Palæos:		Unit 200: Anapsida
The Vertebrates		400: Pleurodira

Anapsida: Pleurodira

REPTILOMORPHA
|--SYNAPSIDA
`--+--EUREPTILIA 
   |   
   Anapsida
   |--Bolosauridae 
   `--Procolophonia
      |--Procolophonoidea 
      `--+--Hallucicrania 
         |  |--Lanthanosuchidae 
         |  `--Pareiasauria 
         `--Testudines
            |--Pleurodira 
            |  |--Proterochersis 
            |  `--Eupleurodira
            |     |--Chelidae 
            |     `--Pelomedusoidea
            |        |--Podcnemoidae
            |        |  |--Bothremyidae 
            |        |  `--Podocnemidae
            |        `--Pelomedusidae
            `--Cryptodira

1. Bothremyidae X
2. Chelidae
3. Eupleurodira
4. Pelomedusidae
5. Pelomedusoidea
6. Pleurodira
7. Podcnemoidae
8. Podocnemidae
9. Proterochersis X

Descriptions

Pleurodira: Platychelys. 

Range: from Late Triassic.

Phylogeny: Testudines:: Cryptodira + *: Proterochersis + Eupleurodira. 

Characters: Skull roof not complete; $ no epipterygoid (not even cartilage); $ major jaw adductor passes over enlarged lateral trochlear process of pterygoid; retracted neck is S-shaped horizontally; neck retraction by ball-and-socket joints; cervical postzygapophyses closed or fused; $ fusion of all pelvic elements to shells (ilium fused to carapace; pubis & ischium to plastron); sacrum may involve up to 4 vertebrae (2 in cryptodires). Now S hemisphere only; dominant in Aus; formerly world-wide (until Cz). Almost all fresh water species. 

Links: Anapsida -- The Dinosauricon (cladogram); Animal Fact Sheets; Pleurodira; PLEURODIRA; Order Testudines; Higher Reptile Taxa; Yahoo! Groups : Pleurodira; Dr. Christian Beisser; ETI - Turtles of the World: Classification; histo (Spanish); DigiMorph - Elseya dentata (and related pages); Phylogeny of Turtles/Dr. E. S. Gaffney (Best on the Web); I. ¿qué tan viejas son las tortugas? (Spanish).

Discussion: One startlingly odd thing about turtle phylogeny is the ability of all turtles, except clearly derived specialists, to withdraw the head inside the carapace. This appears to be a unique ability, but not a synapomorphy, i.e. not a shared derived character. Pleurodires diverged from cryptodires perhaps as early as the late Triassic (perhaps 220 My), and certainly by the late Jurassic (150 My). However, no turtle is known to have possessed the specializations needed to withdraw the head into the carapace until the Cretaceous (roughly 100My), at which point both major turtle lineages developed the same ability using quite different biomechanical strategies. In fact, there is enough variation among the methods used by Cryptodires that the ability may have developed more than twice, entirely independently.

On its face, this is a bizarre set of facts. Why would two separate lineages develop the same strategy at roughly the same time, long after the two had diverged? It is not particularly strange that both groups would develop the ability to withdraw the head. Turtle morphology is quite conservative and, even after 50-100 My, all turtles had a great deal in common. But what conceivable selective pressure would emerge for the first time in the Cretaceous to force both lineages to develop the same strategy and eliminate virtually all turtles which could not adapt in this unique fashion? 

There are a number of possible speculative answers -- bird predation comes to mind, for example. However, turtles cover a good many ecological roles in a wide variety of environments, both terrestrial and marine. Any explanation must apply to all of these cases.

References: ES Gaffney (1975), Bull. Am. Mus. Nat. Hist. 155:387-436. Basic changes in skull morphology may have occurred prior to pleurodire- cryptodire split. ES Gaffney & JW Kitching (1994) Nature 369:55. 020225.

Proterochersis: oldest known pleurodire.

Range: Late Triassic of Europe. 

Phylogeny: Pleurodira: Eupleurodira + *. 

Characters:  many standard pleurodire features, e.g., pelvis fused to shell. 

Links: Studium Integrale Journal 6/2 (Oktober 1999): Streiflichter (German); Untitled Document; Pleurodira. 010412.

Eupleurodira: 

Phylogeny: Pleurodira: Proterochersis + *: Chelidae + Pelomedusoidea. 

Characters: cervical centra not amphiplatyan. 

Links: Pleurodira. 020202.

Chelidae: Chelodina, Emydura, Elseya (all Aus), Platemys, Hydromedusa, Phrynops (all S.Am.). Small to medium-sized "snake-neck" aquatic turtles of Aus & S. Am.

Range: from the Late Cretaceous of Australia & South America. 

Phylogeny: Eupleurodira: Pelomedusoidea + *. 

Characters:  Quadratojugal absent; cheek emargination from ventral margin of skull, extending more posteromedially than in any other turtle group; unique arrangement of the articulations among the neck vertebrae, cervical centra five and eight are convex, mesoplastron absent; intergular (extra scute on plastron) present; extremely long necked ambush predators of fish, or shorter-necked molluscovores with typical durophagous dental specializations. 

Links: infoplease; Turtle, Tuatara, Crocodile Checklist--13; Chelidae; THE TURTLE GALLERY; Turtle, Tuatara, Crocodile Checklist--Literature Cited; CTTC - Matamata, Chelus fimbriatus by William H. Espenshade; Testudo hermanni site. Chelidae; chelidae (Japanese); ETI Turtles Info - Temperature-dependent sex determination; Molecular Evidence for Higher Relationships Among Turtles; DGHT-AG Schildkröten: Arbeitskreise (Deutsch); Mag. Christian Beisser (surprisingly cool research!); pintroduction (Français); side-necked turtle. The Columbia Encyclopedia; ????????? (Chinese); WWW.chelidae.COM / home; ??????? chelidae (Japanese, I think); Chelidae, Schlangenhalsschildkröten (German); Familie Chelidae, Schlangenhalsschildkröten: Bilder (German); familia Chelidae; Testudo hermanni site; Digimorph - Elseya dentata (northern snapping turtle); tierdach.de: Halswender-Schildkröten (German); Digimorph - Chelus fimbriatus (mata mata); pintroduction (French); Turtles and Tortoises; CTTC - Matamata, Chelus fimbriatus by William H. Espenshade; ??? - ??????? Chelodina longicollis (Chinese).

Image: Chelodina novaguinea, courtesy Darrell Senneke of the Tortoise Trust, USA. ATW020910.

Pelomedusoidea: 

Phylogeny: Eupleurodira: Chelidae + *: Pelomedusidae + Podcnemoidae. 

Characters: See Pelomedusidae.  ATW030416.

Podcnemoidae: 

Range: fr up(?)K. 

Phylogeny: Pelomedusoidea: Pelomedusidae + *: Bothremydidae + Podocnemidae. 

Characters: Splenial absent(?); nasals absent; $ quadrate contacts basioccipital; parietal & jugal in contact; post-otic antrum small or moderate; cervical centra generally biconvex; cervical scute absent; mesoplastra reduced and lateral; neural series on carapace not complete, interrupted by costals; pectoral scutes usually contact entoplastron. 

Links: Pleurodira; Araripemys barretoi. 

References: Gaffney & Meylan (1988); Meylan (1996). 

Note: This is the clade that unites the extant podocnemids with the Cretaceous bothremyids. The original cladogram of Gaffney & Meylan (1988) described the bothremyds as the sister group of living pelomedusids, and united the two based on the occipital condyle being formed only by the exoccipitals (that is, the basioccipitals were excluded from the condyle). Meylan (1996) reanalyzed the data with the inclusion of new material from very basal pelomedusoids and determined that the bothremyds were, instead, intermediate between the pelomedusids and podocnemids. Given the extensive re-engineering of neck joints in all turtles in the late Cretaceous, it makes good sense to suspect that podocnemids secondarily re-integrated the basioccipitals with the occipital condyle at about that time, when they diverged from the bothremyd stock. 011109.

Bothremyidae:  Bothremys, Foxemys, Polysternon, Rosasia, Taphrosphys

Range: Late Cretaceous to Miocene. Cosmopolitan.

Phylogeny: Podcnemoidae: Podocnemidae + *.

Characters: $ stapedial canal open anteriorly; $ eustachian tube separated from columellar notch; $ quadrato-basisphenoid covers prootic in ventral view; marine & fresh water.

Links: Museé. Dinosaures - les Collections (in French. English at MuséeDinosaures - Types; Chéloniens (in French); Pleurodires - research in progress (Best on the Web).

Image: Foxemys Late Cretaceous of France, from Museé. Dinosaures - les Collections.

Notes: There are a completely unreasonable number of excellent web resources for such an obscure taxon. 011001.

Podocnemidae: (=Podcnemidae): Erymnochelys, Neochelys, Peltocephalus, Podocnemis. 

Range: Fresh water turtles from South America. 

Phylogeny: Podcnemoidae: Bothremyidae + *. 

Characters: Carotid canal greatly enlarged, forming large channel; large pterygoid flange extends posteromedially covering prootic. 

Links: Pleurodira; Podocnemidae; Untitled Document. ATW010518.

Pelomedusidae: Pelomedusa, Pelusios, Platycheloides, Teneremys.  Medium to large FW aquatic turtles from Africa and South America. Defined polyphyletically as all living pelomedusoids.

Range: from the Early Cretaceous. 

Phylogeny: Pelomedusoidea: Podcnemoidae + *. 

Characters: $ frontals have transverse anterior margin rather than projecting anteriorly between the prefrontals; $ quadrate completely surrounds stapes; quadrate does not contact basioccipital; carotid enters the skull through the prootic; $ posterior pleurals meet on midline between neurals & suprapygal. 

Links: Pelomedusidae; Turtle, Tuatara, Crocodile Checklist--15; Summary sequence view; Zardoya, R & A Meyer (1998); UTDMG - Pelusios sinuatus; Pelomedusidae; CTTC - Matamata, Chelus fimbriatus by William H. Espenshade; Pelomedusidae (Spanish); pelomedusidae (Japanese); ETI Turtles Info - Pelomedusa subrufa example; side-necked turtle. The Columbia Encyclopedia, Sixth Edition. 2001; Schildkröten (German); pelomedusidae, pelomedusenschildkröten, reptilien, reptilia (German); ??????? pelomedusidae (Japanese); Digimorph - Pelusios sinuatus (East African serrated mud turtle); Testudo hermanni site (Italian); pintroduction (French).  

Image: Pelomedusa: Swiss Federal Veterinary Office regulations § 820.112.46(1). ATW020802.

checked ATW030416