neural arch 



costal foramen 



centrum'' 



haemapophysrs / parapophysis\ 



notochord 



A AMPHIBIAN "URTYPE" 



vertebral artery^ 



horizontal septum 

 drapophysis \ ,„-^ 



B 



AMNIOTES 



Figure 6-73. Dorsal and ventral ribs and their relationships to the 

 centrum and neural arch in the primitive amphibian vertebra. A, and 

 the typical amniote vertebra, B. (After Naef, 1929) 



thoracic cavity which in the mammal functions in respiration; 

 in other amniotes, these rib walls function in support of the 

 body, which frequently lies pressed against the ground. In 

 aquatic forms or small types such as the living amphibians, 

 the ribs are not important for support but are utilized 

 in the functioning of the axial musculature. 



Support and protection of the body cavity in the creeping 

 primitive forms was aided by ventral scales and gastralia. 

 The latter are present in the alligator and Sphenodon. Gas- 

 tralia (Figure 6-6) mark the reptile level of differentiation 

 and are presumed to be developed from the rows of bony, 

 ventral scales present in the early amphibians. These ven- 

 tral rows of scales are well marked and fish-like in the 

 small branchiosaurs. 



Support for the posterior part of the body cavity is given 

 also by the forward-projecting plate of cartilage in Cryplo- 

 branchus (Figure 6-55), the pubis of many reptiles, and the 

 marsupial bones of monotremes. Each of these seems to be 

 an independent response to a common problem. 



RESUME OF THE BODY SKELETON 



The vertebrate body skeleton tells essentially the same 

 evolutionary story as the head skeleton, clarifying some 

 areas while being less useful in others. In terms of "verte- 

 brae," it is evident that some sort of column was forming in 



that there is evidence of reduction of the ventral ribs — 

 Lalimeria has none. With reduction of the ventral rib, dorsal 

 ones appeared, and in Eusthenopleron there are very short 

 dorsal ribs, retaining only the base of the ventral one as their 

 ventral head. In the primitive amphibian there are well- 

 developed, two-headed ribs. 



Among tetrapods the costal or ventral segments of the 

 ribs were acquired along with the sternum. At first, the 

 sternum appears to have been a medial plate for the rib at- 

 tachment, much like that observed in living salamanders 

 and some frogs (arciferous), as well as reptiles; in mammals 

 it is a line of separate ossified units. The sternum in the 

 embryo develops as an expansion of the medial ends of the 

 costal rib blastemas formed along the line of contact of the 

 myosepta with the dermis (Figure 6-74). Paired blastemal 

 strands appear and fuse to form the definitive structure 

 made up of anterior manubrium, which may include an 

 interclavicle component, the line of sternebrae between the 

 ends of the ribs, and the posterior xiphisternum or meta- 

 sternum. The sternum of the bird is of special interest be- 

 cause of its broad plate-like form with a deep keel. In some 

 birds there are three or more distinct ossification centers, a 

 pair of centers in the region of the procoracoid articulation, 

 and one in the keel. The latter is probably of interclavicular 

 origin. In the chicken there are five centers, two bilateral 

 pairs and a keel center. 



In amniotes, ribs and sternum enclose a well-defined 



interclavicle clavicular facet interclaviclp clavicle 



, procoracoid^ 

 -corocoid^' 



Figure 6-74. The interclavicle in mammals. A, ventral view of sternal 

 area of Ornithorhynchus; B and C, ventral and dorsal views of sternal 

 region of a half grown Echidna; D, ventral view of sternal region of 

 1 8-day rabbit in which the cartilaginous manubrium supports the clavi- 

 cles much as the interclavicle of more primitive forms. (A, B, C after 

 Porker in Remane, 1936) 



RESUME OF THE BODY SKELETON 



191 



