ELASMOSAURIDAE (Cope, 1869 sensu Ketchum and Benson, 2010)

Sassoon, Judyth, 2019, Congenital and late onset vertebral fusions in long necked plesiosaurs: The first report of spondylosis deformans in Sauropterygians, Palaeontologia Electronica (1 A) 22 (1), pp. 1-15 : 4-6

Family ELASMOSAURIDAE (Cope, 1869 sensu Ketchum and Benson, 2010)

Type Genus Callawayasaurus (Carpenter, 1999) Species Callawayasaurus colombiensis (Welles, 1962)

Type species. UCMP 38349, Callawayasaurus columbensis formerly Alzadasaurus colombiensis described by Welles (1962). Revised under a new generic name Callawayasaurus , by Carpenter (1999, p. 172).

Type locality. Paja Formation (Lower Aptian faunal stage, 125-112 Ma ), Leiva Shale , early Cretaceous of Columbia. 120 km N and 60-70 km Bogota. ( Welles , 1962; Goñi and Gasparini , 1983; Carpenter , 1999) .

Description

Specimen UCMP 38349, Callawayasaurus columbensis , has been extensively described by Welles (1962). However, no reference was made to the pathological condition which is clearly present on a few of the cervical vertebrae. Since UCMP 38349 has been described previously, only a brief account of the cervical vertebrae is included, with additional details of the previously undescribed pathology.

Thirty-five vertebrae from the series belonging to UCMP 38349 were studied. They were identified as anterior and medial cervical vertebrae on the basis of size and position in the preserved series. The exact position of these vertebrae in the spinal column has been debated. O’Keefe and Hiller (2006) stated that the cranialmost vertebrae are missing from this specimen. However, Sven Sachs (Sachs, personal commun. 2018) observed that the series was complete. The numbering system used here is as marked on the specimen. Callawayasaurus typically has 56 cervical vertebrae (Carpenter, 1999).

The anterior and medial cervical centra of Callawayasaurus are longer than high or broad, and broader than high: on average 1.13x longer than high and 1.29x broader than high. The centra are slightly amphicoelous with a thickened rim surrounding the outer region of the articular facet, which is the condition in many elasmosaurid taxa (Sachs and Kear, 2015). Intervertebral discs (soft tissue) were replaced by matrix so the original intervertebral spacing is preserved. Vertebrae are separated by approximately 10-15 mm. Ventrally placed foramina subcentralia are clearly present on some vertebra. Transversally flattened doubleheaded cervical ribs are attached ventrolaterally on the anteroposterior midlength of each centrum. In the more anterior vertebrae the ribs taper and lack an anterior process. The neural spines are short, i.e., less than half the height of the centra, inclined posterodorsally and bear a posterior projection. The projection does not touch the spines of adjacent vertebrae, and there is no dorsoventral bending of neural spines (Kubo et al., 2012).

The spinal pathologies of UCMP 38349 are present on two series of four vertebrae, numbered 15-19 and 23-27 on the specimen. The centra in series 23-27 are fused only on the ventral surface at the intervertebral joint by small osteophytes ( Figure 3 ). In the series 15-19, vertebrae are joined together by unusual expansions around the lateral margin of the intervertebral joint, appearing as smooth, thick, spur-like swellings. They are most pronounced on the left lateral side of the ventral surface, close to the cervical rib especially at the junctions between vertebrae 15-16 and 16-17 ( Figure 4 ). The fusions are not due to ossification of intervertebral ligaments but rather arise from proliferation of the ventral bone surface and ossification around the annulus fibrosus. No infection holes or bone density changes are apparent, and the surfaces of all vertebra studied are free of tooth marks. The pathology is highly localized and appears only on the ventral surfaces of intervertebral zone. It is very similar to the pathology observed on Colymbosaurus CAMSM J 63919. However, in CAMSM J63919, the projection arises from the posterior surface of the vertebrae, contacting the anterior surface of the vertebral element following. The anterior and posterior surfaces of the first and last vertebra in each pathological series are normal, and articulate normally with their neighbours. A normal series of cervical vertebrae from UCMP 38349 with clear separations between centra is shown for comparison ( Figure 4.3 ).

Figures

FIGURE 3. Cervical vertebral series from Callawayasaurus columbensis UCMP 38349 in ventral aspect. 1. Vertebrae 23,24,25, anterior end, to right. Scale bar, 5 cm. 2. Enlargement of osteophytic bridging between vertebrae 23-24, 24- 25. Abbreviations: ns, neural spine; cr, cervical rib. Scale bar equals 5 cm.

FIGURE 4. Cervical vertebral series from Callawayasaurus columbensis UCMP 38349 in ventral aspect. 1. Vertebrae 15-18, anterior end to right. Scale bar equals 5 cm. 2. Enlargement showing claw-like ventral expansion projecting antero-posteriorly between vertebrae 16-17, 17-18. 3. Ventral view of cervical series from UCMP 38349 without pathologies, vertebrae 32-35. Abbreviations: ns, neural spine; cr, cervical rib; ve, pathological ventral expansion. Scale bar equals 5 cm.