ZYZOMYS RACKHAMI SP. NOV. (RODENTIA, MURIDAE) A ROCKRAT FROM PLIOCENE RACKHAM'S ROOST SITE, RIVERSLEIGH, NORTHWESTERN QUEENSLAND H. GODTHELP Godthelp, H., 1997:06:30. Zyzomys rackhami sp. nov. (Rodentia, Muridae) a rockrat from Pliocene Rackham's Roost Site, Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 41(2): 329-333. Brisbane. ISSN 0079-8835. Zyzomys rackhami sp. nov. from the Pliocene Rackham' s Roost Site, Riversleigh, northwest- ern Queensland is the first fossil member of this genus and only the second Tertiary murid described from Australia. It is known from many hundreds of dental fragments recovered as a part of an ancient megadermatid roosting cave. It appears to be the most plesiomorphic member of the genus and is part of a diverse suite of extinct murids from this site. M Rodentia, Muridae, Zyzomys, Pliocene, Riversleigh. H. Godthelp, School of Biological Science, University of New South Wales, New South Wales 2052, Australia; 4 November 1996. The murid genus Zyzomys contains 5 living spe- MI,2; QMF10821, partial right maxill¥y with MI-3; cies which are restricted to tropical Australia. QMF23325, partial right maxillary MI-3; QMF10819, Zyzomys has been placed in the tribe Conilurini left MI; QMF24001, partial left maxillary with MI,2; QMF24004, partial left maxillary with MI,2; (Baverstock, 1984) and is an 'Old Endemic' QMF24002, right Mi; QMF24003, right Mi. sensu Ride (1970) or an 'Older Immigrant' of Tate (1951). Along with Mesembriomysand Con- All from Rackham's Roost Site (19°02'09" S, ilurus, Zyzomys comprises a group of taxa unified 138°41'60' E) at Riversleigh, NW Queensland. The by a number of cranio-dental characters, phallic site represents the remnants of an ancient cave which has largely eroded away leaving the indurated floor morphology (Lidicker, 1987) and chromosomes sediments exposed. The sediments of the floor contain (Baverstock et al., 1981). myriad bones and teeth, mostly fragmented, which are Dental nomenclature follows Musser (1981). interpreted to be megadermatid (Macroderma and Measurements are in millimetres. Unless other- Megaderma spp.) prey remains (Godthelp, 1988; wise stated material is housed in the Queensland Hand, 1994). Its age is Pliocene on the basis of a Museum (QMF). macropodid similar to Protemnodon snewini Bartholamai (1978) which species occurs in the early to middle Pliocene Bluff Downs Local Fauna (Archer SYSTEMATICS & Wade, 1976; Bartholamai, 1978). Order RODENTIA Bowdich,1821 DIAGNOSIS. Zyzomys rackhami differs from Suborder MYOMORPHIA Brandt, 1855 other species of the genus by the following com- Infraorder MYODONTA Schaub, 1958 bination of characters: Relatively well-developed Superfamily MUROIDEA Miller & Gidley, series of cusps (T3,6,9) particularly T3, relatively 1918 small proportions of the lingual series of cusps Family MURIDAE Gray, 1821 ( 1,3,7), reduced molar gradient, frequent TI bis; Subfamily MURINAE Gray, 1821 tooth row short and narrow. Zyzomys Thomas, 1909 DESCRIPTION. Small to medium-sized dental Zyzomys rackhami sp. nov. arcade arcuate and concave lingually with the (Figs 1-2, Table 1) internal edge of M3 as the most lingual point of the tooth row. Mllonger than M2. M3 small and MATERIAL. HolotypeQMFIO818, partialleftmaxil- relatively reduced. All cusps and cusp complexes lary with MI-3. with marked posteriorly inclined slant except in ETYMOLOGY. For Alan Rackham, the discoverer of M3 with cusps nearly vertical. Molar overlap the Rackham's Roost Site. Paratype QMFIO819, left minimal. maxillary fragment with MI and zygomatic plate. Mi. Relatively long and narrow often with a Other material QMF23365, partial left maxillary with prominent anterior cingulum which forms a semi- 330 MEMOIRS OF THE QUEENSLAND MUSEUM in the holotype , becoming more with increased 1'2 large and arcuate with a straight edge, more than the width of the ante- T3 at its buccal T3 smal1 and circular, its posterior third be- the posterior edge of , almost entirely incor- , into the T2 com- with wear. T4 with its long axis obliquely to the the T5,6 complex .T4 a~proxi- unworn, larger in worn ..~ ~ jo~ned to TS !n in .TS large, a su?angu~ar arcua~e concave posterior with~ a smaller~~..~-T2.T6 occlu- attached to TS in early stages of with junction of these ... anterior yand r - ,- the at the and smaller than T3, approxi- FIG. I. Zyzomys rackhami sp. nov., Rackham's Roost Site, holotype, mately half of T6 behind QMFIO8I8, SEM (stereopair). the posterior edge ofT5.17 larger than T4, elliptical, circle around the T2,3 complex in the holotype. with its long axis inclined obliquely to the axis of Anterior cingulum with a series of small but the T8,9 complex but in reverse to the angles of apparently occlusally functional accessory cusps TI and T4 and as such is directed forwards. 17 randomly positioned. TI elliptical with its long and T4 in very close proximity, forming a single axis obliquely inclined to the axis of the T2,3 complex after moderate to extreme wear. T8 complex, directed to the rear of the tooth and large, with a nearly circular occlusal surface. T9 positioned posterior to the base line of the T2,3 barely discernible, incorporated into the T8,9 complex. TI joined to the T2,3 complex only in complex at the onset of wear. Slight furrow in the extreme stages of wear and in some specimens anterior surface of the T8,9 complex marking the via a variably sized and shaped TIbis. TIbis anterolingual edge of T9 (would remain even ZYZOMYS RACKHAMl SP. NOV. FROM RIVERSLEIGH 331 wear. with an axis directed across the of the tooth. T5 small, subtriangular . toward the buccal edge of the T6 absent. with only a poorly defined of the lingual furrow forming a buccal cingulum at the anterior edge. moderate in size. shaped as a bisected T8 a mirror image of 17 , with .T9 absent. MI with 4 roots. 3 wel1~developed. fourth , and probably reduced; anterior root directed forwards, exposed occlus- narrow. posi- -' r .. root a remnant of a more sub- specimens examined even though there not always a root. Posterior root large- ' as wide as the tooth at its origin, M2 with 3 roots; anterolinqual and roots of equal size, together root wide, running obliquely with re- to the anterior roots, with lingual -, its ~o~t p'?-steri.orpoi~t. ~3 w~th with the highest foints being the anterior with moderate wear). Posterior cingulum (z) ab- third of Mi and the M .Anterior palatal vacuit( sent. extending distally as far the anterior edge of M . M2. TI large, tear-shaped, with a long axis Attachment node for the origin of the superficial obliquely inclined to the anterior edge of the tooth masseter large and well defined. Anterior edge of and directed posteriorly. T2 and T3 absent. T4 of the node alligned with the anterior edge of the moderate size, elliptical, posterior to the domi- nant T5. T4 joined to T5 in very early stages of zygomatic plate and just behind the maxil- wear. T5 subtriangular in occlusal outline, with lacy/premaxillary suture. Zygomatic with ante- its anterior most edge of the enamel boundary rior edge rising vertically and straight, of contacting the posterior edge of Mi in extreme moderate width, with one nutrient foramen dis- wear. T6 small, circular, joined to T5 with mod- tally near its base. erate wear. As in the T6 structure of MI, the connection associated with a well-defined fur- COMPARISON .Although the nearest species in row, this cusp always retaining its identity. T7 an size Z. argurus, Z. rackhami is more similar to Z. elliptical cusp of moderate size, with its axis pedunculatus because of the relatively cuspidate running almost parallel to the main axis of the nature of the teeth, the development of the buccal tooth, not observed to merge with the T8,9 com- series of cusps and the relatively unreduced ap- plex in M2 even with extreme wear. T8 large, pearance of M3. These similarities are probably nearly circular, with distal edge extending well simplesiomorphies. A nutrient foramen is found beyond the distal edge ofT7 , giving the posterior anterior to the tooth row in all species with the region of the tooth an arcuate shape.T9lost, with exception of Z rackhami and Z. maini, in which a remnant of the furrow that marked the position the foramen is on the zygomatic plate. Unlike Z of the lingual side of the cusp. maini however Z. rackhami has a longitudinal M3. TI small, almost circular. T2 and T3 ab- palatal crest as in all other species of Zyzomys. sent. T4 small, tear-shaped, joined to T5 after Zyzomys rackhami differs from Pseudomys, 332 MEMOIRS OF THE QUEENSLAND MUSEUM TABLE I. Measurements (mm) of Zyzomys rackhami concentration of these elements elsewhere in the sp. nov. L=Iength; W=width. deposit. Lower jaws and teeth of other mammal groups( i.e., marsupials, bats) found in the deposit MI-3 MI-2 MI appear to be as abundant as their upper counter- SPEC. NO. L L L w parts. Z. rackhami is the only species of Zyzomys MF10818 -2:15.6 Q ~ 1.5 MF10821 4.4 2.7 1.6 recovered from the Rackham's Roost Local Fauna and is represented by hundreds of speci- MF23325 5.3 4.3 2.7 1.7 mens. This is in contrast to modern and MF10819 2.7 1.7 Pleistocene faunas, in which there are usually at MF23365 4.2 2.6 1.5 least 2 species present and sometimes, as in the MF24001 4.3 2.6 1.6 Nouralangie Rock area, N.T., 3 (Kitchener MF24002 2.5 1.5 (1989) reported only 2 but the author has sighted MF24003 2.7 1.5 2 specimens of Z.