atlas centrum
facets for atlantal
intercentrum
atlantal intercentrum
axial intercentrum
facets for neural arches
facets for atlantal
atlas centrum
for axial intercentrum
axis centrum
3rd cervical centrum
for intercentrum of
3rd cervical centrum
atlantal intercentrum
axial intercentrum
atlas centrum
axis centrum
3rd cervical centrum
facet for ihtercentrum of
3rd cervical centrum
facet for axial
intercentrum
Fig. 5. Atlas-axis complex with fused third cervical centrum of Ichthyosaurus communis (BMNH R6697). A-C, without intercentra;
D-F, with intercentra attached. A,D) anterior, B,E) left lateral, and C,F) ventral views. Scale equals 20 mm.
Fig. 6. Atlas-axis complex and third cervical vertebra of Ichthyosaurus communis (BMNH R6697). A-C) neural arches and spine of the
atlas and axis; D-F) neural arch and spine of the third cervical vertebra; G) first three vertebrae in their entirety. A,D) anterior, B,E,G)
left lateral, and C,F) posterior views. Scale equals 20 mm.
lost. In one specimen (BMNH R1205), the atlantal intercentrum
has been lost, but the smaller axial ossification has
been retained (Fig. 4E). In another specimen, acid-prepared
from nodules (BMNH R6697) and identifiable as
Ichthyosaurus communis, both ossifications are present as
separate, unfused elements (Fig. 5). Here the third cervical
centrum is fused with the axis centrum. Like the axis
centrum, its ventral surface is drawn out, giving the centrum
a heart-shaped outline in posterior view. Furthermore,
a small angular facet is formed between its ventral
prominence and that of the adjacent axis centrum (Fig. 5C).
This is undoubtedly the articular facet for an unfused
intercentrum (of the third cervical vertebra), but this small
element has not been preserved.
Fig. 7. Neural arch and spine, probably from the posterior dorsal
series, showing well separated posterior zygapophyses (BMNH
R1201), in posterior view. Scale equals 20 mm.
atlas centrum
axis centrum
A
facet for atlantal intercentrum
floor o f neural
canal
axis centrum
atlas centrum
diapophysis
parapophysis
axis centrum
facet for atlantal
intercentrum
Fig. 8. Atlas-axis complex of Ophthalmosaurus (BMNH 32701). A) anterior, B) posterior, C) dorsal, D) left lateral, and E) ventral
views. Scale equals 30 mm.