TURTLES

 

EVOLUTIONARY MODEL

If evolutionary theory is correct, then modern turtles have not always existed. Modern lineages would have developed gradually over time. Traits associated with turtles (such as dermal armor or even a shell) could develop in other groups. Signficant variation could occur within specific turtle lineages.

CREATIONISM MODEL

If the creationism model is correct, modern turtle have always existed and there is no reason to expect fossils which are transitional between modern and more primitive forms. Turtles should not vary within a kind and, given that separate kinds are completely unrelated, it should be easy to identify the number of kinds of turtles.

INTELLIGENT DESIGN

If intelligent design is correct, then any adaptations which are identified as irreducibly complex should not be represented by transitional forms in early turtles or other ancient reptiles.


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).

proganochelys


The early Jurassic turtle Australochelys is intermediate between the earliest turtles (such as Proganochelys) and modern groups (Gaffney, 1994).

austrachelys

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).

 

TURTLE DESIGN
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.

archelon

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.

ribs

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.

scutosaurus
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, 1997).
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 and Stagonolepidae.

Phytosaur

phytosaur

Aetosaur

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).

placodontplacodont

placodont
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.

TURTLE VARIATIONS
REDUCED SHELL
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, 2006).
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).
OTHER VARIATIONS
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, 1956).

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).
(Robinson;1997)
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).

jaw muscle attachment skulls