Biology of the Reptilia: Lab #2. Testudines. Alex V and Jeff W

Logger-head sea turtle skull - Cheloniidae

The exact ancestry of turtles is hotly debated. The phylogeny of turtles is poorly understood; with their anapsid skulls (lacking any temporal fenestre), turtles could be closely related to the Parareptiles (an ancient extinct anapsid lineage of reptiles) OR they could be derived Euryapsids (those reptiles with the ancestral condition of a subtemporal fenestre) OR they could even be highly derived Crocodylotarsiians (a group of diapsids, that includes todays crocodiles).

Aside from the confusing phylogeny, turtles are one of the most recognizable groups of reptiles. A turtle is a turtle is a turtle is a turtle. The most definitive character of a turtle is the turtle's shell. Below is a Alex (a jolly herper) holding a turtle shell, comprised of a dorsal carapace, and a ventral plastron (these characteristics set turtle shells apart from shells of other creatures). Turtle shells are composed of highly derived vertebral, dermal, and fused rib bones. Layered upon the dermal bones are scutes, a scale-like keratinous layer arising from specialized epidermis. Some soft-shelled turtles, however lack scutes and other aquatic turtles shed their older scutes as new ones grow.

The enormous copulatory organ

The primary benefit of the shell is protection, but it also presents a unique obstacle in copulation. To combat this, males have a concave plastron and an enormous copulatory organ.

Neck anatomy is a characteristic that divides all turtles into two distinct clades, the Pleurodirans (side-necked turtles) and Cryptodirans (hidden-neck turtles). Three families belong to the Pleurodirans and eleven to the Cryptodirans. Side necked-turtles are found primarily in southern hemispheric areas; there are no side-necked turtles native to our area.

Pelomedusiadae (African side-necked turtle)

Podocnemidae (madagascar big-headed turtle)

Chelidae (Australoamerican side-necked turtle)

Hidden-necked turtles are more common and more diverse with wider distributions. We have four families whose ranges overlap our area here in NE Ohio. These are the Chelydridae, Kinosternidae, Trionychidae, and Emydidae.

Baby alligator snapping turtle

The Chelydridae are the snapping turtles and they eat fingers. Characteristics defining of this family are large heads, reduced plastrons, and serations on the posterior end of the carapace. The common snapping turtle, Chelydrida serpentine, is found in NE Ohio, while the larger alligator snapping turtle resides farther to the south.

The Kinosternidae have a NE Ohio representative, affectionately called the stinkpot turtle (Sternotherus odoratus). This group is collectively called the mud and musk turtles because they produce strong odors from their scent glands. Other characteristics include relatively large heads and sometimes the presence of a hinged plastron.

The Trionychidae are the soft-shelled turtles, defined by the absence of keratinous scutes and a softer shell than most turtles. They are aquatic and exhibit cutaneous respiration. These turtles can be distinguished from the similar-looking pig-nosed turtles (Carettochelyidae) by the presence of a fleshy lip. In NE Ohio, we have two species of soft-shelled turtles, the spiny soft-shell (Apalone spinifera)
and the smooth soft-shell (Apalone mutica)

Smooth soft-shell (above), Spiny soft-shell (right)

The Emydidae are a highly diverse group of turtles including sliders, cooters, box turtles and other close relatives. In Ohio, we have eight native species belonging to the the Emydidae: the map turtle, false map turtle, red-eared slider (below), painted turtle (below), spotted turtle, Blanding's turtle, the hieroglyphic river cooter, and the eastern box turtle.