Palaeos Vertebrates : 200.300 Anapsida : Testudines (original) (raw)
| Pal�os: |  |
Unit 200: Anapsida |
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| The Vertebrates | 300: Testudines |
Taxa on This Page
The Origin of Turtles
The origin of turtles and tortoises (Chelonia) from ancestral reptiles is still unclear. There are many different rival theories of where Chelonians fit on the evolutioinary tree. These can be summarised briefly as follows:
Eunotosaurs: The strange Permian reptile Eunotosaurus was originally proposed as the ancestor to chelonia, on the basis of the broadly expanded ribs which form a shell-like structure covering its body. It was since discovered that the shell of turtles is made up of narrow ribs covered by dermal bone, not broadly expanded ribs. This, plus the presence of an ectopterygoid bone in the eunotosaur skull, indicate that this creature was not a turtle ancestor
Captorhinids: A more common view, until recently, was that the closest extinct relatives of turtles were the primitive Permian "cotylosaurian" reptiles of the family, Captorhinidae. Like all turtles, members of this group lack the ectopterygoid bone and have a large medial process of the jugal bone of the skull. Gauthier_et al_. (1988) argue strongly for close affinities between captorhinids and turtles. However, no known captorhinid shows any sign of even a partly developed shell. There is also a gap of many millions of years between the last known captorhinid and the earliest known turtle.
Procolophonids: It has recently been proposed that the Triassic herbivorous anapsids called Procolophonids are the closest relatives of turtles. (for some characters unique to procolophonoids and turtles see Procolophonoidea: Morphology) Once again, no known procolophonid shows any tenedency of even a partly developed shell.
Placodonts: It had been suggested some time ago that turtles are closely related to the turtle-like placodonts. This idea has recently been revived by Rieppel (1995), and would certainly explain why chelonids are unknown in the fossil record until the late Triassic. This means that turtles are not derived from an extinct anapsid group but evolved within diapsids and are thus more closely related tolizards, plesiosaurs, etc than to Permo-Triassic anapsids.
Pareiasaurs: 
Lee (1996) has argued that the large armoured herbivorous Permian pareiasaursare the closest known relatives of turtles, rather than the small procolophonids. This is indicated by the fact that towards the end of the Permian some pareiasaurs became reduced in size, and some, such as the smallish genus Anthodon (left) developed a turtle-like pattern of boney plates from the dermal armour (osteoderms) embedded in their skin. This indicates that the pareiasaurs are the direct ancestors of the turtles. Ther are however two objections to this persuasive hypothesis. First, the last known pareiasaurs died out at teh end of the Permian, millions of years before the earliest known turtle appeared. It is of course possible that turtles may have reamained rare and insignificant all that time (and hence avoided fossilisation). A much more serious objection is raised by G. O. Cherepanov:
"Generally, the bony armor of reptiles consists of separate osteoderms which develop in the dermis independently from the internal skeleton. Such a dermal armor is characteristic of ancient pareiasaurs that are closely related to Testudines. On the basis of this fact it has been assumed that the turtle shell is built up as the fusion of the osteoderms with some elements of the internal skeleton. However, this concept is not confirmed by morphogenetic data. During turtle ontogeny the reduction of the trunk myomeres leads to sinking of the vertebrae and ribs into the dermis. The neural and costal plates of the dorsal disk form as the outgrowths of these endoskeletal bones on inside the dermis. There are no independent superficial ossicles over the general plastral bony primordia in the ventral region of the body. Each plastral plate develops only from a single primordium like the clavicles, interclavicle and gastralia of other reptiles. Most likely, Testudines progressed in a unique evolutionary direction. Their bony shell is mainly the result of modification and consolidation of internal skeletal elements. The real osteoderms develop only on the body margins as connections between the dorsal and the ventral discs."
abstract: THE ORIGIN OF THE BONY SHELL OF TURTLES AS A UNIQUE EVOLUTIONARY MODEL IN REPTILES - Gennady O. Cherepanov from Abstracts of Russian Journal of Herpetology No.2(97)
The origin of turtles thus remains a mystery. (MAK)
A Primer on the Swiss Loi de Douane and Turtle Shell Anatomy
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| 1 | Cervical (1) | 1 | Nuchal (1) |
| 2 | Marginals (12L+12R) | 2 | Peripheral (11L+11R) |
| 4 | Pleurals (4L+4R) | 3 | Neural (8) |
| 5 | Vertebral (5) | 4 | Costal (8L+8R) |
| 6 | Pygal (1) | ||
| 7 | Suprapygal (1) |
Only the Swiss would would define turtles by law. Then again, perhaps only Americans would find this amusing. In any case, the Swiss Federal Veterinary Office of the Department of Economic Affairs has adopted very detailed regulations (in both French and German) describing the various turtle species. It is unclear just what would occur if a turtle evolved, or developed some unregistered mutation. Undoubtedly, the legal consequences would be quite severe. Let it be clearly understood that all reptiles are well-advised not to behave in such a disorganized fashion within the borders of the Swiss Republic.
Fortunately, this loss to Chelonian liberty is partially offset by the public availability of a certain amount of zoological information. Turtle shells are constructed with a layer of epidermal scutes overlying a layer of dermal bone. Pursuant to � 820.112.41(1), the FVO has decreed that the scutes of all turtle carapaces (dorsal shells) shall have the form shown in the figure at right.
The names have been changed to protect the innocent and, more to the point, to reflect the generally accepted names of the scutes. Unfortunately, the original regulations confuse the names of the epidermal scutes (shown here) with the names of the underlying dermal bones. The bones do not conform to the same pattern and have rather different names. The marginal scutes are numbered from the cervical, in both directions, so that the marginal immediately to the left of the cervical, for example, is the First Left Marginal. The scute numbered "2" is thus the Second Right Marginal. Similarly, the scute labelled "4" is the Second Right Pleural and "5" points to the Third Vertebral. This pattern, like the pattern of the dermal bones and the pattern of the scutes and bones of the plastron (ventral shell) is remarkably consistent across all turtles, paticularly extant forms.
The underlying dermal bones are shown in the right half of the figure. These are numbered in the same fashion as the scutes. The first four pairs of costals each fuse with a pair of ribs.
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| 1 | Gular | 1 | Epiplastron |
| 2 | Humeral | 2 | Entoplastron |
| 3 | Axillary | 3 | Hyoplastron |
| 4 | Pectoral | 4 | Hypoplastron |
| 5 | Abdominal | 5 | Xiphiplastron |
| 6 | Inguinal | ||
| 7 | Femoral | ||
| 8 | Anal |
The corresponding pattern of scutes and bone in the plastron is shown in the figure at left.
What is perhaps most extraordinary about this arrangement is that turtles have maintained this particular dual structure for well over 100 My -- even without regulatory supervision under the ever-vigilant eyes of the Federal Veterinary Office. There appears to be no satisfactory explanation for the remarkable stability of this particular arrangement. Bear in mind that turtles inhabit an extraordinary number of different environments and have developed some impressive adaptations. Thus for example, the pleurodire Rheodytes has evolved the ability to breath through its cloaca, a talent rarely found except among politicians. Yet, something as apparently trivial as the pattern of epidermal scutes has remained almost completely unaffected from the Triassic Proganochelys to the present day.
Either this arrangement has some unknown, but obviously critical, functional significance, or we are missing an important point about evolution. Of all the many fundamental unanswered questions in vertebrate paleontology, one of the most strange and difficult is the mundane constancy of turtle shells.
Descriptions
Testudines **:**Many workers prefer "Chelonia" for the stem group, reserving "Testudines" for the crown group (LCA extant turtles). For various reasons, the traditional nomenclature is used here.
Range: from the Triassic.
Phylogeny: Procolophonia :: Hallucicrania+ * : Proganochelys+ (Pleurodira+ Cryptodira).
Characters: Double shell (carapace & plastron) with dermal scutes. Beak covers maxilla, premaxilla & dentary. Retracted neck in K; otic capsule enlarged & jaw adductor passes over this or trochlear process of pterygoid, giving more vertical pull on lower jaw; carapace with 59 dermal bones, 8 fused to neural arches, 8 pairs fused to widened ribs; ribs external to girdles; plastron dermal bone plus clavicles & interclavicals; shell kinesis in some species;
Notes: Mesozoic forms, even in the Cretaceous, apparently had shell fontanelles, less remodeling of ribs, and underdeveloped shell elements compared to modern forms.
Image: from Herp Pictures - Turtles (former site) by permission.
Links: Higher Reptile Taxa; Turtle and Tortoise Ring (with 224 sites at last count!); Introduction to Testudines; Order Testudines (ADW); Testudines Turtle Skull Reproductions; Higher Relationships of Testudines; Order Testudines (Czech & English); Tortoise Trust Web - Start Page; Willkommen beim Reptilien Projekt (German & English); Turtles and Tortoises; The Center for North American Herpetology - Web Portal; Testudines(Tree of Life); Testudines Turtle Skull Reproductions(great collection of skull casts); Georgia Wildlife Web Site; reptiles- testudines(nice summary of diversity); Testudines(Spanish); APUS.RU | ????? �???????? - Testudines(Russian); NatureServe Explorer Species Index- Order TESTUDINES; BiologyBrowser(links); Testudines (turtles, tortoises)(South African turtles); Link to NCBI Front Page;The EMYSSystem (Best on the Web); Testudines;Order Testudines.
References: Gauthier et al. (1988); Lee (1996); Rieppel (1995); Shaffer et al. (1997). ATW030422.
Proganochelys**:**most primitive known turtle.
Range: Late Triassicof Europe, related forms in SE Asia & NAm.
Phylogeny: Testudines : (Pleurodira+ Cryptodira) + *.
Characters: turtle skull and beak; but denticles on vomer, palatine and pterygoid; maxilla, premaxilla, and dentary edentulous; vomers paired (primitive); interpterygoid vacuity present; moveable basipterygoid articulation (primitive); nasals paired; lacrimal small, but present, with lacrimal duct (primitive); supratemporal present (primitive); expanded posttemporal fenestra (allows jaw muscles to become longer and stronger -- muscle origin on occiput); quadrate concave posteriorly and exposed laterally on cheek; postparietals and postfrontals absent; stapes solid, rodlike, without foramen or processes; middle ear without bony lateral wall (primitive); **$**paroccipital process of opisthotic attached to braincase only at its distal end (primitive); paroccipital process only loosely attached to squamosal (note: this is slightly inconsistent with previous character); squamosal & postorbital tightly sutured; cervical vertebrae not specialized; 10 trunk vertebrae; 10th trunk vertebra not in sacrum; $ tail club (? or at least osteoscutes on tail); fully developed bony shell consisting of a carapace (formed from costal bones with fused ribs, neural bones with fused thoracic vertebrae, and marginal bones), a plastron (formed from interclavicle, clavicle, and five paired bones sutured together); carapace and plastron enclosing shoulder girdle and pelvic girdle; cleithrum absent; gracile scapulocoracoids and pelvic girdle; pelvic girdle not fused to carapace; **$**phalangeal formula (manus and pes) of 2-2-2-2-2 (maybe 2 digits with 3 phalanges on manus?); fresh water habitat, but not exclusively aquatic.
Links: Proganochelys(Best on the Web); Anapsida: Life History and Ecology; Phylogeny and Classification of Amniotes (figs 1D, 2D, & 3D are Proganochelys);Molecular Evidence for Higher Relationships Among Turtles; Biology 356; tortugas.html(Spanish); qui (French -- with homemade Bach-omorph fugue & variations!); Turtle Origins; The Paleo Art of D.W. Miller; Turtle.htm;Proganochelys; American Museum of Natural History: Proganochelys; turtle;Paleontology and Geology Glossary: Pr;I. �Qu� tan viejas son las tortugas? (Spanish). ATW010606.
checked ATW040118
Text marked 'MAK' by M. Alan Kazlev
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