Dakotasuchus kingi Mehl, 1941
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publication ID |
https://doi.org/ 10.4202/app.00338.2016 |
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persistent identifier |
https://treatment.plazi.org/id/03FBF377-FFCF-E22D-9C05-5AC3FCBF1EB7 |
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treatment provided by |
Felipe |
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scientific name |
Dakotasuchus kingi Mehl, 1941 |
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Figs. 2 View Fig , 3A, E View Fig ; SOM.
Holotype: KWU uncatalogued, an articulated partial skeleton in a concretion missing the skull and limb elements
Type locality: West of Salina, Kansas, USA.
Type horizon: Dakota Formation, Cenomanian (Late Cretaceous).
Material.— OMNH 34500, a disarticulated partial skeleton, consisting of a right coracoid, right radius, four partial to complete dorsal vertebrae, six caudal vertebrae, three cervical ribs, four nearly complete and many partial dorsal ribs, and multiple ventral and dorsal scutes. From OMNH locality V828, ca. 26 km S of Emery, Emery County, Utah; Cenomanian (Late Cretaceous), Mussentuchit Member of the Cedar Mountain Formation. OMNH V828 is on Federal land administered by the US Bureau of Land Management, which maintains and restricts access to specific locality coordinates.
Diagnosis.— Mehl (1941) never provided a diagnosis for Dakotasuchus kingi . This taxon resembles other North American medial Cretaceous coelognathosuchians such as Woodbinesuchus byersmauricei and Terminonaris robusta , but can be differentiated based on a unique combination of postcranial characters not uniformly present in either taxon: (i) the presence of triangular processes on the caudodorsal surface of the cervical ribs; (ii) dorsal vertebrae with a supporting buttress which forms at the caudal margin of the neural arch and continues through the ventral side of the transverse processes; (iii) the presence of vertebrae with a deep groove on the dorsal surface of the centra, giving the neural canal a heart-shaped appearance; (iv) a scapula with a deeply concave cranial margin with a thickened and expanded distal end that terminates in a ventrally-directed hook; (v) coracoid with dorsal and ventral heads that are rounded and relatively expanded caudally; (vi) a relatively robust radius that possesses a large hook-shaped tubercle on the medial side of the proximal condyle; (vii) ischium possessing a broad pubic process that is oriented more perpendicular to its shaft; and (viii) dorsal scutes with a rounded and concave caudolateral edge. Characters (i), (ii), (iii),
iv), (v), (vi), and (vii) are not shared with Woodbinesuchus byersmauricei , whereas characters (i), (ii), (iv), (v), and (vii) are not found in Terminonaris robusta .
Description.— OMNH 34500 preserves multiple elements on the right side of the body, including a coracoid and radius, as well as cervical ribs, together with vertebrae and osteoderms. In addition, at least 12 teeth were found in association with the specimen, with the largest possessing a maximum height of 18.3 mm; though many of these appear too small (<10 mm in height) to belong to this individual. The bones are relatively well-preserved; however some elements, such as the radius and coracoid, possess fragmented bone surfaces. Broken edges of the bones are relatively sharp, with little smoothing evident on any of the larger elements, implying that taphonomic transport was minimal. A single theropod tooth and a possible mammal canine were also found in close proximity to OMNH 34500.
The most cranially-located elements preserved in OMNH 34500 are three cervical ribs. Overall, the general morphology agrees with descriptions given by Mehl (1941); however, the cervical ribs of the KWU specimen are rather incomplete. Supplemental descriptions provided here are based on a complete, right cervical rib of unknown position ( Fig. 2A View Fig ). The ovate capitular articular surface is nearly three times larger than that of the nearly circular tubercular articular surface. In dorsal view, the capitulum is much thicker than the tuberculum, whereas the tuberculum is longer. A thin posteriorly-directed ridge descends off of the capitulum, ending adjacent to the ventral margin of a triangular process. This conspicuous triangular process ( Fig. 2A 2 View Fig ), on the caudodorsal surface, is oriented caudomedially and is either absent or incipient on other North American Cretaceous crocodilians, as well as modern crocodilians. The cranial process is thick, anterodorsally compressed with a rugose craniodorsal surface, and has a large groove on the ventral side. The posterior process is thin and is curled medially to accommodate the corresponding cranial process of the following cervical rib.
The dorsal vertebrae ( Fig. 2C View Fig ) are similar in size and shape to those described from the holotype specimen of Dakotasuchus kingi . The centra are amphicoelous, with a shallow central depression and flange-like processes on the margin of the articular surfaces, giving the centra a constricted appearance in lateral view ( Fig. 2C View Fig 3 View Fig ). The cranial articular surface is circular ( Fig. 2C View Fig 1 View Fig ), while the caudal articular surface is dorsoventrally taller and subcircular in outline ( Fig. 2C 2 View Fig ). Mehl (1941) describes a conspicuous buttress that arises from the caudodorsal margin of the centrum and continues through the transverse process on the holotype specimen; this is also seen in the newly-referred specimen ( Fig. 2C View Fig 3 View Fig ). In OMNH34500 this feature is present on all dorsal vertebrae, varying in thickness along the vertebral column from notably thickened in cranially-placed vertebrae to nearly flush with the rest of the ventral face of the transverse processes in more caudal vertebrae. The neural canal is heart-shaped in both Dakotasuchus specimens, with a deep groove incised into the dorsal face of the centrum ( Fig. 2G View Fig ). As described by Mehl (1941), the zygapophyses in OMNH 34500 are nearly continuous with the dorsal surface of the transverse processes. Both pre- and postzygapophyses are generally wide, elongate, and finger-like ( Fig. 2C View Fig 4). The neural spines are short and robust, with a mediolateral and craniocaudally expanded apex. Multiple nondescript fragmentary and incomplete dorsal ribs were recovered; these are uninformative and are ignored herein. The caudal vertebrae are largely incomplete, missing the majority of the neural arches; in known respects they are morphologically similar to caudals of other mesoeucrocodylians.
For the appendicular skeleton, only the right coracoid and radius are known in OMNH 34500. Mehl (1941) was able to make a cast of the right coracoid from a natural mold in the holotype specimen. Unfortunately, sometime in the following seven decades this section of the concretion was damaged; however, much of the left coracoid is still visible and descriptions (and figures) from Mehl (1941) appear to accurately reflect the morphology of the now broken right element. In OMNH 34500, the midline of the coracoid (taken in the coronal plane) measures 235 mm (greatest length along midline = 279 mm). The coracoid is long, with expanded dorsal (152 mm wide) and ventral (114 mm wide) ends ( Fig. 2B View Fig ). In profile, the coracoid is curved ventromedially and, like the holotype, is at its greatest curvature at about one third of the way from the dorsal head; the distal two-thirds of the shaft is straight ( Fig. 2B 2 View Fig ). The diameter of the shaft at the apex of the curve (34 mm) is the most constricted portion of the shaft, which despite its larger size, is the same as the holotype described by Mehl (1941). The outline of the dorsal head is mushroom-shaped in lateral view. The scapulocoracoid junction is straight, and gently curves cranially at the terminus of the junction. Cranial to the scapulocoracoid junction, the dorsalmost edge is gently rounded. The coracoid foramen is positioned cranial and dorsal to the glenoid ( Fig. 2B View Fig 3 View Fig ), similar to the position seen in other Cretaceous mesoeucrocodylians ( Mehl 1941; Lee 1997; Wu et al. 2001). The articular surface of the glenoid on the coracoid is highly rounded ( Fig. 2B 2 View Fig ). The glenoid curves ventrally, forming a discrete ridge with a lip along the lateral perimeter of the glenoid. The ventral end of the coracoid flattens into a sharp ulu-like blade that approaches the ventral midline of the body. The caudal edge of the shaft is much straighter than the rostral edge, but both end in a noticeable pointed flare.
The right radius is nearly complete but bears multiple large, radial and longitudinal fractures. In profile, it is a robust, elongate element (248 mm long) with a flattened proximal condyle and a rounded distal condyle ( Fig. 2D View Fig 1 View Fig ). In lateral view ( Fig. 2D 2 View Fig ), the shaft has a sigmoidal shape and the lateral corner of the distal end is craniomedially rotated. As in modern crocodilians, the humeroarticular surface is kidney shaped with an enlarged proximal condyle, whereas the distal condyle is rounded with a central depression on the cranial face for the articulation with the ulna. There is a conspicuous hook-shaped tubercle for the insertion of the humeroantebrachialis inferior muscle on the medial side of the proximal condyle ( Fig. 2D View Fig 1 View Fig ). The insertion tubercle for the brachialis muscle, also present but expressed as a low mound rather than a tubercle, is located a quarter of the way down from the proximal end of the shaft.
Three dorsal osteoderms (two complete and one partial) are preserved in OMNH 34500. These are of typical coelognathosuchian (comprising Pholidosauridae and Goniopholididae ; Martin et al. 2014) morphology, roughly rectangular in outline and paired along the midline of the body. The dorsal surface of each osteoderm is irregularly ornamented with deep oval pits varying in size from 1.0 to 13.0 mm in diameter ( Fig. 2E View Fig ). The cranial edge of each dorsal scute has a smooth margin that articulates with the caudal end of the preceding scute. In addition, a large, broadly triangular cranial dorsal process ( Fig. 2E View Fig ) is present on the craniolateral side of the scute. The lateral edge is ventrally deflected, creating a concavity to interlock with the aforementioned cranial dorsal process of the following dorsal scute. The caudolateral corner is also rounded, with a discrete notch for receiving this process ( Fig. 2E 2 View Fig ). The ventral surface is relatively smooth with a shallow groove running on the caudomedial and extending a third of the way along the length of the scute. Like the dorsal scutes, the ventral scutes compare favorably to the description provided by Mehl (1941) for Dakotasuchus kingi . The scutes range from pentagonal near the midline to hexagonal on the flanks. The ventral surface of each scute is only weakly pitted and is generally more flattened near the center of the body. On the dorsal surface, the bone fibers are arranged in a random orientation. The outer margins are covered in radial crenulations that are morphologically reminiscent of sutures on cranial bones ( Fig. 2F View Fig ).
The largest teeth associated with this specimen have an oval base, with two distinct carinae and multiple fine striae on the crown ( Fig. 2H View Fig ). Other large teeth from the Mussentuchit Member (31 mm, OMNH 32090; 32.9 mm, OMNH 34601) likely also come from this taxon as well, and are presumably synonymous with the “pholidosaurid” teeth described by Garrison et al. (2007: fig. 23E, F). Even so, these teeth are shorter than a sizeable crocodilian tooth (41.9 mm, OMNH 27825) that was discovered in the underlying, Lower Cretaceous Ruby Ranch Member of the Cedar Mountain Formation in Grand County (V857), suggesting that even larger crocodilians were present in the medial Cretaceous of Utah.
Stratigraphic and geographic range.—Cenomanian (Cretaceous), Dakota Formation of Kansas, USA and Mussentuchit Member of the Cedar Mountain Formation of Utah, USA.
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