Suborder Pleurodira (Side-necked turtles)
Family Chelidae (Austro-American Side-necked turtles):
Distribution is Old and New World, but there are no species native
to North America. Unfortunately, there is only one example of
these interesting turtles in lab for you to see: the matamata
(the one you will see is a juvenile, but the herp house at the
Detroit Zoo has a large adult, so check it out if you are ever
there). Some of the best characters for this family are skeletal
or best observed in life (side-necked turtles, as their name implies,
pull their heads in to the side rather than retracting them straight
in as cryptodirans do), but the matamata is rather distinctive
and you should have no trouble learning to recognize it.
MI species: none
Suborder Cryptodira (Hidden-necked turtles)
Family Cheloniidae (Sea turtles): This family includes
all the sea turtles except the Leatherback. Distribution is worldwide,
extending from the tropics well into the temperate oceans. Characteristics
include streamlined bodies and flipper-like limbs. Sea turtles
cannot retract their heads. Of the six species in this family,
four are listed as Endangered Species. In lab you will see a baby
Green Turtle, Chelonia mydas, and an Atlantic Hawksbill
Turtle, Eretmochelys imbricata.
MI species: none
Family Chelydridae (Snapping turtles): Both species in
this small family are found almost exclusively in the United States.
The Alligator Snapping Turtle, Macroclemys temminckii,
lures its prey by means of an appendage on its tongue which resembles
a worm. Characteristics of this family include a large, unretractable
head with hooked jaws and a small, underdeveloped plastron (snapping
turtles make up for this lack of protection by being extremely
aggressive and are capable of inflicting serious wounds on would-be
attackers). In addition, snappers have very long tails and pronounced
bumps and ridges on the carapace. Once again, the examples you
will see in lab are a bit on the small side, but as before, large
adults can be found in the Detroit Zoo herp house.
MI species: Chelydra serpentina (Common Snapping
Turtle)
Family Emydidae (Pond and river turtles): This is by far
the largest family of turtles, and many of the more familiar turtles
belong here. Distribution is Old and New World. Most of these
animals can retract their heads and limbs. Where the plastron
meets the carapace, there are no inframarginals between the marginal
scutes (carapace) and pectoral scutes. All but three of Michigan's
turtle species belong to this family. Sexual dimorphism :
One good character for distinguishing between males and females
in Chrysemys picta marginata is the length of the front
claws, which are longer in males because they are used in this
turtle's mating ritual. In lab you will see three males and three
females. See if you can tell them apart--you will need to be able
to do this for your practical.
MI species: Clemmys insculpta (Wood Turtle)
Clemmys guttata (Spotted Turtle)
Terrapene carolina carolina (Eastern Box Turtle)
Emydoidea blandingii (Blanding's Turtle)
Graptemys geographica (Common Map Turtle)
Chrysemys picta marginata (Midland Painted Turtle)
Trachemys scripta elegans (Red-eared Slider--juvenile
only; no adult available in lab)
Family Kinosternidae (Mud and Musk Turtles): Like snapping
turtles, these New World turtles tend to have an underdeveloped
plastron and compensate by being nasty (mostly musk turtles).
In addition, musk turtles give off a foul odor, which is why they
are often called "stinkpots". For help telling mud turtles
from musk turtles, see Fig. 4 on p. 43 of your field guide. Sexual
dimorphism: The common musk turtle, Sternotherus odoratus,
is sexually dimorphic in the tail and plastron. Check out plate
2 in your field guide and examine the specimens in lab--you will
need to be able to tell males from females on your lab practical.
Incidentally, some members of this family are excellent climbers
and often climb trees to bask. They position themselves on limbs
overhanging the water so they can dive in for a speedy getaway
when they sense danger. If you are ever out on the water and a
turtle suddenly drops into your boat from above, chances are it
belongs to Family Kinosternidae.
MI species: Sternotherus odoratus (Common Musk Turtle)
Family Testudinidae (Tortoises): These are the most terrestrial
of all turtles. Family characteristics include a high, domed carapace
and elephantine feet. The two examples you will see in lab are
relatively small; however, tortoises can be quite large, with
the largest being the Aldabra tortoise, which can weigh over 250kg.
MI species: none
Family Trionychidae (Softshell Turtles): These turtles
are perhaps the most interesting looking of all, but many are
sensitive to impurities in water and are therefore difficult to
keep in captivity. This is because they have highly vascularized
patches inside their cloaca for gas exchange, allowing them to
remain submerged for long periods without air. In addition, trionychids
rarely bask, so they spend more time in the water than other turtles.
They have very long necks and are dorsoventrally compressed (many
of their common names and nicknames reflect this), but the most
obvious character of this family is the absence of scutes on the
carapace and plastron. If you look at the Smooth Softshell, Apalone
mutica, you will see the outline of the vertebral column and
ribs in the anterior portion of the carapace. This is not an artifact
of preservation-- it is actually a field character for this species.
MI species: Apalone spinifera (Spiny Softshell)
TURTLE EGGS
Turtle eggs come in two basic shapes: spherical and eliptical.
Among cryptodirans, families which lay spherical eggs include
Trionychidae, Chelydridae, and Cheloniidae.
Families which lay elliptical eggs include Emydidae and
Kinosternidae. Page 77 in Halliday and Adler offers a possible
explanation as to why different turtles lay differently shaped
eggs--check it out if you're interested.
HATCHLING TURTLES
Hatchling turtles often look quite a bit different from adults
of the same species, and often have more distinct markings (as
in the handout I gave you on aposematic coloration in hatchling
Trachemys scripta elegans). Your field guide has a special
section devoted to young turtles. Examine the hatchling turtles
in lab and pick out the following Michigan species:
Apalone spinifera
Chelydra serpentina
Trachemys scripta elegans
Chrysemys picta marginata
Graptemys geographica
SKULLS
Examine the skulls of the emydid turtle, sea turtle, and snapping
turtle. As you will recall from last week, turtles have anapsid
skulls. You will need to be able to find the following bones on
all three skulls:
squamosal
jugal
postorbital
parietal
CARAPACE/PLASTRON
You learned last week that the size, shape, and placement of turtle
shell bones often give important clues for identification. This
is also true of the scutes (or "laminae") that
cover the shell. For a good example, examine the two subspecies
of painted turtle in the lab: Chysemys picta picta (Eastern
painted turtle) and Chrysemys picta marginata (Midland
painted turtle). Chysemys picta picta has scutes arranged
in rows across its back, while C. picta marginata has its
scutes arranged differently (your field guide calls this "alternating
fashion"). In addition, the size and shape of the 9th marginal
scute in the Yellow Mud Turtle, Kinosternon flavescens,
is one of the distinguishing characteristics for this species
(check out the example in lab to see what I mean). Because scutes
are so important in identifying turtles, you will be responsible
for knowing all of them as outlined on the green page inside the
front cover of your field guide. Because scutes differ from turtle
to turtle, try to identify them on a variety of turtles in the
lab, including the empty shell. Also, note that the arrangement
of turtle scutes generally does not match the arrangement of the
underlying bones. Why do you think this is?
SKELETON
You will not be responsible for knowing the bones of the turtle
skeleton (notable exceptions being the bones of the skull discussed
above and the nuchal bone from last week). However, notice that
the limb girdles have shifted inside the rib cage, which
is now part of the carapace. Among the vertebrates, only turtles
have this condition. The next time you see a live turtle, watch
the way it breathes. Notice that its "ribcage", such
as it is, does not expand and contract like that of other vertebrates.