Anatomical study of two previously undescribed specimens of Clevosaurus hudsoni (Lepidosauria: Rhynchocephalia) from Cromhall Quarry, UK, aided by computed tomography, yields additional information on the skeleton and hitherto undescribed bones
Author
O, Aileen
Author
Brien
Author
Whiteside, David I.
Author
Marshall, John E. A.
text
Zoological Journal of the Linnean Society
2018
2017-12-15
183
163
195
journal article
3394
10.1093/zoolinnean/zlx087
fa156df1-0882-4b3c-9146-cc38770e4b1d
0024-4082
5714674
GENUS:
CLEVOSAURUS
SWINTON, 1939
Type
species
:
Clevosaurus hudsoni
Swinton, 1939
Included species
:
Clevosaurus minor
Fraser, 1988
;
C. latidens*
Fraser, 1993
;
C. bairdi
Sues, Shubin & Olsen, 1994
;
C. wangi
Wu, 1994
,
C. mcgilli
Wu, 1994
and
C. petilus
Wu, 1994
;
C. convallis
Säilä, 2005
;
C. brasiliensis
Bonaparte & Sues, 2006
and
C. sectumsemper
Klein
et al.
, 2015
.
Remarks
: The most recent cladistic analysis by
Hsiou
et al.
(2015)
resolved a
Clevosauridae
clade with the following apomorphic features: antorbital region forming one-quarter of the skull length (reversed to between one-third to one-quarter in
C. brasiliensis
,
C. wangi
and
C. petilus
); a narrow and elongated dorsal process of the jugal; palatine teeth forming a single row, plus one isolated tooth.
Hsiou
et al.
(2015)
define the clade as ‘all taxa more closely related to
Clevosaurus
than to
Sphenodon
’ (
Hsiou
et al.
, 2015: 4
). *
Clevosaurus latidens
is positioned outside of
Clevosauridae
in this analysis.
Characters that occur in clevosaurs, but are not restricted to the genus include: a lateral forked flange of the premaxilla preventing contact between the maxilla and the external naris [horizontal posterior flange not present in
C. convallis
(
Säilä, 2005
)
]; a dorsally expanded lateral process of the premaxilla; suborbital fenestra bounded solely by the ectopterygoid and palatine; a high, steeply inclined coronoid process of the dentary; flanged teeth; a broad maxillary-jugal contact (
Säilä, 2005
;
Bonaparte & Sues, 2006
;
Jones, 2006
;
Hsiou
et al.
, 2015
;
Klein
et al.
, 2015
).
Diagnosis
: Based on
Swinton (1939)
,
Robinson (1973)
and
Fraser (1988)
, specimen NHMUK PV R36832 can be diagnosed as
C. hudsoni
. The diagnosis is based on the following principal features, identified in the specimen:
• Acrodont dentition. The maxilla bears four large additional teeth that increase in size posteriorly; the teeth are conical in form and have posterolingual flanges. Three smaller conical teeth occur posteriorly to that additional set on the maxilla. The dentary has four large, conical, additional teeth which increase in size posteriorly; these additional teeth have anterolateral flanges. • Teeth are evident on the pterygoid and palatine. Two rows of teeth are present on the pterygoid. A single lateral row of large teeth is present on the palatine but because of incomplete preservation of the medial region of the palatine, the small isolated tooth medially offset from
Figure 3.
Photographs and surface models of
Clevosaurus hudsoni
specimen NHMUK PV R36832. A, entire specimen. B, diagrammatic representation of the specimen with key showing location of views in (C–F) and in Figure 4. Skull bones in (C, D) dorsolateral view and (E, F) left lateral view (surface models are in artificial colour in all figures).
170 A. O’BRIEN
ET AL
.
Figure 4.
Postcranial bones of
Clevosaurus hudsoni
specimen NHMUK PV R36832. (A) photograph and (B) surface model (refer to Fig. 3B for location on specimen).
the lateral row, typical of clevosaurs, is not recorded.
• Incomplete lower temporal bar. There is a gap between the posterior process of the jugal and the quadrate on this specimen.
Fraser (1988)
notes that in some specimens of
C. hudsoni
, weak contact is made between the jugal and quadratojugal. There is no contact on this specimen.
• A flattened, plate-like quadrate present.
• Large pineal foramen.
• Postorbital triangular in shape.
• Supratemporal present.
Specimen NMHUK PV R36846 is designated the hind limb of
C. hudsoni
based on its great similarity to
C. hudsoni
, figured and described by
Fraser (1988
: fig. 35). Identifiable features on this specimen include:
• Size of tibia falls within size range of elements measured by
Fraser (1988)
.
• Hooked fifth metatarsal.
• Fifth tarsal fused to fifth metatarsal to form a single tarsometatarsal.
A single astragalocalcaneum, formed by fusion of ankle bones, is considered diagnostic of clevosaurs but on this specimen the elements appear to be separate (see description of NMHUK PV R36846 below).