Rhomaleosauridae Kuhn, 1961
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publication ID |
https://doi.org/ 10.4202/app.00167.2015 |
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persistent identifier |
https://treatment.plazi.org/id/03ED87B8-FFB8-386C-14D7-FBA6DE38DFD6 |
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treatment provided by |
Felipe |
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scientific name |
Rhomaleosauridae Kuhn, 1961 |
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Genus et species indet.
Figs. 2–9 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig .
Material.— CAMSM X.50215 comprises an associated left ilium and left ischium collected by Henry Keeping in 1898 from the Oxford Clay Formation at Fletton, Peterborough Member (Callovian), Peterborough, Cambridgeshire, United
Kingdom.Evidence for the association of these bones is given by the fit between the iliac facet of the ischium, and matrix attached to the ventral surface of the ilium, covering its ischiadic facet, and part of the acetabulum. SGM 1445-9–20, 97−120 comprises a partial postcranial skeleton collected by Soviet geologist Eugeny Vladimirovich Milanovsky in the summer of 1920 during geological investigations in the basins of two small inflows of Medveditsa River; “Konnyi barak” ravine, flowing into the river Dobrinka on the left bank, located 3.2 km south of the village Verkhnaya Dobrinka, Zhirnovsk district, Volgograd region, Russia ( Fig. 1 View Fig ). Currently a significant part of the ravine is recultivated (personal observation MSA). Bed J 2 cl (after Milanovsky 1921; Pervushov et al. 1999; Fig. 2 View Fig ), Cadoceras elatmae Zone, Hlebnovka Formation , Lower Callovian ( Mitta et al. 2012). Dark purplish-gray clay, including numerous gypsum crystals, sometimes forming ellipsoidal concretions, belemnites, fossil wood and the ammonites Cadoceras elatmae , Chamoussetia chamousseti , and Keplerites sp. also occur in this horizon.
Description
CAMSM X.50215.—Ilium: The ilium of CAMSM X.50215 is 375 mm long dorsoventrally ( Fig. 2 View Fig ), and is large compared to cryptoclidid ilia (181–225 mm in Muraenosaurus ; 172–200 mm in Cryptoclidus ; Andrews 1910), but comparable in size to the ilia of some pliosaurids (116–206 mm in Peloneustes , 242–310 in “ Pliosaurus ” andrewsi [as Peloneustes evansi ]; Andrews 1913; Liopleurodon is larger than Peloneustes or “ P.” andrewsi, but no ilium is preserved). The ilium of CAMSM X.50215 comprises anteroposteriorly expanded proximal and distal ends, joined by a straight, constricted shaft, with a minimum anteroposterior diameter of 64 mm. The shaft cross section is suboval, and slightly mediolaterally compressed, with a mediolateral diameter of 54 mm at the point of minimum anteroposterior diameter. This morphology differs from that in cryptoclidids and most xenopsarians, in which the shaft is not straight, but curves anterodorsally (e.g., Andrews 1910; Wegner 1914; Sato 2002: character 170; Albright et al. 2007; Hampe 2013).
The ventral expansion of the ilium has a suboval outline in ventral view, which tapers posteriorly. It is 165 mm long anteroposteriorly, 95 mm wide mediolaterally, and is divided into two approximately flat facets; the ischiadic facet posteriorly, and the iliac portion of the acetabulum anteriorly. As in all plesiosaurians ( Storrs 1991), the ilium lacks a pubic articular facet. The ischiadic facet faces posteroventrally, and is larger than the iliac portion of the acetablum, which faces ventrally. This morphology is similar to the condition in Early Jurassic plesiosaurians, including rhomaleosaurids, microclidids and early pliosaurids (e.g., Fraas 1910; Bardet et al. 1999; Vincent 2011; Benson et al. 2012). However, it differs from the condition in thalassophonean pliosaurids Andrews 1913; an ilium described as a scapula by Tarlo 1957; see Halstead 1989; Ketchum and Benson 2011), cryptoclidids ( Andrews 1910) and xenopsarians (e.g., Wegner 1914), in which the ischiadic facet is located on the ventromedial surface of the ilium. Furthermore, in thalassophoneans the ventral end of the ilium is only slightly longer anteroposteriorly than it is mediolaterally ( Andrews 1913; Ketchum and Benson 2011), and in cryptoclidids and many xenopsarians it is wider mediolaterally than it is long anteroposteriorly ( Andrews 1910; Hampe 2013), further differing from CAMSM X.50215.
The dorsal portion of the ilium of CAMSM X.50215 is transversely narrow (maximum thickness = 32 mm; minimum = 19 mm), and expanded anteroposteriorly (170 mm) into a sheet-like dorsal expansion ( Fig. 2 View Fig ). It extends further posterodorsally than anteriorly, and is therefore slightly asymmetrical in lateral view. This also occurs in Rhomaleosaurus ( Smith and Benson 2014) , cryptoclidids, the early pliosaurid Hauffiosaurus (Vincent 2011) , and some thalassophonean pliosaurids ( Peloneustes, Andrews 1913 ; Marmornectes, Ketchum and Benson 2011 ). However, it is distinct from the almost symmetrical dorsal expansion seen in many early Jurassic plesiosaurians (e.g., Thalassiodracon, GSM 51235; Eurycleidus, NHMUK PV OR 2030 * and associated material; Stratesaurus, Benson et al. 2012 ) and from the strongly asymmetrical conditions seen in more derived thalassophoneans such as “ Pliosaurus ” andrewsi (NHMUK R3891; Andrews 1913) and Pliosaurus ( Tarlo 1957) .
The plane of the dorsal expansion is rotated approximately 20° posterolaterally relative to that of the ventral expansion in CAMSM X.50215 ( Fig. 2E, F View Fig ). This is similar to the condition in many Early Jurassic plesiosaurians, including the rhomaleosaurids Eurycleidus (NHMUK PV OR 2030*) and Rhomaleosaurus ( Smith and Benson 2014) , but differs from that in thalassophoneans, cryptoclidids, and xenopsarians, in which the angle is 90° due to the position of the ischiadic facet on the ventromedial surface of the ilium ( Benson et al. 2012: character 173).
Ischium: The ischium of CAMSM X.50215 is almost complete ( Fig. 3 View Fig ), missing only its anteromedial process, which contacts the pubis in some plesiosaurians. The ischium is 550 mm long anteroposteriorly, from its posterior end to the anterior surface of the acetablum. It is 470 mm wide mediolaterally, from the acetabulum to the medial symphysis, and therefore has a length:width ratio of 1.17. This ratio is low compared to thalassophonean pliosaurids, which have anteroposteriorly elongate ischia (ratio = 1.7–2.0; Andrews 1913; Romer and Lewis 1959; Ketchum and Benson 2011), but is similar to the values seen in rhomaleosaurids (ratio = 1.0–1.5; Meyerasaurus , Rhomaleosaurus , Macroplata ; Fraas 1910; Ketchum and Smith 2010; Smith and Vincent 2010; Benson et al. 2012; Smith and Benson 2014) and other Early Jurassic taxa, including pliosaurids ( Bardet et al. 1999; Vincent 2011). It is also within the range of many plesiosauroids, including cryptoclidids (ratio ~ 1.1 in Cryptoclidus and Muraenosaurus, Andrews 1910 ; ratio = 1.4 in Picrocleidus, NHMUK R2739 ). The medial and lateral surfaces of the ischiadic blade are approximately parallel posteriorly, converging only weakly ( Fig. 3A, B View Fig ), and the posterior margin of the outline of the ischium in dorsal view is straight, with a truncated appearance. This is similar to the condition in rhomaleosaurids such as Rhomlaeosaurus ( Andrews 1922; Smith and Benson 2014), Meyerasaurus ( Fraas 1910; Smith and Vincent 2010) and Eurycleidus (NHMUK PV OR 2030*), but differs from the condition in most plesiosaurians, including Middle Jurassic taxa, in which the posterior margin of the ischium is convex, as seen in pliosaurids ( Peloneustes, NHMUK R 3318; Simolestes, NHMUK R 3319; Andrews 1913), and most cryptoclidids ( Cryptoclidus, NHMUK R2417 ; Picrocleidus, NHMUK R2739 ; Andrews 1910), although a specimen of Muraenosaurus (NHMUK R2863) was shown as having a straight posterior margin by Andrews (1910: text-fig. 65).
The acetabular process of the ischium is well preserved in CAMSM X.50215, with a dorsoventral height of 105 mm and anteroposterior length of 215 mm ( Fig. 3 View Fig ). It shows prominent facets for the ilium (facing posterolaterally), acetabulum (facing laterally), and pubis (facing anteriorly). The lateral orientation of the iliac facet of CAMSM X.50215 is similar to that in Rhomaleosaurus ( Andrews 1922; Smith and Benson 2014), but unlike the dorsolateral orientation seen in many other plesiosaurians, including in Early Jurassic taxa such as Eurycleidus (NHMUK PV OR 2030*) and Microcleidus ( Occitanosaurus, Bardet et al. 1999 ), and in well-preserved Middle Jurassic pliosaurids ( Liopleurodon, Andrews 1913 : text-fig. 7) and cryptoclidids ( Muraenosaurus, Andrews 1910 : pl. 5; Cryptoclidus, NHMUK R2860 ).
The blade of the ischium of CAMSM X.50215 is dorsoventrally thin, with a minimum symphysial thickness of 18 mm. It attains its maximum symphysial thickness of 40 mm anteriorly, where the ischiadic symphysis rises anterodorsally. This thicker portion of the symphysis extends laterally as a low buttress that contacts the acetabular process and defines the anterior margin of a gentle depression on the dorsal
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surface of the ischial blade. The ventral surface of the ischial blade is approximately flat.
The anterior and posterior borders of the ischial neck, connecting the acetabular process to the ischial blade, are dorsoventrally shallow and sharply edged. This cannot be attributed to postmortem compression, which appears to be absent. The neck of the ischium is relatively wide due to shallow excavations of anterior and posterior edges (the ischial neck:acetabular process ratio is ~0.73) as in most thalassophonean pliosaurids (ratio = 0.70 in Liopleurodon, NHMUK R 3536; 0.88 in Peloneustes, NHMUK R 3318; and ~1.0 in Simolestes, NHMUK R 3319 and Marmornectes ; Andrews 1913; Ketchum and Benson 2011). This contrasts with the narrow ischial neck seen in rhomalesaurids, which is embayed by the deep anterior excavation of obturator foramen (ratio = 0.6 in Rhomalesaurus thorntoni, NHMUK R4853; Smith and Benson 2014; and 0.56 in Eurycleidus arcuatus, NHMUK R2230 ), or excavation of both the anterior and posterior margins (ratio = 0.5 in Meyerasaurus victor, SMNS 12478; Smith and Vincent 2010). In Early Jurassic pliosaurids (Vincent 2011) and Callovian cryptoclidids, the ischial neck is also relatively thin and mediolaterally elongated ( Cryptocleidus , Muraenosaurus ; Andrews 1910).
SGM 1445-9–20, 97−120.—The fragmented posterior part of the skeleton is preserved, including dorsal, sacral and caudal vertebral centra and ribs, the pelvic girdle, and incomplete fragments of limbs. The bones are light gray-yellowish, and sometimes pyritized, with a covering of gypsum crystals.
Dorsal vertebrae: Two dorsal centra are preserved ( Fig. 4 View Fig ; SGM 1445-108, 107). They have subcircular, weakly concave anterior and posterior articular surfaces that are approximately as wide mediolaterally as high dorsoventrally ( Table 1). The central portions of the centra are slightly constricted between the articular surfaces. The centra are proportionally short anteroposteriorly, approximately 60% of their dorsoventral heights ( Table 1), and have transversely convex ventral surfaces. The dorsal surfaces of the centra bear the neural arch facets and the floor of the neural canal, which is transversely narrowest anteriorly.
Caudal vertebrae: Five caudal centra are preserved ( Fig. 5 View Fig ), forming a non-continuous sequence with specimens numbers listed here from anterior to posterior (SGM 1445- 102−106). The centra are slightly wider mediolaterally than high dorsoventrally, and shorter anteroposteriorly than either their height or width ( Table 1). One proximal caudal centrum (SGM 1445-105) has a ventral surface that is anteroposteriorly longer than its dorsal surface, conferring a wedge-like morphology ( Fig. 5B 5 View Fig ) that does not result from taphonomic distortion. The outlines of the articular surfaces are suboval, with a subrectangular appearance due to ventrolateral expansion contributed by the chevron facets, especially in more posterior centra (SGM 1445-102, 103). Well-defined chevron facets are generally present both anteriorly and posteriorly on the ventral surfaces of the centra. They are semi-oval, and the anterior facets are generally larger than the posterior facets. Rib facets are present anterodorsally on the lateral surfaces of the centra, extending outwards from the lateral surface of the centrum ( Fig. 5 View Fig ), and contacting the neural arch peduncle, as occurs in rhomaleosaurids, some pliosaurids, some leptocleidians, and Early Jurassic plesiosauroids (e.g., Benson et al. 2012; Smith 2013; Smith and Benson 2014). The articular surfaces of the rib facets are concave, and have suboval outlines in lateral view.
Paired subcentral foramina are present on the ventral surfaces of the caudal centra, as in many plesiosaurians ( Storrs 1991), but differing from the condition in early xenopsarians, where often only a single, midline foramen is present ( Benson and Druckenmiller 2013). The dorsal surfaces of the caudal centra bear the neural arch facets laterally, and the ventral floor of the neural canal, centrally. The neural canal is mediolaterally narrower anteriorly than posteriorly, as it is constricted by the neural arch facets.
Dorsal ribs: One dorsal rib is completely preserved, with a length of 270 mm and a dumbbell-shaped cross section ( Fig. 6 View Fig ). The proximal articular surface is oval, and measures 23 mm dorsoventrally. Other rib fragments are very small. Two of them might represent sacral ribs: they are double-headed, and 70 mm in length as preserved.
Ilium: The left ilium is well preserved ( Fig. 7 View Fig ; SGM 1445- 99), although it appears to have been plastically deformed, with the dorsal end bent over laterally. The ilium conforms well to the description of CAMSM X. 50215 in most topological features: the orientations of the acetabular and ischiadic facets, the cross-section of the shaft, and the rotation of the dorsal expansion relative to the ventral end. These morphologies are consistent with affinities to an early-diverging group such as Rhomaleosauridae , but inconsistent with pliosaurid or cryptoclidid affinities (see comparisons above). However, SGM 1445-99 is smaller than CAMSM X.50215 with a dorsoventral length of 255 mm, and is proportionally more robust (the minimum anteroposterior diameter of the shaft is 60 mm, similar to that of CAMSM X.50215). The cross section of the shaft is suboval and its mediolateral diameter at the point of minimum anteroposterior diameter is 40 mm.
The dorsal portion of the ilium is expanded more posterodorsally than anteriorly, although the appearance of this morphology is exaggerated by plastic deformation ( Fig. 7 View Fig ). The plane of the dorsal expansion is rotated approximately 35° posterolaterally relative to that of the ventral expansion. The ventral expansion is 115 mm long anteroposteriorly, with maximum mediolateral width of 60 mm. The ischiadic facet is subtriangular in outline and faces posteroventrally. It has an anteroposterior length is 55 mm, and maximal mediolateral width of 40 mm. The acetabular facet has a mediolateral and anteroposterior diameter of 60 mm.
Ischium: The?left ischium is crushed and incomplete, and only two uninformative fragments are now present ( Fig. 8C, D View Fig ; SGM 1445-97). Measurements of the ischium recorded by Milanovsky (1921) report a mediolateral width of 300 mm, and a preserved anteroposterior length of 450 mm, noting that the posterior process was broken. Milanovsky estimated an original length of 600−650 mm ( Milanovsky 1921). It is not clear that these measurements are reliable, or whether the recorded mediolateral width represents a complete or incomplete specimen. Nevertheless, the length:width ratios resulting from these measurements range from 1.50–2.17, exceeding 1.17 seen in CAMSM X.50215, and overlapping with those of many other plesiosaurians, including rhomaleosaurids and pliosaurids (discussed above).
Pubis: Only the acetabular process of the left pubis is preserved ( Fig. 8A, B View Fig ), with a dorsoventral height of 95 mm and an anteroposterior length of 165 mm. It bears two flat slightly concave equal in size facets, one for the ischium, facing posteriorly and one for the acetabulum, facing laterally.
Hindlimb: Five metatarsals and 17 phalanges are preserved ( Fig. 9 View Fig ), probably representing the hindlimb, inferred from preservation of the pelvic girdle. The length of the metatarsals is 70 mm, with a proximal mediolateral width of 50 mm, and distal mediolateral width of 45− 42 mm.
The phalanges are proportionally short, with proximodistal lengths that are generally less than twice their mediolateral width. They have rugose, approximately flat or weakly concave proximal and distal articular surfaces, which are dorsoventrally low compared to their mediolateral widths, and the central portions of the phalanges are mediolaterally constricted between these articular surfaces.
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