Sigilmassasaurus brevicollis, Russell, 1996

Sigilmassasaurus brevicollis

( Fig. 13 View Figure 13 )

Referred specimen

A completely preserved posterior cervical vertebra (NHMUK PV R 38358).

Morphological description

NHMUK PV R 38358 is an almost complete cervical vertebra, with only part of the neural spine missing. Based on the axial sequence proposed by Evers et al. (2015), NHMUK PV R 38358 can be identified as a C9 (Fig. 13).

The vertebral centrum is c. 125 mm long anteroposteriorly, c. 140 mm wide, and c. 95 mm high. It is strongly opisthocoelous and the anterior articular surface is surrounded by a rim (Fig. 13A, B). NHMUK PV R 38358 has a vertebral centrum that is shorter than it is wide, being 12% wider than it is long, and its dorsal surface is slightly anteroposteriorly shorter than ventral surface. The articular facets are wide, with the anterior and the posterior articular surfaces being 1.4 and 1.6 times wider than they are tall, respectively (Fig. 13). The anterior articular surface is strongly convex and the posterior one is strongly concave. In anterior view, the anterior articular surface is pronouncedly elliptical (Fig. 13A, B) and, in posterior view, the posterior one has a reniform outline (Fig. 13C, D). The ventral surface exhibits a well-developed ventral keel that extends to the anterior and posterior margins of the vertebral centrum, widening slightly transversely towards these edges (Fig. 13G, H). In lateral view, the anterior and posterior parts of the keel project further ventrally, so that it is concave at midlength of the centrum (Fig. 13I–L). Anteriorly, the ventral keel gradually becomes lower and merges into a small, rough, triangular area located immediately anterior to the parapophyses (Fig. 13G, H). Lateral to the ventral keel, a fossa is present on each side of the centrum. These fossae are delimited anterolaterally by the parapophyses and the lamina that connects the parapophyses to a small anterior triangular area, and posterolaterally by a lamina that extends from the parapophyses posteriorly to the posterior articular facet (Fig. 13G–L). No hypapophysis seems to be present in NHMUK PV R 38358.

The parapophyses are robust, 'button-shaped' structures that are long and project ventrolaterally; located anteroventrally on the lateral surfaces of the centrum (Fig. 13A–D, G–L). The articular surfaces of the parapophyses are concave and oval in outline. From the posterior margin of each of the parapophyses, a rounded ridge extends posteriorly; these are the ridges that laterally delimit the ventral fossae (Fig. 13G, H). Above the parapophyses, a single and large central pneumatic foramen is present on each side of the NHMUK PV R 38358 vertebral centrum and it penetrates the bone anteroventrally (Fig. 13I–L). Each foramen has a different shape: the right one is triangular and anteroposteriorly larger than tall (Fig. 13), whereas the left is oval and taller than its length anteroposteriorly (Fig. 13K, L).

The neurocentral suture in NHMUK PV R 38358 is still visible. The neural canal is large, subrectangular to oval in shape, and transversely wider than tall (Fig. 13A–D). The pleurocentral depressions are located ventral to this suture, anterodorsally located on the centrum.

The transverse processes are very large and project ventrolaterally at an angle of c. 40° to the lateral surface (Fig. 13A–D). In anterior view, the transverse processes are quite straight dorsoventrally (their dorsal surfaces are distinctly flat), but towards the diapophyses they curve slightly more ventrally. In dorsal view, the posterior surfaces of the transverse processes are straight and laterally directed (Fig. 13E, F). The anterior margin is gently concave in dorsal view and expands anteriorly near the diapophyses. The diapophyses face ventrolaterally and are convex and triangular in outline. Both prezygodiapophyseal and postzygodiapophyseal laminae are present. These laminae are robust, rounded, developed in a similar way, and are less marked near the diapophysis. The prezygodiapophyseal lamina extends laterally from the anterolateral margin of the prezygapophyses along the anterodorsal margin of the transverse process, reaching the anterodorsal margin of the diapophyses. The postzygodiapophyseal lamina runs from the anterolateral margin of the postzygapophyses toward the diapophyses, forming the posterodorsal edge of the transverse process. The centroprezygapophyseal lamina is directed anterolaterally and merges with the prezygodiapophyseal lamina on the anterior surface of the base of the prezygapophyses, with the lamina being less marked, but more robust and rounded than the prezygodiapophyseal lamina.

The centrodiapophyseal lamina is present on the ventral surface of the transverse process. This is very prominent and it seems not to be bifurcated into the anterior centrodiapophyseal and posterior centrodiapophyseal laminae. However, there is another lamina that runs from the ventral surface of the transverse process toward the posterodorsal margin of the centrum [probably the posterior centrodiapophyseal lamina according to Evers et al. (2015)]. The centrodiapophyseal lamina runs from the diapophysis, extending from the ventral surface of the transverse process toward the lateral side of the centrum, resulting in a 'T-shaped' cross-section of the transverse process.

Regarding the pneumaticity of the vertebra, the prezygocentrodiapophyseal fossa is open ventrolaterally and extends throughout the anteroventral part of the transverse process (Fig.13A,B).Thisfossaisdelimitedbythecentroprezygapophyseal lamina anteriorly, the centrodiapophyseal lamina posteriorly, and the prezygodiapophyseal lamina dorsally. The prezygocentrodiapophyseal fossa presents a 'slit-shaped' foramen, quite high, narrow, and oval, which penetrates the pedicle of the prezygapophysis. The postzygocentrodiapophyseal fossa is located on the posterior surface of the transverse process, being larger than the prezygocentrodiapophyseal fossa (Fig. 13C, D). This fossa is delimited anteriorly by the centrodiapophyseal lamina, posteriorly by the centropostzygapophyseal lamina and dorsally by the postzygodiapophyseal lamina. Moreover, it exhibits another pneumatic foramen, which penetrates posterolaterally into the transverse process, and is also more circular than that of the prezygocentrodiapophyseal fossa (Fig. 13C, D).

The prezygapophyses are large and considerably separated, projecting more laterally than the vertebral centrum itself, being projected mainly dorsolaterally and slightly anteriorly (Fig. 13A–F). The prezygapophyses are located dorsal to the transverse process in the anterior half of the transverse processes and their anterior margin is placed slightly anterior to the anterior margin of the transverse process (Fig. 13I–L). The prezygapophyseal articular facets of the prezygapophyses are oval to subcircular in dorsal view, being wider lateromedially. They are dorsomedially facing, with an angle of 125° between both facets. These facets are slightly convex but almost flat, and slightly posteriorly inclined. There is no intraprezygapophyseal lamina between the prezygapophyses.

The postzygapophyses are also large, but more compact compared to the prezygapophyses and without an epipophysis. They extend posterodorsally beyond the posterior margin of the centrum, with half of the postzygapophyses posterior to the posterior margin of the centrum (Fig. 13I–L). The postzygapophyseal facets are concave, with an inverted 'teardropshape', posteroventrally facing, and laterally oriented. They are connected anteromedially by the spinopostzygapophyseal laminae, but they are not connected ventromedially because there is no intrapostzygapophyseal lamina. Nevertheless, from the posteromedial margin of the postzygapophyses, two narrow spinopostzygapophyseal laminae extend ventrally, reaching the dorsolateraledgeoftheneuralspine.Thespinopostzygapophyseal laminae are well developed, robust, and rounded, being delimited dorsolaterally by the spinopostzygapophyseal fossa, which is quite triangular and open ventrally. This fossa is laterally delimited by postzygapophyses and ventromedially oriented laminae.

The neural spine is not completely preserved, but its base suggests that it was a 'spike-like' process that projected posterodorsally (Fig. 13A–F). The cross-section of its base is subcircular in dorsal view, and is connected to the prezygapophyses by the spinoprezygapophyseal laminae. These laminae are poorly developed in comparison to the other laminae present in the vertebra; besides that, they are rounded and extend from the anterior surface of the neural spine to the posteromedial margin of the prezygapophyses, forming an inverted 'V-shaped' structure (Fig. 13A–F). A prespinal lamina (sensu Evers et al. 2015) is also present on the anterior surface of the neural arch, situated between the spinoprezygapophyseal laminae. This lamina is low, dorsoventrally oriented, and projects slightly ventrally into the neural canal. This lamina, together with the spinoprezygapophyseal laminae, delimits a shallow triangular depression on the anterior surface of the neural arch.

Morphological comparisons

Specimen NHMUK PV R 38358 shares with many earlybranching tetanuran theropods the single pneumatic foramen on the lateral surface of the vertebral centrum ( Carrano et al. 2012). Furthermore, this posterior cervical vertebra shares with Megalosauroidea taxa the bordered (or rimmed) anterior articular surface of the centrum ( Carrano et al. 2012, Evers et al. 2015, Malafaia et al. 2020, Barker et al. 2021). However, this trait may be also present in Allosaurus ( Rauhut and Pol 2019) . In the specimen studied here, the parapophyses are enlarged and exhibit a strongly concave facet, similar with the spinosaurids Baryonyx , Ichthyovenator , Sigilmassasaurus , and Suchomimus ( Allain et al. 2012, Evers et al. 2015). Another feature shared between NHMUK PV R 38358 and spinosaurids is the presence of a ventral keel on the vertebral centrum, with the anterior end projecting anteriorly (a synapomorphy for Spinosauridae — Schade et al. 2023). The neural arch of NHMUK PV R 38358 lacks epipophyses, similar to the posterior cervicals of Baryonyx [considering the arrangement proposed by Evers et al. (2015)], and Sigilmassasaurus ( Russell 1996, McFeeters et al. 2013, Evers et al. 2015). The neural spine is inferred to be 'spike-like' in NHMUK PV R 38358, a feature also shared with Baryonyx and Sigilmassasaurus ( Evers et al. 2015) .

The vertebral centrum of NHMUK PV R 38358 is very wide, with the anterior articular surface being 1.4 times wider than it is tall, similar to the condition observed in Ichthyovenator and Sigilmassasaurus , in the latter being more than 1.5 times larger than high ( Russell 1996, McFeeters et al. 2013, Evers et al. 2015). Specimen NHMUK PV R 38358 also shares with Ichthyovenator and Sigilmassasaurus the lack of intraprezygapophyseal and intrapostzygapophyseal laminae ( Allain et al. 2012, Evers et al. 2015). The posterior cervical vertebra (C9) studied here further resembles the posterior cervicals of Sigilmassasaurus brevicollis by having large, transverse processes that exhibit pneumatic foramina deep beneath their base ( McFeeters et al. 2013, Evers et al. 2015).

If the emended diagnosis of Sigilmassasaurus brevicollis proposed by Evers et al. (2015) is considered, specimen NHMUK PV R 38358 shares with the former the reduced lamination of the neural arch with the centrodiapophyseal laminae not being divided into anterior and posterior centrodiapophyseal laminae [autapomorphy proposed by Evers et al. (2015)]. Nevertheless, the specimen described here lacks the anterior tubercle that is present on the anterior articular surfaces of the posterior cervical and anterior dorsal vertebrae of Sigilmassasaurus brevicollis ( McFeeters et al. 2013, Evers et al. 2015), but this feature is more subtle in the BSPG 2011 I 115 cervical vertebra ( Evers et al. 2015) and also absent in ROM 65537 ( McFeeters et al. 2013). The posterior vertebra NHMUK PV R 38358 can be safely assigned to Spinosauridae , furthermore, due to the above-mentioned features shared with Sigilmassasaurus brevicollis , we here assign this specimen to the same taxon.