Conilurus capricornensis, Cramb & Hocknull, 2010

Cramb, Jonathan & Hocknull, Scott, 2010, New Quaternary records of Conilurus (Rodentia: Muridae) from eastern and northern Australia with the description of a new species, Zootaxa 2634 (1), pp. 41-56 : 46-51

Conilurus capricornensis sp. nov. Cramb & Hocknull

(fig. 4)

Holotype. QMF 54017 left maxilla with M 1-3.

Type locality. QML1091S Dodgey’s Cave surface collection, Broken River, north east Queensland.

Paratypes. QMF 54033 right mandible with M 1-2, 54034 right mandible with M 1-3 .

Referred specimens. QML1457: QMF 54018 left maxilla with M 1-3, 54019 right M 1, 54020 left M 2, 54022 left M 2, 54025 right M 3, 54026 left M 1, 54027 right M 1, 54028 right M 1; QML 1456: QMF 54021 left M 2, 54023 left M 2, 54024 left M 3, 54029 left M 1, 54030 left M 1, 54031 right M 2, 54032 right M 2, 54037 left maxilla, 54038 right maxilla fragment with M 1, 54039 left maxilla fragment with M 1, 54040 right mandible with M 1; QML1315: QMF 54041 right M 1.; QML1469: QMF 54035 left mandible with M 1-3, 54036 right mandible with M 1.

Etymology. capricornensis from Capricorn Caves, eastern central Queensland, the first site to produce identifiable specimens.

Diagnosis. Large Conilurus with broad molars; T3 present on M 1; anterior cingulum present on M 1; posteriorly narrow anterior palatal foramina; anterolingual cusp on M 1 not projecting posteriorly; posterior cingulum on M 1-2 small to absent.

Description. Upper dentition. M 1: proportionally broad. Anterior cingulum present as a rough depression on T2. T1 large, circular and distinct but still closely associated with T2. T1-3 and T4-6 lophs straight and parallel to each other, both perpendicular to the long axis of the tooth row. T2 laterally elongate, anterior surface sloped but posterior surface vertical. T3 small, circular and approximated to T2, becoming indistinct with wear. T1 bis absent. A small bulge on the buccal side of the tooth present at the base of T3 on some specimens (e.g. QMF54019). Accessory cusp absent.

T4 large, circular when unworn, oval-shaped when worn (laterally elongate); made distinct from T5 by a slight infolding of enamel but dentine is continuous between the two cusps. T5 laterally elongate, largest cusp in T4-6 loph. T6 small and only distinct from T5 when unworn. T5-6 complex extends further buccally than T2-3 complex. T7-9 loph sloped posteriorly, overlapping M 2. T7 large and circular, oriented more vertically than T8-9 complex and thus remaining distinct until advanced stages of wear. T8-9 complex large, T9 not distinct from T8 on any specimens regardless of wear.

M 2: broad like M 1, but shorter due to absence of T2-3 and more vertical orientation of T7-9 loph. T1 large, circular, closely associated with T8-9 complex of M 1. T2-3 absent. T4-6 loph slightly curved, more obviously when worn. T4 closely approximated to T5, only distinct when entirely unworn.

T4 slightly posterior of T5. T6 barely distinct when unworn, otherwise indistinguishable from T5. T7-9 lophs sloped more posteriorly than T4-6 loph, overlapping slightly with M 3. T7 circular, oriented more vertically than T8-9 complex. T9 not distinct from T8. T8-9 complex the most bulbous cusps on M 2, and the first to become worn.

M 3: T1 large, circular and distinct. T4-6 loph arc-shaped, bulging antero-buccally. T4-6 loph a single mass, cusps indistinct. The size and position of the T7 and T8 is variable, and wear can join the T4-6 loph to the T7 or T8. T7 and T8 commonly differ in size. T7 can be circular or oval shaped in occlusal view. One specimen (QMF54025) has T7 large and oval shaped with a very small T8. T9 absent. Posterior cingulum absent.

Maxilla: angle of zygomatic bar less than that of C. albipes and similar to that of C. penicillatus . Anterior palatal foramina posteriorly narrow. Holotype has a groove in the zygomatic plate not seen in other specimens. A large swelling is present on the posterior edge of the zygomatic plate, buccally of the M 1.

M 1 has three roots: anterior root, plunging anteriorly; long lingual root and smaller postero-buccal root. M 2 has a laterally broad anterior root, a long lingual root and a smaller postero-buccal root. The anterior and lingual roots are joined in the antero-lingual corner in some specimens (e.g. QMF54023). M 3 has three major roots and one minor root. Major roots are situated antero-buccally, antero-lingually and posteriorly. The two major anterior roots are joined in some specimens (e.g. QMF54024). A minor root is situated between the antero-lingual and posterior root.

Lower dentition. M 1: longest lower molar. Broad, increasingly so from the anterior to the posterior loph. Antero-lingual cusp circular, larger than anterio-buccal cusp. Antero-buccal cusp subtriangular, extending posteriorly of anterolingual cusp. Anterolingual anf anterobuccal cusps become fused after heavy wear. Protoconid appears larger than metaconid when unworn. After wear protoconid and metaconid become subequal in size. Protoconid subtriangular shaped, metaconid oval shaped. Hypoconid laterally narrower than broad entoconid. Hypoconid and entoconid completely fused after moderate wear. Posterior cingulid commonly small, but some specimens have it moderately well developed (QMF54026) or absent (QMF54030). Buccal cuspules absent.

M 2: slightly broader than M 1. Consists of two lophs. Protoconid more robust at base than metaconid. Metaconid laterally elongate and completely fused to protoconid after moderate wear. Hypoconid-entoconid loph slightly narrower than protoconid-metaconid loph. Hypoconid fused to entoconid after minimal wear. Protoconid-metaconid loph oriented in parallel with hypoconid-entoconid loph. Entoconid more laterally elongate in comparison with hypoconid. Posterior cingulid variably small or very small.

M 3: All specimens worn. Smaller than M 2. Hypoconid-entoconid loph narrower than protoconidmetaconid loph, making the tooth appear triangular in shape. Protoconid more robust than metaconid. Unknown whether hypoconid is distinct from entoconid. Posterior root sloping anteriorly, forcing the tooth to the anterior of the posterior root alveolus.

Alveoli laterally broad, mirroring molar lophs. All molar roots laterally broad but longitudinally short, and commonly bifurcating at the tips. M 1 shows a different pattern, with a laterally narrow anterior root (matching the narrow loph formed by the anterobuccal and anterolingual cusps). Orientation of roots is commonly vertical, the exception being the posterior root of M 3, which follows the slope of the entoconid.

Remarks. Distinguishing features of the molars of Conilurus and Mesembriomys species are shown in appendix 1.

Conilurus capricornensis differs from C. albipes in the following characteristics: 1. Possession of a T3 on M 1; 2. More gently sloping zygomatic bar; 3. Molar crowns less crowded; 4. Molars proportionally broader; 5. Presence of an anterior cingulum on M 1; 6. Posteriorly narrow anterior palatal foramina; 7. An anterolingual cusp on M 1 that does not project posteriorly; and 8. The posterior cingulid on M 1-2 is small to absent.

Conilurus capricornensis differs from C. penicillatus by its larger size in all molar dimensions and possession of a posteriorly more narrow palatal foramen, a feature less common in C. penicillatus . These features do not vary in available specimens, and are interpreted not to be ontogenetically or allometrically variable.

Two forms initially described as separate Conilurus species deserve special consideration: C. hemileucurus and C. randi are both currently considered to be synonymous with C. penicillatus (as a subspecies in the case of C. randi ). Both of these forms were noted by Tate (1951) as having larger teeth than other forms of C. penicillatus . A third form, C. p. melibius, is smaller than C. p. penicillatus , and thus easily distinguished from C. capricornensis . Comparison with the type specimen of C. hemileucurus (BM no. 57.10.24.17) demonstrates that C. capricornensis is much larger. No specimens of C. randi were available for comparison. However, measurements of the dentition listed by Tate (1951) indicate that the type specimen is within the size range observed for C. penicillatus at Chillagoe and thus much smaller than C. capricornensis . Kemper and Schmitt (1992) gave an M 1 length measurement for a specimen of C. p. randi (4.90) that overlaps slightly with the smallest M 1 length recorded for C. capricornensis (4.89). The M 1 width of this specimen was not given, so it could not be included in figure 5.

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The size and proportions of the molars gives C. capricornensis the broadest molars of any Conilurus species (fig.5). This feature led to the misidentification of some incomplete specimens as Mastacomys ( Hocknull et al. 2007) . Conilurus capricornensis is morphologically most similar to C. penicillatus . Both species have similar molar proportions; orientation of the zygomatic arch; possession of a T3 and anterior cingulum on M 1; and a small posterior cingulum on M 1-2. Conilurus capricornensis also has a swelling on the posterior edge of the zygomatic plate, buccally of the M 1. A smaller swelling is commonly seen in C. penicillatus and C. albipes , suggesting that this feature is related to size. Some questions currently cannot be resolved. For example, the buccal swelling of the maxilla is larger in C. capricornensis than both other Conilurus species. This could be no more than a function of size, or it could potentially provide reinforcement for the M 1, perhaps facilitating dietary specialisation.

Some measurements of the dentition (e.g. length and width of M 1 and M 1) of C. capricornensis specimens from the Broken River are on average slightly smaller than specimens from Mount Etna (table 2). This may be an example of Bergmann’s rule, which states that body size has a positive relationship with latitude ( Bergmann 1847). Alternatively, this result may merely reflect the small number of specimens available.