyracotherium was very small (small-dog size) and had four toes on its front legs. Although this genus gave rise to various lineages of horses that included successively larger and faster horses, Hyracotherium was not a runner. Instead, it moved slowly about in forests and relied on its own camouflage rather than swift running in order to protect itself from predators.

Hyracotherium (length 60 cm = 23 inches).
Hyracotherium was the first name given to this genus, based on specimens found in England. Later, based on specimens found in North America, it was mistakenly named again; this time as “Eohippus,” which means “dawn horse.” Eohippus is thus a secondary junior synonym of Hyracotherium, and in the international rules of naming of organisms, Hyracotherium has priority because it was named first.

 The tooth shown below is a single molar from the lower jaw of Hyracotherium angustidens of Eocene age from Wyoming. Its teeth were low-crowned with low cusps, which enabled the animal to eat both fruit and soft leaves.

Side view of a molar of Hyracotherium angustidens; specimen is 10 mm length, 5.5 mm height.

Top view of molar shown above.

The next picture shows how small an H. angustidens molar tooth is in comparison to a molar tooth of the modern horse Equus caballus.
Side view of a molar  (50 mm height = about 80% complete because the base is broken off) of Equus caballus versus the molar of H. angustidens shown above.

Thursday, July 16, 2015

A Late Cretaceous early mammal tooth

Multituberculates were early rodent-like mammals (see my sketch below) that lived from the Late Jurassic to early Oligocene. They have the longest mammalian lineage (approximately 125 million years) and are the only branch of mammals to have become extinct. They did survive, however, the mass extinction at the end of the Cretaceous Period 65 million years ago. In the early Oligocene, multituberculates were most likely out-competed by true rodents.

They were rat- to squirrel-size and were most likely the first herbivorous mammals. They were widespread, but most are known from Europe and North America.

An early multituberculate. P4 refers to its most forward molar tooth
 (see remarks given below).
Their fossil record consists of minute teeth, which are scarce. As shown below in the plaster replica, they had rodent-like front teeth (incisors), then a gap, followed by a series of gnawing cheek teeth (premolars and molars). The cheek teeth possess many cusps, or tubercles; thus, the derivation of the name “multituberculates.” The first premolar (P4) in the lower jaw was commonly of large size and used as a massive, blade-like tooth for shearing purposes.

Side view of plaster replica of jaws and teeth of Ptilolus? sp., an early multituberculate. This replica is about 15 times larger than the actual jaws and teeth.

                                                                top view of the lower jaw

An actual P4 molar tooth (3 mm width, 3.75 mm length) of  Ptilodus? sp., a Late Cretaceous squirrel-like multituberculate is shown below. This Cretaeous tooth was found in modern-day anthills in the Bug Creek  area of Montana. Paleontologists search these anthills for fossils because ants burrow into Upper Cretaceous rocks and bring the microscopic-sized fossils to the Earth's surface.

   side view of a P4 molar (3 mm width)

Thursday, July 2, 2015

Amazing eyes of the trilobite Phacops rana from Ohio

Trilobites are some of the most beautiful and interesting fossils. They are extinct arthropods and belong to the same phylum that includes scorpions, spiders, "horseshoe crabs," barnacles, and crabs, etc. Trilobites lived during the Paleozoic and especially flourished during the Devonian.

Trilobites were the first animals in the fossil record with preserved ocular lenses (arranged in vertical-arranged bundles of calcite rods, commonly referred to as compound eyes), which, by the way, provided sharp vision for detecting predators. One genus has many specimens with these well-preserved eyes. It is Phacops, a genus that ranges from Silurian to Devonian, in North American and Europe. The photograph shown below is Phacops rana (Green, 1832) from the Silica Formation in Ohio. This species, which is of Middle Devonian age (about 390 million years), is the state fossil of Pennsylvania. The species is found also in northeastern USA, Ontario, and Morocco, Africa. This last occurrence can be explained because during North America was once attached to the African tectonic plate.

Dorsal (top) view of a specimen of Phacops rana. There are two areas of compound lenses (eyes), on both sides of the head. The large, swollen and bumpy area in between the eyes is the stomach region. The entire trilobite body is 30 mm in length and 20 mm in width; each eye area is about 3 mm in width. There is attached inarticulate-brachiopod shell (6 mm width) on the central right side of the body.

The compound eye of P. rana differs from most other trilobites by having relatively fewer and more widely lenses, as well as rounded lenses, rather than being largely flat. The lenses of P. rana were elevated, thereby allowing for an almost 360° field of view. This type of eye is called the schizochroal eye.

Oblique, close-up view of the eyes on the right side of the specimen shown above. The width of the eye (the area with the aligned bumps) is 3 mm.