Alierasaurus ronchii Romano and Nicosia, 2014
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publication ID |
https://doi.org/ 10.26879/684 |
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persistent identifier |
https://treatment.plazi.org/id/5A41CC5D-442E-FFD3-18E7-FA174F7A5D1F |
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treatment provided by |
Felipe |
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scientific name |
Alierasaurus ronchii Romano and Nicosia, 2014 |
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Alierasaurus ronchii Romano and Nicosia, 2014
Figures 2-8 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8
Revised diagnosis. Very large caseid with an overall size comparable or greater than Cotylorhynchus hancocki , and characterized by the following autapomorphies: metatarsal IV with distinct axial region, length about twice that of the corresponding proximal phalanx, not short and massive as in other large caseids; metatarsal IV proximal head not orthogonal to the bone axis, forming an angle of 120° with the shaft: with this conformation, the proximal and distal heads are much closer along the medial side of the metatarsal; clawshaped ungual phalanges proportionally shorter than in Cotylorhynchus , with a double ventral flexor tubercle very close to the proximal rim of the phalanx; ungual phalangeal axis bent downward and medially; distal transverse section subtriangular, not spatulate as in Cotylorhynchus . Ventrodistal orientation of the phalangeal distal articular surface (synapomorphy of Caseasauria ); sacral and dorsal neural spines with a transversely expanded dorsal tip, forming a spine table (synapomorphy of caseids more derived than Casea broilii ).
New described material. NS 151/7, NS 151/8, NS151/9, NS151/10, NS151/11, NS151/12, NS 151/33 (caudal vertebrae); NS 151/15, NS 151/ 16A, NS 151/16B, NS 151/17, NS151/18, NS 151/ 29, NS 151/30 (isolated zygapophyses); NS 151/ 48, NS 151/74 (isolated neural spines); NS 151/59, NS 151/72 (partial haemal arches); NS 151/39, NS 151/40, NS 151/42, NS 151/54 and NS 151/55, NS 151/75, NS 151/76 (dorsal ribs).
Description
Vertebrae. Specimen NS151/10 is the largest caudal vertebra among those recovered to date, thus representing the most anterior one ( Figure 2.1-4 View FIGURE 2 View FIGURE 3 View FIGURE 4 ). The specimen is fairly laterally compressed, with the centrum sub-elliptical in shape. The left side, probably exposed for a certain period to the erosion, looks damaged and eroded so that it exposes the notochordal portion of the centrum. The left side of the centrum, the left pedicel and consequently also the left prezygapophysis are missing, probably due to breakage. The better preserved right side shows a centrum markedly depressed in the lateral portion, rimmed by stout lips, strongly bent inwards. Anterior and posterior lips have an average thickness of about 1 cm; this value seems to not vary significantly throughout the perimeter of the centrum. The latter has a length, measured along the ventral border, of about 5 cm, while along the dorsal one measures about 5.5 cm; its height is estimated at about 6.5 cm. The interruption of the notochordal canal occurs at a depth of about 2 cm on the anterior side and of about 3 cm on the posterior side. On the ventral side, the centrum being incomplete makes it impossible to observe the presence of the characteristic longitudinal groove seen in other caseids (see Romano and Nicosia, 2014). The vertebra total height, measured from the ventral edge of the centrum to the base of the neural spine, is about 15 cm. Due to the substantial compression, the right prezygapophysis shows an upwards rotation of about 90°. Consequently, the neural arch is fairly flattened and stretched upward. The only articular facet that is visible is that on the left postzygapophysis, characterized by a sub-elliptical shape with a major axis of about 2.5 cm and a minor axis of about 1.5 cm, dipping towards the vertebral body at an angle of about 32°. The horizontal distance between the posterior termination of the postzygapophyses and the anterior termination of the right prezygapophysis is about 9 cm. The neural spine shows a basal section with a very flattened elliptical outline (length of about 2.5 cm; width of 0.7 cm). Given the size and the shape of its base, the missing neural spine is estimated to be about 5–7 cm high, or about one third of the whole vertebra.
The vertebra NS151/12 ( Figure 2.5-6 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ) consists of three pieces found separated at a certain distance on the outcrop surface. The specimen is better preserved on the right side. As for the above described vertebra, we can assume that NS151/ 12, at the time of exhumation by atmospheric agents, laid with the right side embedded in sediment, exposing the left side to erosion. The specimen has undergone lateral compression that deformed the vertebral articulations of the centrum into flattened ellipses and pushed the postzygapophyses decidedly upwards relative to their original position. The left pedicel and a portion of the postzygapophysis are missing, probably due to breakage (also the front lip and the left half of the centrum are missing). The centrum has a length of about 5 cm along the dorsal margin, a width of about 3 cm and a height of 6 cm; due to fractures it is impossible to obtain the exact length along the ventral margin. The posterior lip is a quite stout structure, raised with respect to the body of the centrum, and with a thickness in the median part of about 1 cm; it thickens dorsally and ventrally until reaching a width of about 2 cm. The notochordal canal stops on the posterior side to a depth of about 1.5 cm, whereas it is impossible to observe its termination in the anterior side. The neural arch is fairly compressed, with the right prezygapophysis that has undergone an upwards rotation of about 90°, until it reached the height of the postzygapophyses dorsal tip. The right prezygapophysis is incomplete due to fracture, whereas the postzygapophyses are complete, and show two articular facets, best preserved in their posterior half. These facets have a sub-elliptical shape, with a major axis of about 2.5 cm and a minor axis of 1.5 cm, and dip toward the center of the vertebral body with an angle of about 33°. The complete absence of the left pedicel makes it impossible to estimate the exact shape of the neural canal. The neural spine, missing due to breakage, shows a sub-rectangular base with beveled edges, and a length of about 2.5 cm and a width of about 0.8 cm.
The caudal vertebra NS151/11 is composed of three pieces found separately on the outcrop surface. The vertebral body is compressed laterally with a sub-elliptical centrum ( Figure 2.7-8 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 ). The left side of the centrum is well preserved while the midposterior portion of the right side is missing (also the pedicel and the left prezygapophysis are missing due to breakage). The lip on the anterior side is almost continuous and has an average thickness of 1.5 cm; it re-expands ventrally and dorsally up to reach a width of 2 cm. The lips are stout and bent inwards. The centrum has a length of 4 cm along the ventral edge and 5 cm along the dorsal one. The notochordal canal stops at a depth of about 2 cm, both on the anterior and on the posterior sides. The neural arch is laterally flattened with an arrangement of zygapophyses similar to that described above for vertebrae NS 151/10 and NS 151/12. The only visible articular facet, although incomplete, is that of the right postzygapophysis. Overall, the facet has an elliptical shape with a major axis of about 2.5 cm and a minor axis of about 2 cm, and dip at an angle of about 30°. The base of the neural spine has a sub-rectangular outline, rounded at the posterior edge, while it tapers up to a lanceolate shape in its anterior termination. The base has a length of about 2.5 cm and a width in the median part of about 0.6 cm.
Specimens NS 151/7, NS 151/8, NS151/9, and NS 151/33 represent broken caudal vertebrae missing their neural arch due to breakage ( Figure 2.9 View FIGURE 2 -31). The centra are quite laterally compressed, with a shortening of the average width between 30% (NS 151/7) and 16% (NS 151/33). NS 151/7 is the largest of the four centra, with a length of 3.7 cm, therefore representing the most anterior one. Considering the rate of decrease in the length of the caudal centra reported by Olson (1962) for Cotylorhynchus hancocki , the distal centrum NS 151/7 was very likely separated from the more anterior one (i.e., NS 151/10) by at least 7-8 caudal vertebrae. The smaller of the centra, hence the more posterior among those recovered, is represented by NS 151/33 with a length of 2.6 cm. Dorsally in all the centra the hemi-cylindrical groove of the neural canal is clearly visible, with an average transverse width ranging from 0.6 cm in NS151/7 to 0.4 cm in NS 151/33. The typical groove on the centrum ventral side, previously described for the eight caudal vertebrae of Alierasaurus in articulation (see Romano and Nicosia, 2014), is clearly observable only in NS 151/7 where it assumes an average transverse width of about 0.8 cm. Specimen NS 151/33 shows a structure typical of distal caudal centra, being much longer than high, then taking overall the appearance of an elongated spool (e.g., distal centra in specimen in C. hancocki FMNH UR 578, FMNH UR 703, FMNH UR 883, FMNH UR 884, M.R. pers. obs.).
The above described vertebrae (NS 151/10- 12), which completely lack transverse processes or co-ossified ribs, can therefore be identified as anterior-middle caudal vertebrae, in any case successive to the disappearance of even the smallest bony protuberance on the centrum (see below).
Among the recovered fragmentary zygapophyses, specimens NS 151/16B, NS 151/17, and NS 151/30 ( Figure 3.1-8 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 ) represent prezygapophyses with a total size compatible with the more anterior caudal vertebrae recovered (NS 151/10 to12). Specimen NS 151/17 ( Figure 3.4-5 View FIGURE 3 View FIGURE 4 View FIGURE 5 ) is a left prezygapophysis slightly larger than that observed in NS 151/10, then referable to a more anterior caudal vertebra. NS 151/30 and NS 151/16B are right prezygapophyses, the latter showing a very well-preserved articular facet, with a teardrop shape expanded distally and gently tapered proximally (long axis of approximately 3 cm, maximum transverse width equal to 2.1 cm). By vertically orienting the neural arch proximal edge, the prezygapophyses articular surface are decidedly tilted thus confirming the attribution of these specimens to caudal vertebrae (see Reisz, 1986), mostly contiguous to NS 151/10 to 12.
Specimens NS 151/15, NS 151/16A, NS151/ 18, and NS 151/29 are fragmentary postzygapophyses ( Figure 3.9 View FIGURE 3 -21) which retain, proximally, the base of the neural spine (thus allowing to observe the spine section). NS 151/16A and NS 151/18 have a size consistent with the more anterior caudal vertebrae NS 151/10-12 and show a long and narrow neural spine section, with a maximum transverse width of about 0.7 mm. NS 151/29 are postzygapophyses significantly smaller than those just described ( Figure 3.20 View FIGURE 3 -21) and compatible with those characterizing the articulated series (NS 151/19-28, see Romano and Nicosia, 2014, p. 902, figure 2), therefore probably referable to the anterior-middle portion of the tail. Worthy of note are the postzygapophyses NS 151/15 which show a much greater size than that observed in all the recovered and described caudal vertebrae ( Figure 3.9 View FIGURE 3 -12). The base of the neural spine (the remain- der is missing) has a sub-oval section, not transversely flattened as in the typical caudal occupying the middle portion of the tail in large caseids. In particular, the neural spine section is 3.5 cm long and 2 cm wide transversely, a considerable size even for an anterior caudal vertebra. As reported above, the close arrangement of the postzygapophyses and the consistent tilt of the articular facets indicate that the specimen NS 151/15 would belong to a caudal vertebra (an anterior one, given
ROMANO ET AL.: NEW MATERIAL OF ALIERASAURUS its large size). However, even in the posterior sacrals the postzygapophyses may well be narrower than the prezygapophyses of the same vertebra (according to Reisz et al., 2011 in Ruthenosaurus russellorum the narrowing of the postzygapophyses and the increase of tilting up to about 55° starts from the first sacral). Thus, taking into account the general great size of postzygapophyses and neural spines section and robustness, an assignment of NS 151/15 to a sacral vertebra cannot be ruled out. The zygapohyseal material described above can be referred to at least seven distinct caudal vertebrae (still interpret-
PALAEO- ELECTRONICA.ORG ing the large specimen NS 151/15 as a caudal one).
Among the newly recovered material one of the most diagnostic bone is represented by the neural spine NS 151/74 ( Figure 4.1-5 View FIGURE 4 View FIGURE 5 ), which corroborates the assignment of Alierasaurus ronchii to the Caseidae . The spine central portion (its base is missing due to breakage) has the usual synapsid blade-shape. A very diagnostic feature of the spine results a marked expansion at the distal end, with two bony protrusions, which depart from the spine side and form a sort of plateau, sub-circular in shape, when the neural spine is observed dorsally. According to the characters scored in Benson (2012, data matrix character 166) the transverse enlargement of the neural spine tip to form a ‘spines table’ represents a unique synapomorphy, in particular of medium to large caseids; however, a slight lateral expansion of the spines can be observed in an incipient way even in the sacrals and ‘lumbars’ of Casea broilii (FMNH UC 656, holotype; M.R. pers. obs.) and in the dorsals of Casea nicholsi (FMNH UR 86; M.R. pers. obs.) characterized by relatively small body size within Caseidae . Among large caseids, neural spines distally broad to form a plateau, are observable in the cervical, dorsal, lumbar, and sacral vertebrae of Angelosaurus romeri (e.g., FMNH UR 827, FMNH UR 904, FMNH UR 908, FMNH UR 916; M.R. pers. obs.), Cotylorhynchus romeri (e.g., OMNH 655, OMNH 1673, OMNH 4188, FMNH PR 272; M.R. pers. obs.), Cotylorhynchus bransoni (e.g., FMNH UR 915, FMNH UR 925; M.R. pers. obs.) and Cotylorhynchus hancocki (e.g., FMNH UR 622, FMNH UR 703, FMNH UR 706, FMNH UR 710, FMNH UR 720, FMNH UR 821, FMNH UR 893, FMNH UR 894; M.R. pers. obs.). Dorsally expanded neural spines are also recognized in the French caseid Ruthenosaurus russellorum , however, according to Reisz et al. (2011), the top of the spines is not classically anteroposteriorly elongate but shows an autapomorphic diamond-shaped outline. Even if in Benson (2012) the distal plateau is defined as a character only for the dorsal and sacral vertebrae, the relatively small size of the neural spines NS 151/74 is compatible with a proximal caudal vertebra (5 to 8) or a cervical one. The latter hypothesis can be reasonably discarded taking into account that in the cervical region at least in Cotylorhynchus romeri the bifid dorsal aspect tends to disappear, and the spine assumes a more rounded section ( Stovall et al., 1966). Regarding known caseids, a neural spine distal expansion is observable in the proximal caudal vertebrae at least in Cotylorhynchus romeri (e.g., OMNH 1673, M.R. pers. obs.; see also Stovall et al., 1966, p. 6, figure 2E). In the light of the new better preserved spine, the fragmentary specimen NS 151/48, previously recovered, can be interpreted as neural spine distal end ( Figure 4.6-10 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10 ). The specimen has a considerable size, with a length of 3.5 cm and a transverse width of at least 3.8 cm (the left side has been broken). Starting from the most proximal preserved portion, which shows a sub-oval section (length 2.5 cm, width 1.7 cm), the spine widens consistently on both sides, with two longitudinally
ROMANO ET AL.: NEW MATERIAL OF ALIERASAURUS elongated expansions and separated by a groove in dorsal view. Thus, dorsally, the spine assumes a typical bifid aspect very well appreciable in the anterior dorsal of Cotylorynchus romeri (e.g., OMNH 655; M.R. pers. obs.). According to Stovall et al. (1966) the dorsal termination of the spines, expanded laterally and medially indented, is a typical feature of the mid-dorsal vertebrae in C. romeri , as well as the general knobby appearance of the spine. On the posterior side of spine NS 151/48 a longitudinal groove, about 0.4 cm wide, is observable between two strong bony ridges ( Figure 4.8 View FIGURE 4 ). A very similar groove is present in the dorsal spine FMNH UR 925 referred to Angelosaurus romeri (M.R. pers. obs.). The overall robustness of the spine and the relative size, indicate a possible attribution of the spine to a sacral or a dorsal vertebra. Specimens NS 151/59 and NS 151/72 represent two anterior portions of haemal arch ( Figure 4.11 View FIGURE 4 -17). Although the specimens resemble in some way the above described neural spine end, the haemal arch anterior head is fairly asymmetric, with the lateral expansion more developed than the medial one. The approximately flat articular surface for the caudal centra has a sub-oval shape, broader in the proximal half and distally tapered (length equals to 3.3 cm, maximum width in the anterior portion equals to 1.7 cm).
Ribs. Specimens NS 151/39, NS 151/40, NS 151/ 42, NS 151/54, and NS 151/55 represent fragmentary ribs that, given their absolute size and the overall robustness, can be referred most likely to the dorsal region ( Figure 5 View FIGURE 5 ). In fact, being larger than the ribs described by Romano and Nicosia (2014, p. 5, figure 3), the new material can probably be identified as mid-dorsal ribs. All preserved portions show sharp transverse fractures, straight and clean perpendicular to the long axis and parallel to each other, most likely occurred after permineralization (therefore referable to a tectonic phase consistently later than final burial; see Ronchi et al., 2011). On average, the maximum transverse width of the ribs is approximately 3.7 cm while the maximum length of preserved portion is found in NS 151/40, with a length of 24.5 cm ( Figure 5.3-4 View FIGURE 5 ). With the exception of the slightly curved NS 151/ 54, the rib axes are mostly straight, therefore representing the rectilinear portion of the dorsal ribs, comprised between the proximal and the distal curvatures (see Stovall et al., 1966). The rib shows a typical section characterized by a convex anterior side and a flat posterior one. The ribs outer margin is tapered and ends with a sort of bony blade, developed more or less continuously throughout the preserved length. This structure, clearly visible in NS 151/54 and NS 151/40, probably served as the insertion of the M. ilio costalis dorsi (see Romano and Nicosia, 2014). In contrast, the inner border is more rounded, or, as in the case of NS 151/55, consists of two continuous bony ridges separated by a central longitudinal groove. Specimen NS 151/39 is a rib fragment (11.2 cm in length, Figure 5.1-2 View FIGURE 5 ) which, with respect to the just described portions, is overall more flat and with both edges fairly tapered. On the basis of its general structure, the rib may be referred to the last presacrals, being fully compatible with the ‘lumbar’ region of C. romeri (OMNH 1673, M.R. pers. obs.).
Specimen NS 151/75 represents a consistent portion of dorsal rib, with a preserved length measured along the curved axis equal to 27 cm ( Figure 6 View FIGURE 6 ). As for the above described material, the preserved portion shows sharp fractures parallel to each other and perpendicular to the rib long axis. Distally, the rib portion expands, increasing in thickness anteroposteriorly, and shows a weakly concave elliptical surface which probably marks the articulation with the terminal cartilaginous portion of the rib (medio-lateral width equal to 3.7 cm, anteroposterior width equal to 1.5 cm). Thus, this structure indicates that the preserved portion represents very likely the distal segment of a dorsal rib. As described above for the dorsal rib portions, the anterior rib surface appears weakly convex while the posterior one is, on the whole, flat. Along both the medial and lateral edge, the rib ends with a beveled hull, thus indicating that the two longitudinal keels, observed in some proximal portions of recovered ribs (e.g., NS 151/69) for the insertion of the M. levator costae (see Romano and Nicosia, 2014), coalesce distally with a single keel; the latter runs until the termination of the rib.
The new finding NS 151/76 represents by far the largest dorsal rib among those recovered to date, with a preserved portion approximately 52.5 cm long ( Figures 7 View FIGURE 7 , 8 View FIGURE 8 ). The bone results quite flattened however maintaining a mostly convex anterior and an approximately flat posterior one. Proximally, a system capitulum-tuberculum of considerable size is preserved, with a maximum width of the complex equal to 13.5 cm (in the largest rib described by Romano and Nicosia, 2014 NS 151/ 62 this distance measures 10.5 cm). The capitulum is well preserved, proximally expanded, and tapered distally up to form a kind of collar before re-expanding towards the tuberculum. The concave articular surface has a sub-rhomboid outline, probably due in part to rib post-mortem compression. The long side of the articular area measure 3.1 cm, the short orthogonal one 1.8 cm in correspondence of the wider central area. The tuberculum is poorly preserved, its proximal portion missing in part due to breakage. However, as typically observed in caseid dorsal ribs, the tuberculum is very close to the axis without forming a welldeveloped process. The rib width immediately distal to the tuberculum (zone of maximum rib width in C. romeri according to Stovall et al., 1966) is 5.3 cm, a very impressive size even for a large caseid.
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