American Museum of Natural History

OSBORN, THE REPTILIAN SUBCLASSES DIAPSIDA AND SYNAPSIDA. 501
Composition of Vertebra?.- The primitive component parts of each vertebra
are 6, namely: 2 hypocentra, 2 pleurocentra (centra), and 2 neurocentra (neural
arches) as seen in embryos of Sphenodon. The opposite pairs of pleurocentra
and neurocentra early coalesce. Pleurocentra and neurocentra, or centra and
neural arches, are either suturally separate (Protorosauria, Pelycosauria, young
Rhynchocephalia), or firmly coalesced (Proganosauria). The centra are primitively
amphiccelous and perforated by a notochordal canal, as seen in certain
Protorosauria (Palaohatteria, Kadaliosaurus), in Pelycosauria, in Proganosauria,
and in certain Rhynchocephalia. In other forms they are imperforate. Another
exception recorded is in the elongate cervicals of Protorosaurus, which are
opisthoccelous. Hypocentra are figured as bilaterally paired in certain cervicals
of Protorosaurus and are described as bilaterally paired in the three anterior
cervicals of Procolophon, also as paired 'primary hypocentra' in Sphenodon embryos.
Hypocentra are more frequently single, transversely placed or lunate
elements, primitively beneath the presacrals and anterior caudals. Hypocentra
secondarily disappear beneath the posterior cervicals and dorsals of certain Protorosauria,
of Rhynchosauria, of Proganosauria, of Choristodera, and of certain
Rhynchocephalia.
RIBS.
Ribs primitively single-headed on atlas and all succeeding cervical, dorsal,
sacral, and anterior caudal vertebra. On all presacrals the heads of ribs are
primitively hypocentral or intervertebral in attachment. The sacral and anterior
caudal ribs, however, are apparently vertebral or pleurocentral in attachment in
all Diaptosauria; this would appear to render it probable that the sacrals were
derived from modified anterior caudal ribs, and that the sacral and caudal ribs
were, very early in the evolution of the Diaptosauria, shifted from the hypocentra
to the pleurocentra; at all events, in all Diaptosauria, so far as known, the sacral
and caudal ribs arise from the sides and not from between the vertebrae.
The capitula are adaptively shifted from this primitive hypocentral position
to various parts of the pleurocentra (centra) and neurocentra (neural arches).
For example: in Protorosaurus, in which all ribs are single-headed, the cervical
ribs are hypocentral, the dorsal ribs are high and neurocentral as in Dinosaurs;
in Pala?ohatteria and Kadaliosaurus the ribs are apparently attached to the
anterior part of the pleurocentra; in Proganosauria the ribs are single-headed,
loosely attached to the anterior part of the pleurocentra.
By the development of the tuberculum, two-headed ribs arise. In Protorosauria
and Proganosauria the rudimentary separation of capitula and tubercula is
observed in certain ribs; in Pelycosauria the presacral ribs are all two-headed
except on the atlas; in Choristodera (Chamfisosaurus) the presacral ribs are all
two-headed except on the atlas and axis. Two-headed ribs will undoubtedly be
found also on the atlas, on which they are known to occur in certain higher
Diapsida.