dorsal
facet for radius
facet for radius deltopectoral
crest
facet
for
ulna
erosional
dorsal
process
ulna for
intermedium
facet
for
intermedium
dorsal
Fig. 48. Isolated left humerus of an ichthyosaur from the Upper Greensand (BMNH 43989). A) dorsal, B) ventral, C) proximal, and
D) distal views. Scale equals 40 mm. This specimen was described as the holotype of Ophthalmosaurus cantabrigiensis Lydekker, 1888;
it is here referred to as Brachypterygius cantabrigiensis.
(Fig. 47D). Distally, the humerus is broadly elongated in
the plane of the fin, and has a distinctive profile, pointed
anteriorly and bluntly pointed posteriorly, which is not
seen in any other ichthyosaur (Fig. 45E). There are two
large and fairly deeply excavated articular facets, the posterior
one for the ulna, the anterior one for the radius
(Kdrton, 1983). A third facet, which is small and often
indistinct, occupies the pointed leading edge of the distal
end, and has been interpreted by Kirton as being for a
preaxial accessory element. A ndrews (1910) identified this
facet, which he considered to be postaxial, as for a pisiform
element. The entire distal surface is rugose, again suggestive
of an extensive covering of cartilage. In many individuals,
there is a prominent tubercle on the dorsal and ventral
surfaces, immediately proximal to the distal edge and
close to the ulnar facet. Sometimes only one tubercle is
present (Fig. 47A), sometimes both are absent. In some
specimens, such as BMNH R2856 and R3702, where the
epipodials are attached to the humerus, the tubercles are
on the proximal end of the ulna instead (Kirton, 1983).
Andrews (1910) and A ppleby (1958) interpreted these processes,
which are found in about half the specimens, as
being sexually dimorphic, a suggestion rejected by Kirton
(1983). The anterior margin of the shaft is bluntly rounded,
but the posterior one is raised into a moderately sharp edge.
Because the humeri are fairly robust, they are often
found as isolated elements, much like the basioccipitals.
Again, as for the basioccipitals, there are many isolated
humeri from the Upper Greensand that do not conform to
Ophthalmosaurus (Fig. 48). Included here are six specimens
in the Natural History Museum, London: BMNH R2343
[three specimens with the same number], R2342 [two specimens],
and 43989. They all share the distinctive feature of
having three almost equal-sized distal facets (Fig. 48D).
Ophthalmosaurus also has three distal facets, but the most
anterior of these is very small compared with the others,
and is often poorly differentiated. Furthermore, its leading
edge is sharply pointed (Fig. 47E). Platypterygius, in contrast,
has only two distal facets (Kiprijanoff, 1881; McGowan,
1972C). However, Brachypterygius has three well-differentiated
distal facets (McGowan, 1997). Furthermore, as
noted earlier, some basioccipitals are known from the
Upper Greensand that bear a strong resemblance to those
of Brachypterygius. It is therefore concluded that the Upper
Greensand humeri, as well as the basioccipitals, are referable
to Brachypterygius. L ydekker (1888) designated the
humerus figured here (BMNH 43989) as the holotype of a
new species, Ophthalmosaurus cantabrigiensis L ydekker,
1888, which is referred to here as Brachypterygius cantabrigiensis.
Remainder of the Forefin
The forefins of typical Early Jurassic ichthyosaurs, like
Ichthyosaurus, exemplify extreme hyperphalangy and varying
degrees of hyperdactyly (Fig. 49). As Motani (1999A)
has shown, digit I has been lost, but the acquisition of
additional digits has more than compensated for this. The
individual elements are subrectangular or polygonal, tending
towards being discoidal towards the tip. Discoidal
humerus
radius
radiale
distal carpals
ulna
intermedium
ulnare
postaxial
accessory
digits
Fig. 49. Isolated forefin (left) of Ichthyosaurus communis (ROM
337), shown in dorsal view. Scale equals 50 mm.
Fig. 52. Isolated phalanx of Ophthalmosaurus icenicus (BMNH
R3702), A) planar and B) marginal views. Scale equals 20 mm.
B articular facets
Fig. 50. A single phalanx, from a disarticulated forefin of Ichthyosaurus
communis (ROM 12814). A) planar and B) marginal views.
Scale equals 5 mm.
Fig. 51. Right forefin of Ophthalmosaurus icenicus (BMNH R3702),
dorsal view. Scale equals 100 mm.
elements are also found in the accessory digits - a descriptive
term applied to those digits that extend for only a part
of the fin length, and which are preaxial, and postaxial, in
position.
The fins of Early Jurassic ichthyosaurs are invariably
embedded in matrix, and only exposed in dorsal or ventral
aspect, enhancing the impression that their individual
components are fairly flat. However, they are quite thick,
providing a substantial area of contact for their articular
surfaces (Fig. 50). Most of the fin elements are closely
packed, forming a tightly interlocking mosaic and making
for a rigid structure.