Ecol 483/583 – Herpetology Lab 11: Reptile Diversity 3: Testudines and Crocodylia Spring 2010
P.J. Bergmann & S. Foldi
Lab objectives The objectives of today’s lab are to: 1. Familiarize yourselves with extant diversity of the Testudines and Crocodylia. 2. Learn to identify species of Testundines that live in Arizona. Today's lab is the final lab on "reptile" diversity, and will introduce you to the Testudines, or turtles and tortoises, Crocodylia. Although there are no crocodylians in Arizona and the Testudine diversity is lower than that of the lizards or snakes, there is still a fair amount of material to learn, so use your time wisely.
Tips for learning the material At this point in the course, your skills for learning herp diversity should be well honed, so continue with the strategies you have already learned during the semester. Learn how to differentiate the three major clades of Crocodylians, and the more numerous Testudine clades. There is another keying exercise in this lab, focusing on the Testudines.
1 Ecol 483/583 – Lab 11: Testudines & Crocs 2010 Exercise 1: Testudines (Modified from Bonine & Foldi 2008; Bonine, Dee & Hall 2006; Edwards 2002; Prival 2000)
General information Turtles are probably the most instantly recognizable groups of all reptiles because of their shell and the ability to withdraw their heads and limbs into this protective structure. Turtles are a monophyletic group comprising the order Testudines also called Chelonia . Testudines is the term used to denote all members of the order (extant and extinct) whereas Chelonia is often used to denote extant turtles. There is some confusion in common terminology used for various groups of turtles. The term ‘tortoise’ refers to terrestrial species that have high domed shells, large size and the hind legs are elephantine (columnar). Tortoises belong to the clade Testudinidae. ‘Terrapin’ is used for freshwater species that are generally harvested and eaten, however in Britain it is used for all pond turtles. Terrapins belong to more than one clade of the Testudines.
There are 13 extant families of Testudines, containing about 300 species. Turtles are further divided into two distinctive clades: the Pleurodira and Cryptodira . These are distinguished by their method of neck retraction. The Pleurodira, or side-necked turtles, bend their head and neck to the side ( pleuro – Gr. for ‘side’). The Cryptodira withdraw their necks in a vertical plane into their shell ( krypto – Gr. for ‘hidden’). North American turtles all belong to the Cryptodira. Turtles are found on all continents (with the exception of Antarctica) and reach the greatest species diversity in two areas of the world: Southeastern United States (35 species) and Southeastern Asia (about 90 species). In Arizona there are five native species and four introduced species (all North American species).
The phylogenetic relationships of the Testudines are shown to the right and in your textbook. As you go through the material today, try to think about what sorts of characteristics make some clades of turtles more closely related to each other than to others. A key difference between the Cryptodira and Pleurodira has already been discussed.
What is one species of Pleurodira and one species of Cryptodira that is on display today?
2 Ecol 483/583 – Lab 11: Testudines & Crocs 2010 Generalized morphology The turtle shell is comprised of bony plates that are generally covered with keratinized scutes . These scutes are the portion of the shell that you see, not the actual bones underneath. The shell is comprised of two general sections, the dorsal carapace and the ventral plastron . The scutes that cover both halves of the shell are grouped and named. These features are used in species recognition so it would be good if you familiarize yourself with them. The head is covered with horny plates whereas the neck generally lacks scales. The limbs may be covered with scales (as in the desert tortoise) or not (as in the Sonoran mud turtle). In aquatic species the feet are webbed with clawed toes. In terrestrial species the feet lack webbing.
Label the different scutes on the shells below. Do all species of turtles on display have the same scutes? Have any species lost some? Which ones?
Pough et al. 2004, Fig. 4 -5
Evolutionary trends and conservation Turtle evolution is the source of much inquiry and debate. The debate is dicussed in some detail in your textbook and in lecture. Study this for lecture exams, but don’t worry about it for the lab.
The marvelous feature of the shell, from a comparative evolutionary and anatomy standpoint, is the fact that the shell is comprised of the rib cage and the limb girdles, with the collar bone and pelvis inside the rib cage. Somehow the ancestral form for the turtles managed this feat and it has been so successful very few changes have been made to the basic turtle morphology in over 200 million years. In fact, the genus Podocnemis from South America has not changed in shell morphology for over 60 million years. This evolutionary ‘conservatism’ suggests just how successful life in a suit of armor is. Turtles have life history strategies that reflect this morphological success. Most turtle mortality occurs during the egg and hatchling stage, when
3 Ecol 483/583 – Lab 11: Testudines & Crocs 2010 the shell is still soft. Once the shell hardens, turtle survivorship is high. In addition, sexual maturity is delayed in turtles and reproductive success is highly correlated with increased age. As a result, the removal of adults for food, medicine or pet trade can have devastating effects on wild populations.
Turtle populations everywhere are declining. Although there have been turtle conservation successes (for example, halts in population declines in some sea turtle populations) the opposite is the rule. Turtle products are so popular in Asia that three quarters of Asia’s native turtle populations are considered threatened and two Chinese turtles are thought now to be extinct (Altyherr and Freyer, 2000, Anon. 2001). During the past century 11 turtle species (out of the 300 known) have gone extinct and 49% are considered threatened by the IUCN (The World Conservation Union, Turtle Conservation fund, 2002). Demand for turtle products in Asia is so great that exports of North American turtles to Asia has increased over 1000% in the past 20 years. The pet trade also has been implicated in turtle declines worldwide. This demand combined with worldwide habitat destruction and lack of conservation laws means that turtles face considerable threats to their survival. In the U.S., fifteen species are listed for protection under the Endangered Species Act.