Boremys Lambe, 1906b
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publication ID |
https://dx.doi.org/10.3897/fr.25.85563 |
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publication LSID |
lsid:zoobank.org:pub:1B122F9D-B355-4D4B-808D-A0345404370B |
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persistent identifier |
https://treatment.plazi.org/id/F4A4BF64-FA51-59AB-9843-D1730190DD21 |
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treatment provided by |
by Pensoft |
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scientific name |
Boremys Lambe, 1906b |
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Boremys pulchra Lambe, 1906a
Type specimen.
CMN 1130, a plastron and anterior half of carapace ( Lambe 1902: fig. 8; Lambe 1906a: pls. 3.4, 4).
Type locality and stratum.
Near the mouth of Berry Creek, Red Deer River, Dinosaur Provincial Park, Alberta, Canada; Dinosaur Park (formerly Judith River) Formation, Judith River Group, Campanian, Late Cretaceous ( Lambe 1906a; Brinkman and Nicholls 1991; Eberth and Hamblin 1993).
Material.
RAM 27109 View Materials , a near-complete anterior plastral lobe, comprised of co-ossified entoplastron, epiplastra and partial hyoplastra (Fig. 2 View Figure 2 ) .
Description.
RAM 27109 is a well preserved, mostly complete anterior plastral lobe (Fig. 2 View Figure 2 ). The pattern of sulci at the anterior end of the ventral side of the lobe closely matches that of Boremys pulchra , as reconstructed by Brinkman and Nicholls (1991: fig. 7B) (Fig. 2A-C View Figure 2 ). The epiplastra, entoplastron and hyoplastra of RAM 27109 are completely fused, with no visible sutures (Fig. 2A, C View Figure 2 ). Archibald (1977) considered lack of shell fusion a derived character in Boremys , but most Boremys pulchra shells examined by Brinkman and Nicholls (1991) were fused. Considering that baenid turtles exhibit determinate growth and co-ossify as adults and that RAM 27109 is similar in size and proportions to other B. pulchra specimens, RAM 27109 can be considered a nearly full-sized adult ( Hutchison 1984) (Fig. 3 View Figure 3 , Table 1 View Table 1 ). Smaller size is a primary diagnostic character for this species and Brinkman and Nicholls (1991) regard 320 mm as the maximum mid-line carapace length for B. pulchra . Similarly, Joyce and Lyson (2015) found 300 mm as the carapace length dividing B. pulchra from the congeneric B. grandis . Using the proportions of the reconstructed shell of B. pulchra from Brinkman and Nicholls (1991), the mid-line carapace length of RAM 27109 is estimated to be ~ 200 mm, similar to Campanian B. pulchra specimens ( Brinkman and Nicholls 1991; Joyce and Lyson 2015) (Fig. 3 View Figure 3 , Table 1 View Table 1 ). The presence of anterior plastral scalloping (lateral epiplastral projections) and straight extragular-humeral sulci further differentiate RAM 27109 from B. grandis ( Gilmore 1935). I consider the reconstruction of Brinkman and Nicholls (1991: fig. 7B) probably more representative than that presented by Gaffney (1972: fig. 40) because it is more recent and based on more specimens (NMC 2281, ROM 5115, TMP 90.119.6, UALVP 9, UCMP 130155). The current specimen and reconstructions of B. pulchra vary in the degree of curvature between the gular-extragular and extragular-humeral sulci, as well as the degree of lateral projection on the epiplastra ( Gaffney 1972; Brinkman and Nicholls 1991). Given the uncertainty regarding sutures due to shell fusion, I interpret these last minor morphological differences as probable individual variation ( Gaffney 1972; Brinkman and Nicholls 1991).
RAM 27109 is subtriangular in shape with a bilateral pair of rounded lobes (the anterior of which is smaller) projecting laterally from the epiplastra to form distinct anterior plastral scalloping (Fig. 2B-E View Figure 2 ). Each laterally projecting lobe is upturned dorsally and protrudes slightly beyond a rounded ridge on the dorsal surface of the plastron which runs along the bases of the lobes (Fig. 2D-E View Figure 2 ). This 3-4 mm wide ridge likely marked the transition between the scale-covered lobes and the body wall (Fig. 2E View Figure 2 ). The dorsal surface of the anterior plastral lobe is otherwise flat and even, apart from a ~ 1 mm tall ridge along the posterior portion of the anterior plastral lobe mid-line, which likely represents a reduced homologue of the posterior process of the entoplastron that is present in basal testudinatans ( Gaffney 1990: figs. 91-92; Szczygielski and Sulej 2019: fig. 6) (Fig. 2D-E View Figure 2 ). The mid-line ridge reaches a maximum width of 4.3 mm and it diminishes anteriorly across the inferred length of the entoplastron, terminating at approximately the anterior end of this bone (Fig. 2D-E View Figure 2 ). At this level, there is a small round pit (diameter = ~ 3 mm) on each side of the mid-line ridge (Fig. 2D-E View Figure 2 ). Anterior to the hyoplastra, the muscles supracoracoideus anterior and deltoideus clavicularis possibly attached to the anterior plastral lobe near the mid-line ( Zhu 2011). The functions of these muscles are adduction and retraction of the humerus, which are important in aquatic locomotion of turtles ( Walker 1973; Zhu 2011).
The posterior end of the anterior plastral lobe is broken cleanly in an approximately straight line perpendicular to the mid-line on the left side (Fig. 2F View Figure 2 ). On the right side, the break angles anterolaterally at approximately 53° from the mid-line (Fig. 2B-E View Figure 2 ). Both breaks provide a view into the interior anatomy of the anterior plastron, but few details of the microanatomy can be assessed (Fig. 2F View Figure 2 ). The bone exhibits a diploë structure typical of turtles, with compact external and internal cortices enclosing an interiormost region of cancellous bone ( Scheyer 2007) (Fig. 2F View Figure 2 ). The primary comparative data available regarding baenid shell histology is from carapacial and plastral material of Boremys sp. from the Late Cretaceous Dinosaur Park Formation in Alberta, Canada ( Scheyer 2007). Compared to Neurankylus sp. and Plesiobaena sp., Boremys sp. (and Chisternon sp.) is reported to have less compact bone, thinner cortices and a well-developed cancellous interior, with larger marrow cavities in cancellous bone and higher vascularisation in compact bone ( Scheyer 2007). Such detailed microanatomical morphology cannot be ascertained from RAM 27109; however, there is distinct cancellous bone with prominent marrow cavities in the interiormost region of approximately the middle third of the lobe (Fig. 2F View Figure 2 ). Trabeculae are larger towards the mid-line and are generally round to ovoid and the lateral plastral margins consist of only compact bone, without a cancellous interior (Fig. 2F View Figure 2 ). Near the mid-line, the well-developed external cortex is approximately 2.7 times thicker than the internal cortex (Fig. 2F View Figure 2 ). This disparity between external and internal cortical thickness is similar to that of a costal from Denazinemys nodosa Gilmore, 1916 in the late Campanian Fruitland Formation, where the exterior cortex is significantly thicker than the internal cortex and trabeculae are mostly small and circular ( Lichtig and Lucas 2017).
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