Comments on the Serial Homology and Homologues of Vertebral Lateral Projections in Crocodylia (Eusuchia)

Abstract

The literature on crocodylian anatomy presents the transverse process in an ambiguous meaning, which could represent all lateral expansions derived from the neural arch, including vertebrae from cervical to caudal series, or in a more restrictive meaning, being applied only to lumbar vertebrae. The lateral expansion of sacral and caudal vertebrae usually referred to as the transverse process has been discovered to be fused ribs, bringing more ambiguity to this term. Therefore, with the lack of a definition for transverse process and other associated terms, the present work aims to propose a nomenclatural standardization, as well as definitions and biological meaning, for vertebral rib related structures. Vertebra obtained from museum collections from a total of 87 specimens of 22 species of all extant Crocodylia genera were studied. All vertebrae, except cervical and first three dorsal, exhibit transverse processes. The transverse process is more developed in dorsal and lumbar vertebrae than in sacral and caudal vertebrae in which it is suppressed by the fused ribs. The serial homology hypotheses here proposed can also be aplied to other Crurotarsi and saurischian dinosaurs specimens. This standardization clarifies the understand of the serial homology among those homotypes, and reduces the ambiguity and misleadings in future work comparisons. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

The postcranial skeleton of Cerrejonisuchus improcerus (Crocodyliformes: Dyrosauridae) and the unusual anatomy of dyrosaurids

“…Hyposaurine dyrosaurids possess at least 22 pre-sacral vertebrae ( Langston, 1995 ; Schwarz-Wings, Frey & Martin, 2009 ), but there is evidence from Rhabdognathus ( Storrs, 1986 ; Langston, 1995 ) and Dyrosaurus maghribensis ( Jouve et al, 2006 ) that some dyrosaurids had at least 25 pre-sacrals. Modern crocodylians possess eight to nine cervicals and 15 to 16 dorsals (thoracics and lumbars) ( Mook, 1921 ; Grigg & Kirshner, 2015 ; de Souza, 2018 ), whereas crocodylomorphs are considered to possess nine cervical vertebrae ( Steel, 1973 ). Jouve et al (2006) interpreted that Dyrosaurus maghribensis possesses nine cervicals (including the atlas–axis complex as two separate vertebra).…”

“…The cervicals were identified following the presence of a parapophysis and a diapophysis on the lateral sides of the centrum, or the presence of a cervical rib as it is the case for modern crocodylians ( Grenard, 1999 ; Grigg & Kirshner, 2015 ; de Souza, 2018 ).…”

“…The dorsal vertebrae were identified as such using the shape and position of the lateral process: it is generally single (i.e. single base) and borne on the neural arch, just like in crocodylians where it is often called ’the transverse process’ ( Romer, 1956 ; Grenard, 1999 ; Grigg & Kirshner, 2015 ; de Souza, 2018 ). Among thoracics, the lateral process splits into two processes distally which resemble two rami of a single structure: the parapophyseal process (anterior), and the diapophyseal process (posterior).…”

“…Among thoracics, the lateral process splits into two processes distally which resemble two rami of a single structure: the parapophyseal process (anterior), and the diapophyseal process (posterior). Each process bears a distinct end, the parapophysis and diapophysis, corresponding to two different attachment sites on the thoracic rib just like modern crocodylians ( Mook, 1921 ; Grigg & Kirshner, 2015 ; de Souza, 2018 ). We chose to follow the terminologies from de Souza (2018) because we found the definition of transverse processes of dorsals too ambiguous for this work on a more basal dyrosaurid; also we decided to use the general term of ‘lateral process’ for all bony structures of the vertebrae laterally emerging (either from the centrum or neural arch) instead of sporadically using ‘transverse process’ which has a restricted meaning among Crocodylia (and it is not always possible to meet with the necessary requirements with fossils to use this definition) ( de Souza, 2018 ).…”

“…Each process bears a distinct end, the parapophysis and diapophysis, corresponding to two different attachment sites on the thoracic rib just like modern crocodylians ( Mook, 1921 ; Grigg & Kirshner, 2015 ; de Souza, 2018 ). We chose to follow the terminologies from de Souza (2018) because we found the definition of transverse processes of dorsals too ambiguous for this work on a more basal dyrosaurid; also we decided to use the general term of ‘lateral process’ for all bony structures of the vertebrae laterally emerging (either from the centrum or neural arch) instead of sporadically using ‘transverse process’ which has a restricted meaning among Crocodylia (and it is not always possible to meet with the necessary requirements with fossils to use this definition) ( de Souza, 2018 ). For each one of the thoracics of Cerrejonisuchus the lateral process is attached to the centrum, this indicating the relative maturity of the specimen ( Hastings et al, 2010 ), even if dyrosaurids are known to possess weak neurocentral sutures ( Buffetaut, 1978b ).…”