anterior zygapophysis 



notochord -"_ 



basiventral 



perichordal tube 



neural canal 



notochord 

 prospondylous ring 



opisthospondylous ring 



vertebral ring 



ntervertebrol ligament 



Figure 6-12. Composition of bird vertebra in terms of Gadowian elements said to be indicated 

 by blostematic condensations. A, lateral view; B, frontal section of vertebra; C to F, cross sections 

 at levels C to F' of A. 



a dorsal and a ventral plate are formed (perichondral or 

 subperichondral). These plates are soon joined by a vertical 

 pillar, perforated by the much constricted notochord, 

 through the cartilage of the centrum. When arch, centrum, 

 and rib are well formed in bone, fusion takes place between 

 arch and centrum. There are no epiphyses. 



In later development of the centrum, the notochord is 

 constricted to a remnant which then disappears. Between 

 the vertebrae the notochord is also constricted and obliter- 

 ated. The articular surfaces of the (movable) vertebral 

 bodies are of a heterocoelous type (Figure 6-10), that is, 

 having a transverse, saddle-shaped surface in front and a 

 vertical, saddle-shaped surface behind. 



Fossil reptiles and amphibians Traced back in time, the 

 reptilian type shows the intercentrum becoming a regular 

 feature and increasing in size. The body of the vertebra (or 

 centrum) becomes amphicoelous, articulating with ball-like 

 intercentral masses in which the notochord plays a part. The 

 spool-like body now becomes perforated by the notochord, 

 which is constricted but remains intact (except, perhaps, at 

 the joint between the head and vertebral column). We have 

 now arrived back in time to the Pennsylvanian genus 

 Cephalerpeton; the Permian amphibian-reptile Seymouna (Fig- 

 ure 6-13); or the Pennsylvanian-Permian forerunners of the 

 mammal, the primitive Pelycosaurs. 



.■\mong the early amphibians (Figures 6-14 to 6-17), an 

 extension of this pattern is observed with further reduction 

 of the size of the central and intercentral blocks or arcs, and 



evidence of their bilateral origin. Exactly what the stem type 

 was like is difficult to say. The most primitive amphibian 

 known, the ichthyostegid (Figure 6-16), has the same type 

 of vertebra as the crossopterygian fish (Figure 6-25). Be- 

 cause of this similarity, it is generally supposed that this is 

 the basic type. Figure 6-18 shows a presumed radiation of 

 the several types, and Table 6-1 lists the terms (adjectives) 

 used to describe them. 



The use of vertebrae in a classification seems impractical 

 at first glance. For example, among reptiles the lepospon- 

 dylous type appears to be but an ontogenetic or phylo- 

 genetic stage in what is, or was, a continuum. From fossil 

 amphibians, it is suggested that the trend was toward the 

 stereospondylous type (whether from a rhachitomous or 

 embolomerous source). In the stereospondylous vertebra, 

 the intercentrum forms much, or all, of the adult vertebral 

 body. The lepospondylous amphibian Megamnlgophis (Fig- 

 ure 6-17) has the parapophysis apparently on the interver- 

 tebral connective tissue, suggesting that in this group the 

 centrum alone remains and the intercentrum is lost. This 

 lepospondylous type resembles, but is even more advanced 

 than, the amphibian type leading to the reptile. To pene- 

 trate this question of vertebral structure a bit deeper, the 

 living amphibians will be examined. 



Amphibians 



Salamander Cryptobranchus and Neclurus are much alike in 

 terms of the vertebral column. There are 19 presacrals in 



THE VERTEBRAL COLUMN 



151 



