The walls of the neural arch are massive, and the neural canal is large (c. 35 mm in diametre) and round to slightly oval in outline. The base of the massive transverse process is placed entirely on the neural arch and extends for approximately the anterior three-fourths of the centrum. Posteriorly, the transverse process is supported ventrally by a stout, posterolaterally facing posterior centrodiapophyseal lamina, the ventral end of which does not reach the posterodorsal end of the centrum ( Fig. 4B ). Whereas the left lamina forms a sharp, posterolaterally facing edge, the right lamina seems to be more rounded, although this might be due to erosion. An anterior centrodiapophyseal lamina lamina is only indicated by a slight depression on the anterior side of the base of the transverse process. The transverse process was laterally and strongly posteriorly directed, but has almost no dorsal inclination. Posteriorly, a large postzygocentrodiapophyseal fossa is present between the posterior centrodiapophyseal lamina and the lamina extending ventrally between the medial ends of the postzygapophyses and the neural canal ( Fig. 4B ); as the postzygapophyses are missing and the median lamina is poorly preserved, it is unclear if a small hyposphene might have been present, but at least a marked ventral expansion of this lamina was certainly absent. A small, ridge-like lamina extending from the dorsal margin of the transverse process towards the dorsomedial rim of the neural canal subdivides the postzygocentrodiapophyeal fossa into a larger, conical ventral recess and a smaller, much shallower dorsomedial depression ( Fig. 4B ). Anteriorly, a small depression is present on the roof of the neural canal, being offset from the massive dorsal surface of the transverse process by a small, transverse step ( Fig. 4D ). The base of the broken neural spine is transversely narrow and extends over the entire length of the neural arch, showing the eroded bases of the slightly anteriorly diverging spinoprezygapophyseal laminae anteriorly.

The vertebra MUJA-1913 is a large anterior caudal vertebra that has most of the centrum and the base of the neural arch preserved ( Fig. 4 ); the zygapophyses, neural spine and most of the transverse processes are missing. The centrum is notably robust and amphi-platycoelous, with the articular surfaces being oval in outline and slightly higher than wide. The anterior articular surface has suffered from erosion, so that its exact size and morphology cannot be established, but the posterior articular surface is only slightly concave and only slightly higher (c. 150 mm) than wide (c. 140–145 mm as reconstructed; the right rim is eroded). In lateral view, the posterior articular surface is notably offset ventrally in respect to the anterior surface ( Fig. 4A ). The length of the centrum as preserved is c. 140 mm, but approximately 10 mm might be missing anteriorly, so that the centrum was approximately as high as long. In ventral view, the centrum is moderately constricted to a minimal width of c. 90 mm between the articular ends. Ventrally, a broad, but shallow ventral groove is present, which becomes more marked posteriorly between the poorly developed chevron facets ( Fig. 4C ). The lateral sides of the centrum are strongly convex dorsoventrally and offset from the ventral surface by the broadly rounded edges of the ventral groove. On the dorsal part of the lateral side of the centrum, below the base of the neural arch, a notable, large pleurocentral depression is present ( Fig. 4A ). This depression is deeper posteriorly than anteriorly, with the anteroventral part of the depression forming a small lateroposteroventrally facing platform that is offset from the deeper posterior part by a rounded, but notable oblique step.

Asturian theropod tracks

Following the definition of Marty et al. (2017), according to which giant theropod tracks are those of a footprint length (FL) longer than 50 cm, seven Asturian tracks are described in the present study (see Table 1 for measurements). The footprints (all more than 53 cm long), reported from the Kimmeridgian Lastres Formation, are preserved as natural sandstone casts and can be classified into two groups by morphology (Piñuela Suárez, 2015).

Foot FL FW FL/FW II–IV Morphotype A Argüero R 62 70 0.88 36 Oles L 82 66 1.24 38 Tazones L 57 >47 >1.16 38 MUJA-1889 L 53 53 1 40 Morphotype B MUJA-1263 R 62 38 1.63 15 MUJA-0213 R 78 Argüero R 67 DOI: 10.7717/peerj.4963/table-1

Morphotype A is represented by four tracks (Argüero1, Oles and Tazones specimens, and MUJA-1889; Fig. 5), which, although slightly different in morphology, are robust and weakly mesaxonic. The FL/footprint width (FW) ratio is very low (0.88–1.16). The digit impressions are broad and generally show claw marks. The divarication angle (II–IV) lies between 36° and 40°. In some of these tracks the digital pads are visible. Based on the morphology, the Asturian footprints would form part of the Megalosauripus–Kayentapus-group proposed by Piñuela Suárez (2015), The specimens of morphotype A are thought to represent more graviportal theropods (Piñuela Suárez, 2015) than those of morphotype B.

Figure 5: Asturian Jurassic footprints with a weak mesaxony and probably related to very large or giant megalosaurid theropod trackmakers (Morphotype A). (A), (B), and (C) specimens still on Argüero, Oles, and Tazones sea cliffs, respectively. Note that track (C) does not preserved the end of the digit IV. (D) MUJA-1889. (E–H) Same specimens, photographs with outline drawings to better illustrate track morphology. Photographs by José-Calros García-Ramos.

Argüero specimen 1. The poorly preserved track represents a positive hyporelief. It is 70 cm in length and 62 cm in width; thus, the FL/FW ratio is very low (0.88), considerably lower than in the other tracks of the morphotype (Fig. 5A). The digit impressions are broad and relatively short, the best preserved being digits II and III. The claw marks are evident, well developed and medially turned. It is possible to recognize two pads in digit II. Digit IV is not well preserved, but enough is present to measure the divarication angle between digits II and IV, which is 36°. Even though the end of the digit IV is not preserved, the print seems weakly mesaxonic.

Oles specimen. The footprint represents a shallow positive epirelief (Fig. 5B). It is 82 cm in length and 66 cm in width, so the FL/FW ratio is 1.24. The digit impressions are broad, slight less so than in the previous specimen (Argüero specimen 1), and relatively short. Claw marks are evident in the three digits, being long and broad in digit II and shorter and narrower in III and IV. The digital pads are subtly visible, at least in digits III and IV. The divarication angle between digits II and IV is 38°. The track is weakly mesaxonic.

Tazones specimen. The print represents a positive hyporelief (Fig. 5C). It is 57 cm in length and more than 47 cm in width (the end of the digit IV is not preserved), so the FL/FW ratio is at least 1.21. The digit impressions are long and less broad than in the previous specimens. The claw marks, only preserved in digits II and III, are relatively large, especially in digit II. The digital pads are subtly visible in digits II and III. The divarication angle between digits II and IV is 38°. Despite that the end of digit IV is not preserved, the print seems weakly mesaxonic. This footprint might be the best preserved of the morphotype A.

MUJA-1889. The track represents a positive hyporelief (Fig. 5D). It is 53 cm in length and 53 cm in width, so the FL/FW ratio is 1. The digit impressions are broad and short. The print is preserved as a shallow undertrack cast (associated to the true track cast), which might explain the poor definition of the claw marks, the absence of digital pads and the relatively anterior position of the hypeces. The divarication angle between digits II and IV is apparently very high if taken from the undertrack (giving an incorrect value); using the true cast, the divarication angle (II–IV) is 40°. The print is also weakly mesaxonic. A horizontal outward translation movement is seen in this track, mainly in the digits II and III. The maximum depth for the track is 16 cm in the distal part of digit III. The specimen MUJA-1889 was recovered close to the Tazones specimen, and the composition and the thickness of the sandstone beds are similar in both, suggesting that they are derived from the same stratigraphic level. Keeping in mind that MUJA-1889 represents a different preservation (true track and shallow undertrack casts are associated) and is also affected by an oblique foot displacement, the morphology of this footprint does not reflect the foot anatomy of the producer, and thus could have been made by the same trackmacker that produced the Tazones specimen.

Morphotype B is represented by three footprints (MUJA-1263, MUJA-0213, and Argüero specimen 2; Fig. 6), which are much longer than wide and show a strong mesaxony. Pad impressions are only preserved in one specimen. The claw impressions vary from narrow and short to wide and long. The morphology of these footprints does not fit in large or giant known theropod ichnogenera, but rather with smaller ones characterized by a higher mesaxony. This set of tracks seems to represent more cursorial theropods (Piñuela Suárez, 2015) than morphotype A.

Figure 6: Giant Asturian Jurassic footprints, strongly mesaxonic (Morphotype B). (A) MUJA-1263. (B) MUJA-0213, scale bar: 1 m. (C) Specimen still on Argüero sea cliffs. (D–F) Same specimens, photographs with outline drawings to better illustrate track morphology. Photographs by José-Calros García-Ramos.

MUJA-1263. This true sandstone cast represents a positive hyporelief and is associated with a shallow undertrack (Fig. 6A). The print is much longer (62 cm) than wide (38 cm), so the FL/FW ratio is high (1.63). The digit impressions are relatively broad and long, and the claw marks are large. Even though this specimen is interpreted as an undertrack, it is possible to recognize two pads in digit II and three in digit III. The divarication angle (II–IV) is very low (15°). Although the end of digit II is not well preserved, the print is clearly highly mesaxonic. The maximum depth of the track is 10 cm in the distal part of digit III.

MUJA-0213. The track represents a positive hyporelief (Fig. 6B). The posterior part of the track is not well preserved, and although it is difficult to recognize the proximal margin, the footprint is much longer (78 cm) than wide (at least 35 cm, but digit IV is not complete). The impression of digit III is very long and digit II is relatively short, but both of them are broad, due to flattening processes sensu Lockley & Xing (2015). The claw marks are short and narrow. Only two subtly visible pads are preserved in digit II. Digit IV is not complete, but enough to measure the divarication angle between digits II and IV, 34°. The print is highly mesaxonic, even though the digit IV is not complete.

Argüero specimen 2. The footprint (an epirelief) seems to be longer (67 cm) than wide (detailed measurements cannot be taken, because digit IV is not preserved) (Fig. 6C). The impressions of the digits are broad, and digit III is very long, whereas digit II is relatively short. The claw marks in both are long and narrow and medially directed in digit III. Digital pads are not recognizable in the digits. The interdigital angle between digits II and III is high (36°). The print seems to have been highly mesaxonic, although digit IV is not preserved.