According to the creationist model, turtles have always existed. There
is no evidence to support this. Most of the history of life and most of
vertebrate history passed without a single turtle fossil. Prior to the
oldest turtle fossil, ancestral reptiles developed dermal bone as part
of their defenses. Turtles seem to have evolved shortly after the earth's
worst extinction epoch, the End-Permian extinction, a time in which many
lineages were undergoing rapid diversification to fill the ecological
niches left empty by the extinction. The earliest turtles were not members
of the groups which are alive today. Proganochelys represents the oldest
known turtle with a number of primitive traits such as small teeth (on
the edges of the jaws and roof of the mouth) and primitive conditions
of the shoulder, pelvis, and skull. (Some modern turtles develop vestigial
teeth as embryos). Dermal bones covered the neck (Romer, 1956).
The early Jurassic turtle Australochelys is intermediate between the earliest
turtles (such as Proganochelys) and modern groups (Gaffney, 1994).
The turtles of the group Testudinoidea (composed of the families Emydidae
and Testudinoidae) compose half of modern turtle species. This group diversified
only recently, beginning in the Eocene. Skull shape is correlated with
diet and similar dietary adaptations have evolved separately in distinct
lineages (Claude, 2004).
The oldest known sea turtle is Santanachelys from the Early Cretaceous.
Although its skull possesses features shared with modern sea turtles (such
as salt glands around the eyes), the primitive paddles are comparable
to those of freshwater turtles with movable joints (Hirayama, 1998).
Mesozoic turtles are classified in a separate suborder, Amphichelydia.
Within the Mesozoic suborder Amphichelydia, several superfamilies are
recognized. Some turtles within amphichelydia represent transitional forms
ancestral to the two modern suborders, although the modern orders had
separate origins within Amphichelydia. Many extinct species were probably
capable of some retraction of the neck. Cervical ribs were present in
the early turtles but have been reduced and fused to cervical vertebrae
in modern groups. Fossil turtles possessed variations in the shell such
as the degree of fusion of the pelvis and the shell and the fusion between
the two halves of the shell (Romer, 1956). The superfamily Baenoidea is
though to include the ancestors of the cryptodires and some extinct forms
which survived into the Cenozoic (Romer, 1956).
Most modern groups of turtles did not evolve until recently. Turtles
of the families Trionychoididae, Pelomedusidae, and Chelonidae date from
the Early Cretaceous (Krenz, 2005). The earliest fossils of the superfamily
Testudinoidea are known from the Late Cretaceous, and the first tortoise
fossils from the Paleocene. By the Eocene, tortoises had spread to at
least 3 continents (Le, 2006).
Turtles are one of the most recognizable groups of modern vertebrates.
Much of their lifestyle and locomotion is determined by their shell which
has thus limited the modifications of their ancestral form.
Although the turtle shell is complex, it does not indicate design. First
of all, an armored shell is not essential to turtle "design"
since some modern turtles lack epidermal scutes and others (such as the
fossil Archelon and the modern leatherback) have lost the bony regions
of their shells.
Some modern turtles have developed new anatomical arrangements for their
shell (such as a plywood-like orientation of collagen fibers in soft-shell
turtles) which are just as strong as the shell of typical turtles but
much lighter. Dermal armor is not essential for survival as a turtle (as
the leatherback demonstrates) nor are tough epidermal scutes over the
shell essential (as soft shell turtles, pig nosed turtles, and leatherbacks
demonstrate), nor is one type of shell structure essential (as soft shell
turtles demonstrate). Ancestral turtles did not have to evolve the only
type of armor covering which would allow survival, they simply had to
develop one of several ways to augment ancestral complements of armor
(Scheyer, 2007; Alibardi, 2007).
In addition, there are many ways in which an animal increase its dermal
armor apart from the form found in typical turtles. A number of animals
have developed thicker rib cages for support including early amphibians
(such as Ichthyostega) and the early reptile Eunotosaurus.
Dermal bone evolved in early reptiles to deter predation and is retained
in modern crocodilians.
Pareiasaurs evolved osteoderms which formed dermal armor in their skin.
While the earlier pareiasaurs possessed osteoderms only on the dorsal
midline, later species possessed these osteoderms covering much of the
back. In a later group of dwarf pareiasaurs (which some feel are closely
related to turtles), the osteoderms covered the entire dorsal region (Lee,
Among primitive archosaurs, species of the families Euparkeriidae and
Phytosauridae possessed armor. Extensive armor covered both the dorsal
and ventral sides of the body in species of the families Aetosauridae
Some lineages of dinosaurs enhanced and diversified ancestral complements
of dermal armor (such as stegosaurs, nodosaurs, ankylosaurs, and even
a few sauropods). Among mammals, an armor covering has developed in animals
such as armadillos and ground sloths (Romer, 1956).
Of all the armored groups of amniotes, the advanced placodonts are the
most similar to turtles in their armor and in the orientation of the shoulder
to the carapace (DeBraga, 1997). Armor developed in placodonts gradually
over time. Of the four families of Placodonts, the family Helveticosauridae
lacked armor, species of the family Placodontidae possessed little or
no armor, The family Placochelyidae possessed a carapace of dermal bone,
and the family Henodontidae possessed a bony plastron in addition to the
bony carapace (Romer, 1956).
A bony shell or epidermal scutes which cover the shell are not essential
components of turtle "design" given that some modern turtles
lack them. Fossil animals indicate that there are a variety of ways to
develop dermal armor. Ancestral turtles did not have to achieve the only
possible "design" which would allow for protection from predators,
but rather one of a variety of different "designs". Other groups
ranging from armored dinosaurs to turtle-like placodonts indicate that
a covering of armor can develop slowly over a series of stages.
Soft shell turtles have lost their epidermal scutes and their shell is
covered by their leathery epidermis. The bone of the shell has been modified
with an orientation of collagen fibers and mineral layers which has been
compared to plywood. This unique bone structure has lightened their shells
while maintaining strength. As a result, these turtles have improved mobility,
ability to camouflage themselves, and perform cutaneous respiration (Scheyer,
2007). These shells are as strong as those of hard-shelled turtles (Alibardi,
The bones around the periphery of the shell are completely lost in all
soft-shell turtles, except Lissemys punctata which has retained small
remnants of these bones (Scheyer, 2007). The turtle which forms the sister
group to soft-shell turtles also has lost its epidermal scutes (Carettochelys
insculpta) (Scheyer, 2007). Epidermal scutes are also reduced in the recent
turtle Natator depressus of the family Chelonidae (Scheyer, 2007).
The superfamily Dermochelyoidea includes the modern leatherback and a
few fossil relatives. Leatherbacks lost their epidermal scutes of the
shell and lost the majority of the dermal bone of the shell (Romer, 1956).
Some turtle shells are better able to close because of joints which allow
the flexion of the components of the shell. Joints exist in the carapace
of Kinixys and in the plastrons of 2 genera of Kinosternidae, 4 genera
of Emydidae, and 3 genera of Testudinidae (and the latter two seem to
include independently evolved joints) (Zangerl, 1969; Feldman, 2003).
Lifestyle can vary significantly between related turtles. For example,
the superfamily Testudinoidea includes both the majority of aquatic turtles
and the major group of terrestrial turtles. The group includes the related
families Emydidae (which includes both aquatic terrapins and other groups
more adapted to terrestrial life such as some tortoises and box turtles),
and Testudinidae (terrestrial tortoises, including giant tortoises). Thus,
a single family and related families can include species adapted to both
terrestrial and aquatic habitats. The tortoises of the family Testudinidae
can vary significantly. They include Galapagos tortoises which can measure
1.5 m in length and the speckled padloper (Homopus signatus) which measures
10 cm in length. While most have highly domed carapaces, flat carapaces
are known in species of the genera Kinixys, and Malacochersus (Le, 2006).
While most members of the extinct superfamily Pleurosternoidea inhabited
aquatic terrestrial habitats, some were marine (Desmemys of the Family
Pleurosternidae and the marine turtles of the Family Thalassemyidae) (Romer,
KINDS OF TURTLES
Some creationists consider turtles to represent a single "kind"
If turtles belong to a monotypic baramin, perhaps Proganochelys is the
closest ancestor for both of these groups. This is my present viewpoint.
A few years ago I suggested that possibly turtles constituted a polytypic
baramin with as many as four diversification lines. Even though this is
not my current position, I still consider it a reasonable hypothesis worthy
of further consideration (Frair, 1991).
Other creationists have identified multiple turtle "kinds"
(ranging from two to four).
The current viewpoint of Frair is that all turtles form a single phylogenetically
related baramin. A hypothesis by Wise is that turtles are composed of
two unrelated baramins including the pleurodires (side-necked), the cryptodires
(hidden-necked). An earlier suggestion of Frair and the recent conclusion
of Wise suggests that turtles form four baramins including pleurodires
(side-necked), the trionychoids (soft-shells), the chelonids (marine),
the remaining cryptodires (an assortment of terrestrial and aquatic species).
Another author identified the "kind" level in turtles at family
or genus, which would generate significantly more "kinds" (Estes,
2000). Recent works identify the two families of the Pleurodires as separate
"kinds" and the cryptodires as being divided into three "kinds"
(marine turtles, soft shell turtles, and all other groups (Wood, 2006).
In addition to the obvious feature of the shell and modified limbs, additional
traits link turtles as a clade ranging from the structure of the skull
(Romer, 1956) to the use of aragonite instead of calcite in their eggshells,
unlike other amniotes (Winkler, 2006). Molecular evidence indicates that
turtles and their subgroups share a common ancestry. Individual studies
have indicated the monophyly of the marine turtles (Chelonidae as a clade
and also with leatherbacks as a sister group;Dutton, 1996, Chen, 1980;
Krenz, 2005), pleurodirans (Seddon, 1997; Fujita, 2004; Georges, 1998;
Krenz, 2005; Winkler, 2006), cryptodirans (Krenz, 2005), soft shell turtles
and Carettochelys (Chen, 1980; Krenz, 2005), superfamily Testudinoidea
(including the turtle family Geoemydidae composes a quarter of modern
turtle species, primarily pond turtles of the Old World; Sasaki, 2006;
Spinks, 2004; Le, 2006).