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Plesiochelys planiceps

Reptilia - Testudines - Plesiochelyidae

Taxonomy
Chelone planiceps was named by Owen (1842). Its type specimen is OU-JI582, a partial skeleton (nearly complete skull lacking nasals, anterior margins of apertura narium externa, and most of posterolateral skull roof (i.e., jugal preserved only on right si), and it is a 3D body fossil. Its type locality is Plesiochelys planiceps type (?Isle of Portland), which is in a Tithonian marine sandstone in the United Kingdom.

It was recombined as Stegochelys planiceps by Lydekker (1889) and Delair (1958); it was recombined as Plesiochelys planiceps by Gaffney (1975), Pérez-García and Murelaga (2013), Anquetin et al. (2015) and Anquetin et al. (2017).

Synonymy list
YearName and author
1842Chelone planiceps Owen pp. 168-169 figs. first plate 8, figs 1-3
1889Stegochelys planiceps Lydekker p. 229
1958Stegochelys planiceps Delair p. 55
1975Plesiochelys planiceps Gaffney pp. 6-7
2013Plesiochelys planiceps Pérez-García and Murelaga
2015Plesiochelys planiceps Anquetin et al.
2017Plesiochelys planiceps Anquetin et al.

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RankNameAuthor
kingdomAnimalia()
Triploblastica
Nephrozoa
Deuterostomia
phylumChordataHaeckel 1847
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
Sarcopterygii
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda()
Reptiliomorpha
RankNameAuthor
Anthracosauria
Batrachosauria()
Cotylosauria()
Amniota
Sauropsida
classReptilia
Testudinata(Oppel 1811)
orderTestudinesBatsch 1788
Thalassochelydia
familyPlesiochelyidaeRütimeyer 1873
genusPlesiochelys
speciesplaniceps()

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Diagnosis
ReferenceDiagnosis
E. S. Gaffney 1975Member of the Plesiochelyidae (sensu Gaffney, present paper) known only from the skull; distinguishable from Plesiochelys etalloni by the following features: lingual ridge of maxilla higher than in P. etalloni, anterior portion of lingual ridge on lower jaw curving medially (as opposed to anteriorly in P. etalloni), at level of vomer-premaxilla suture the distance between lingual ridges of maxillae being wider than in P. etalloni.
J. Anquetin et al. 2017Plesiochelys planiceps can be diagnosed as a “plesiochelyid” and a representative of Plesiochelys by the full list of cranial characters provided for these taxa above. Plesiochelys planiceps differs from other Plesiochelys spp. by its larger size, a higher lingual ridge on the maxilla, the absence of a contact between the parietal and the quadrate posterior to the foramen nervi trigemini, a strongly developed processus trochlearis oticum, a canalis caroticus internus situated deep within bone, a foramen posterius canalis carotici interni opening on the posterior margin of the pterygoid and not visible in ventral view, and each exoccipital forming one-third of the condylus occipi- talis. In addition, Plesiochelys planiceps differs from Plesiochelys etalloni by having a wider distance between the lingual ridges of the maxilla at the level of the pterygoid-vomer suture, an elon- gated foramen palatinum posterius, and the anterior portion of the lingual ridge on the dentary curving medially (as opposed to anteriorly). It differs from Plesiochelys bigleri by having a higher temporal skull roof, a deeper pterygoid fossa, a more developed processus trochlearis oticum, a foramen anterior nervi abducentis located more posteriorly relative to the base of the proces- sus clinoideus, and the foramina anterius canalis carotici cerebralis located almost vertically below the dorsum sellae.