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Early Cretaceous Mammals Early Cretaceous Mammals from Flat Rocks, Victoria, Australia Thomas H. Rich1, Patricia Vickers-Rich2, Andrew Constantine3, Timothy F. Flannery4, Lesley K00F and Nicholas van Klaveren6 I. Museum Victoria, P. O. Box 666E. Melbourne. Victoria 3001, Australia 2. Earth Sciences Department and Monash Science Centre, Monash University, Clayton, Victoria 3150, Australia 3. Earth Sciences Department, Monash University. Clayton. Victoria 3150, Australia 4. Australian Museum, 6 College Street, Sydney South, NSW 2000, Australia 5. Earth Sciences Department, Monash University. Clayton. Victoria 3150, Australia 6. Earth Sciences Department, Monash University. Clayton, Victoria 3150, Australia Abstract The tribosphenic mammal Ausktribosphenos nyktos together with the eupantothere Teinoloplios trusleri gen. et sp. nov. occur at the Early Aptian Flat Rocks locality in south-eastern Australia. That a eupantothere might occur in the Early Cretaceous of Australia is in accordance with previous expectations. However, quite the opposite is true of tribosphenic mammals, in particular placentals to which group A. nyktos may belong. Analysis of the morphology of Ausktribosphenos nyktos suggests it may be (1) a placental, (2) the sister group of the placentals, or (3) a tribosphenic mammal unrelated to the placentals. On morphological grounds, the first hypothesis appears to be the most probable. Within the placentals, A. nyktos shows a startling resemblance to the Erinaceidae. A. nyktos may be characterised as having a lower dentition remarkably like that of the erinaceids set in a lower jaw which is markedly more primitive, perhaps the condition that might be found in the primitive sister-group of the Erinaceidae. The oldest undoubted placentals known from lower dentitions are the two species of Prokennalestes found in the Early Cretaceous of Mongolia. If Ausktribosphenos nyktos is also a placental, then there are five biogeographic hypotheses divided into two main classes to explain this distribution. The first class is dispersal via Antarctica-South America-Africa-Europe and the second, direct migration between Asia and Australia. Within the first class of hypotheses, the most likely seeming variant is one which posits the extinction of placentals in South America by the Campanian after the interchange had occurred. Within the second class, a one-way transport of faunal elements from Eastern Gondwana to Asia by the mechanism of Noah’s Ark type passage on a northward drifting terrane or terranes is the most likely seeming mechanism. Introduction On 8 March 1997, Ms Nicola Barton, a volunteer working at the Flat Rocks locality, discovered the In 1978, a systematic search for terrestrial vertebrate holotype of Ausktribosphenos nyktos Rich et al. 1997. fossils in the coastal outcrops of the Early Cretaceous As a consequence of that discovery, using a binocular rocks in the Wonthaggi and Eumeralla formations in microscope, Mrs Lesley Kool began to re-examine Victoria, Australia, began. This work eventually resulted systematically every fragment of bone that had been in the discovery of two major sites, Dinosaur Cove in the saved from that locality as well as Dinosaur Cove. As a Eumeralla Formation about 200 km to the south-west of result of that effort, three more mammalian specimens Melbourne and the Flat Rocks site in the Wonthaggi have now been recovered from the Flat Rocks locality. Formation about 120 km south-east of that city (fig. 1). Although the discovery of avian and mammalian fossils Three days prior to Ms Barton’s discovery, an avian was the primary objective of this work, the bulk of the furculum was found at the Flat Rocks locality. Although material collected thus far has been dinosaurs and turtles. feathers were previously known from the contemporane¬ For the most part, these fossils are isolated bones ous Koonwarra locality only about 27 km from Flat and teeth with only two partial skeletons of dinosaurs Rocks (Waldman 1970), this furculum is the first osteo- recovered to date (Rich & Rich 1989, Rich et al. 1997). logical specimen of a Cretaceous bird from Victoria, and the first representative of this avian element ever found in The first mammalian specimen found in the Cretaceous of the Mesozoic of Australia. It is currently being described Victoria was collected in 1993 at Dinosaur Cove. In the elsewhere. field, the specimen was labelled as ‘Turtle? humerus?’ and it was not until it was completely prepared in August A Leica MZ8 binocular microscope was utilised to 1996, that it was identified as a monotreme humerus. examine the fossils described here. These same specimens This specimen and a fragment of a mammal tooth also were collected under permits NP978/11 1 and NP recovered from Dinosaur Cove are to be described 10000241 issued by the Victorian Department of Natural elsewhere. Resources and Environment. Flora and Fauna Branch. 1 Records of the Queen Victoria Museum No. 106 Locality, Stratigraphic Unit and Age of detrital zircons obtained from a volcaniclastic sandstone 50 cm above the bone-bearing conglomerate. The 21 All the mammals described below were found at the Flat analysed zircons yielded a pooled age of 122.5±8 Ma (lo) Rocks fossil vertebrate site. 38° 39' 40±02” S. 145° 40’ with a chi-squared probability of 99.7%. The pooled age 52±03" E (World Geodetic Standard 1984) on the shore represents the age of the source volcanic rocks, because platform of the Bunurong Marine Park, Victoria, vitrinite reflectance measurements of coalified plant Australia. The fossils occur in the Wonthaggi Formation, fragments from the site indicate that the sediments have a non-marine volcaniclastic sandstone and mudstone unit not experienced burial temperatures greater than 77°C. approximately 3 000 m thick. This is well below the partial annealing temperature for zircon (200°C), which indicates they have not been Palynological examination of Five mudstone samples thermally annealed. This interpretation substantiates collected from a 7 m thick floodplain sequence an earlier fission track study by Green (1989) on detrital immediately underlying the Flat Rocks vertebrate fossil zircon from similar volcaniclastic sandstones of site indicates it is Pilosisporites notensis Zone (Aptian) in Crybelosporites striatus Zone age in the Otway Basin age (see fig. 2). This age determination is based on the approximately 130 km to the west of Flat Rocks. Green presence of the spore Pilosisporites notensis in one of the (1989) obtained a pooled fission track age that overlapped five mudstone samples examined. This spore makes its its chronobiostratigraphic age. This overlap suggests the first appearance in Australia at the base of the Aptian lag time between eruption, erosion, transport, and deposition (Morgan 1980) and continues up into the Albian (Helby, was probably in the order of a few million years. The Morgan & Partridge 1987). The site is not Albian in age pooled age for Flat Rocks, therefore, suggests the site is because none of the samples examined contained any probably situated close to the base of the Aptian, which spore or pollen species which make stratigraphically ranges in age, according to the time scale of Gradstein et younger first appearances within the overlying al. 0994). from 121 to 112.5 Ma. Crybelosporites striatus, Coptospora paradoxa, or Pliimopollenites pcinnosus spore-pollen zones. Palaeoenvironnient The Aptian age determination for the Flat Rocks Constantine et al. (1998) presented evidence for the occur¬ vertebrate fossil site is supported by the fission track dating rence of cryoturbation structures 3 m stratigraphically 2 Early Cretaceous Mammals (Ma) Time scale Spore-pollen zones 98.9 L P. pannosus Albian M C. paradoxa E C. striatus 112.2 Aptian y P. notensls y 121.0 Barremian 127.0 tS Hauterivian Pooled age of 132.0 detrltal zircons Valanginlan from Flat Rocks 137.0 with 1 a Berriasian error bars 144.21 Figure 4. Position of south-eastern Australia relative to Antarctica and the South Pole during the Aptian-Albian Figure 2. Age of the Flat Rocks vertebrate fossil site (hatched) (Early Cretaceous). relative to the geologic time scale (Gradstein et al. 1994). Spore-pollen zones modified after Helby, Morgan and Partridge (1987). From Rich et al. (1997). Flat Rocks E" P3 channel sediments HH overtoank sediments Figure 5. Cross section from south-eastern Australia to East -« cryoturbation site Antarctica in the Early Cretaceous. Separation between the two continents began in the Late Jurassic with the initiation of a vertebrate fossil site F fault spreading centre which resulted in a rift valley being formed on the common boundary of the two continental plates. Into this rift valley poured a vast quantity of volcanogenic sediments Figure 3. Simplified stratigraphic column showing the derived from volcanoes which were perhaps to the east in the relationship between the Flat Rocks fossil locality and the vicinity of the Lord Howe Rise (Bryan et al. 1997) or to the cryoturbation site 3 m below it. Modified from Constantine et south along the axis of the rift valley (Felton I997a-b). Large al. (1998). rivers flowed across the floor of this rift valley. They were fed perhaps in part by meltwater of snow, presumably located at below the fossiliferous unit at the Flat Rocks site, where high altitude on mountains along the margins of the rift valley the mammals described here were found. These cryotur¬ or the volcanoes which produced the volcanogenic sediments. bation structures imply seasonally frozen ground was The Early Cretaceous letrapod and plant fossils primarly occur present there during the Aptian (fig. 3). That, plus other in sediments that were laid down in small streams which Bowed evidence they presented, suggests a mean annual into the larger rivers on the Boor of the rift valley. Subsequent palaeotemperature between -2°C and +3°C for the area. to the Early Cretaceous, the sediments which had been deposited This accords well with a previous estimate of the mean on the Boor of the rift valley were lilhified as they were buried under additional sediment. Then late in the Cainozoic, these annual temperature based on the oxygen stable isotope sediments were uplifted to form the Strzelecki and Otway geochemistry of carbonate concretions (± 5°C, Gregory et Ranges. al. 1989) bui contradicted earlier palaeobotanical evidence suggesting a mean annual temperature for the area of 8- 10°C (Parrish et al. 1991). 3 Records of the Queen Victoria Museum No. 106 In the Early Cretaceous, Australia was located at a high Figure 6. (a) Generalised lower molars of Ausktribosphenos southern latitude (fig. 4) and still joined with Antarctica. nyktos. Abbreviations: co, cristid obliqua; encd, entocrislid; The two continents were in the initial stages of rifting end, entoconid; hypld. hypoconulid; hyped, hypocristid; liypd, hypoconid: meed, metacristid; med. metaconid; pad, paraconid; apart. In the Strzelecki and Otway ranges on the southern pacd, paracristid; pred, precingulid; prd, protoconid; ucl, coast of Victoria, occur thick deposits of volcaniclastic unnamed cristid 1: uc2 unnamed cristid 2; uc3, unnamed cristid sediments laid down at this time in the rift valley that 3. (b) Restoration of the wear surfaces on the lower molars of formed as the two continents separated (Dettmann et al. the holotype of A. nyktos. MSC 007. (c) Wear surfaces on the 1992, Frakes et al. 1987) (fig. 5). Braided rivers cut M,, of MSC 145. referred specimen of A. nyktos. Note that the across the floor of this rift valley. It was in the bed of one degree of wear in MSC 145 is generally significantly greater of the lesser tributary streams of such a river that the than on MSC 007 in most but not all areas. fossils found at the Flat Rocks site accumulated, probably as the result of a series of discrete flood events which relatively anteroposteriorly expanded, (b) well-developed caused the accumulation of mud clasts and plant debris, tribosphenic pattern of wear on lower molars, particularly together with isolated bones and teeth (Vickers-Rich & within the talonid basin as well as external to it (fig. 6), Rich 1997). (c) P4 with three trigonid cusps and trigonid sub equal in size to that of M„ (d) except for Tachyglossus aculeata Institutional Abbreviations with its highly reduced dentary, absence of a masseteric canal and masseteric foramen, and (e) absence of an AM F Palaeontology Collection, Australian Museum, internal coronoid process. Sydney, Australia F: AM Frick Collection, American Museum of Natural Distinguished from Aegialodontidae (sensu Fox 1980) by History, New York, United States (a) more anteroposteriorly compressed molar trigonids of America. and (b) the lack of a distal metacristid (.sensu Fox 1975) MSC Monash Science Centre, Clayton, Victoria, on M,.,. Australia. Distinguished from Pappotheriidae (sensu Fox 1980) by NMV P Palaeontology Collection, Museum Victoria, (a) the lower trigonid cusps on the molars and (b) on the Melbourne, Australia. Mh the talonid width being as great or greater than the UCM University of Colorado Museum, Boulder, trigonid width. Colorado, United States of America. Distinguished from the Marsupialia and Deltatheroida Systeniatics (sensu Kielan-Jaworowska & Nessov 1988) by (a) P.,with all three trigonid cusps well-developed, (b) the presence Class Mammalia Linnaeus 1758 of three, rather than four, lower molars and al least four, Subclass Theria Parker and Haswell 1897 rather than three, lower premolars, P, submolariform Infraclass Placentalia Owen 1837 rather than premolariform, and (d) except for Caluirmys, Order Ausktribosphenida Rich et al. 1997 Dastylopsila, Mynnecobius, Phascolcwcios and Tarsipes Family Ausktribosphenidae Rich et al. 1997 (Sanchez-Villagra & Smith 1997), absence of an inflected Ausktribosphenos Rich et al. 1997 angle on the dentary. Type species. Ausktribosphenos nyktos Rich et al. 1997 Figure 7 (Opposite). Holotype of Ausktribosphenos nyktos, MSC 007 [= NMV P208090], Maximum length of specimen. Diagnosis. Distinguished from the Monotremata (Archer 16 mm. (a) Labial view, (b) Occlusal view, (c) Lingual view, et al. 1978, 1985, 1992, 1993, Flannery et al. 1995) by (d) Ventral view, (e) Anterior view, (0 Posterior view. (a) presence of a paraconid on M, and all trigonids Technical art by P. Trusler. 4 Early Cretaceous Mammals 5 Records of the Queen Victoria Museum No. 106 Figure 8. Stereophotographs of the holotype of Ausktribosphenos nyktos, MSC 007 [= NMV P208090], (a) Lateral view, (b) Occlusal view. X8.00. 6 Early Cretaceous Mammals Figure 9. Stereophotographs of the holotype of Ausktribosphenos nyktos, MSC 007 [= NMV P208090], (a) Lingual view, (b) Ventral view. X7.75. 7 Records of the Queen Victoria Museum No. 106 relative to tooth length, (c) the presence of four double- rooted lower premolars instead of five, (d) the presence of a single rather than double-rooted Ch or the presence of a single rather than double-rooted P, (Sigogneau-Russell, Dashzeveg & Russell 1992), and (e) the lack of a labial mandibular foramen. Ausktribosphenos nyktos Rich et al. 1997 Holotype: MSC 007 [= NMV P208090], Right mandible fragment only slightly crushed with P5-M3 plus eight alveoli anterior to P5, area of condyle abraded, remnant of Figure 10. The posterior part of the holotype of base of angular process preserved, base of ascending Ausktribosphenos nyktos, MSC 007 [= NMV P208090], as it ramus preserved (figs 7-10). appeared when partially excavated from the rock in which it had been embedded. Referred material: MSC 145 |= NMV P208228], Right mandible fragment moderately crushed with P4-M2, four tooth roots present anterior to P4, angular process Distinguished from the Marsupialia by lack of a present, base of ascending ramus preserved (figs 11-13). posterolabial cingulum on M,.3. The talonid of the M2 of this specimen broke up during Distinguished from the Deltatheroida (sensit preparation. Fortunately, a mould of most of the crown of Kielan-Jaworowska & Nessov 1988) by (a) strong, this tooth was made by Dr Donald E. Russell prior to that well-developed talonids, (b) strong precingulids. and (c) event (fig. 1 la). paraconids lower than metaconids on M,.,. Diagnosis: That of the genus until other species are Distinguished from all other Placentalia including described. Prokennalestes by (a) the presence of a remnant of the Description: No foramina are present on the labial side surangular facet (Kielan-Jaworowska & Dashzeveg of the dentary. The base of the masseteric fossa is 1989), (b) a crest on M,; linking the hypoconulid and preserved with the anterior edge bounded by a sharp metaconid buccal to the entoconid, which is separately vertical ridge, whereas the ventral margin is more rounded. linked to the metaconid. Distinguished from most other Placentalia by the close proximity of the hypoconulid to As preserved, the symphysial region is confined to the the entoconid on the lower molars. lower half of the lingual side of the jaw beneath the posterior root of P3. Distinguished from all other Placentalia except Prokennalestes by the presence of (a) remnants of a Meckelian groove and (b) the coronoid bone on the Figure 11 (Opposite). Stereophotographs of referred jaw of lingual side of dentary. Ausktribosphenos nyktos. MSC 145, |= NMV P208228). (a) Partial cast of M,. Mould poured by Dr Donald E. Russell Distinguished from Prokennalestes but similar to many prior to original specimen suffering damage to talonid. X7.61, more advanced placentals by (a) the much smaller size of (b) Lateral view, (c) Occlusal view. X8.42. the M, relative to M,.2) (b) the lower trigonid cusps on M,.3 Measurements (in mm) of Ausktribosphenos nyktos Total Length Trigonoid Width Talonoid Width MSC 007 MSC 145 MSC007 MSC 145 MSC 007 MSC 145 p5 1.4 1.4 1.2 1.1 M5 1.6 2.3 1.4 1.4 1.7 1.4 m2 1.8 2.3* ... 1.7 ... 1.3* m3 1.2 ... 0.9 ... 0.9 ... ^Measurements of cast 8 Early Cretaceous Mammals 9 Records of the Queen Victoria Museum No. 106 On MSC 007, a distinct Meckeiian groove is developed precingulid is developed from the labial corner of the from beneath M, posteriorly to the mandibular foramen base of the prevallid almost to its lingual corner. A from which it is separated by a narrow ridge. A shallow postcingulid is developed along the base of the postvallid depression, the surangular facet, presumably a vestigial instead of a full-fledged talonid. No talonid cusps are remnant of a groove for a post dentary bone, begins present. anteriorly at a point just beneath and anterior to the posteriorly facing mandibular foramen as well as below M,. The protoconid is the tallest of the trigonid cusps the posterior end of the Meckeiian groove from which it with the metaconid slightly lower and the paraconid the is clearly separated. This shallow depression extends lowest of all. There is a definite carnassial notch present posteriorly to a point near the anterior edge of the base of in the paracristid. The metacristid is too damaged [MSC the mandibular process. This depression has been slightly 007] or encrusted with matrix [MSC 145] in the expected area of a notch to ascertain if a carnassial notch was affected by crushing, which may have made it more present. A precingulid is developed along the entire prominent than it originally was. A small condylar length of the base of the prevallid. The hypoconid is foramen is developed near the preserved posteroventral lower than the entoconid. Damage and wear to the comer of the jaw. talonids on both available specimens of the M, is extensive, but what remains suggests it was rather similar On MSC 145, there may be a trace of the Meckeiian to those of the MTs which are somewhat better preserved groove preserved just anterior to the mandibular foramen. in both specimens in this area. Although the width of the Anterior to this point, the specimen is damaged to such an talonid of MSC 007 (given in the Table of Measurements) extent that it is impossible to say whether or not a is 0.3 mm greater than the trigonid width, damage to the Meckeiian groove was once present. The separation of talonid makes this measurement suspect. It may well be the mandibular foramen from the surangular facet is that this tooth, prior to being damaged, resembled the M, much broader than in MSC 007. The surangular facet of MSC 145 in having the trigonid and talonid within 0.1 terminates posteriorly about halfway between the mm of being equal. mandibular foramen and the base of the angular process. No condylar foramen is preserved on MSC 145. M2. This tooth is slightly longer and the trigonid slightly wider than on M,. The corresponding trigonid cusps Only the base of the ascending ramus is preserved on are slightly lower than those on the M,. Except that the MSC 145. What remains suggests that the anterior edge paraconid is smaller relative to the size of the metaconid, was nearly vertical, rather than posteriorly inclined. At its the trigonid cusps have the same relative proportions to anterior end is a rugose area, presumably the site where one another as they do on that tooth. Likewise, the the coronoid bone was located. Posterior to that can precingulid is equally developed. On the talonid, the be seen a horizontal ridge that lies above both the hypoconid is lower than the entoconid, as is the case on mandibular foramen and the surangular facet. the M|. A prominent cristid (ucI, unnamed cristid 1, fig. 6a). descends the posterior side of the metaconid and P,.j. Only the roots of these teeth are preserved on MSC extends towards a similar entocristid directed forward 007. On that basis, they seem to have been mediolaterally from the entoconid. Unnamed cristid 1 is not a distal compressed teeth noticeably narrower than the P4_5. P, had metacristid (sensu Fox 1975), because it is nowhere close a single root, whereas P2., were double-rooted. On MSC to the cristid obliqua. Lingual to the entocristid and 145, owing to breakage of the jaw immediately anterior to subparallel to it is a second, higher cristid (uc2, fig. 6a) P4, it is uncertain to which teeth the four visible roots in extending forward from the entoconid along the lingual that region should be assigned. margin of the tooth. Along the length of this cristid are one or two small cuspules, the one at the anterior end On all five teeth known, P4-M3, the crowns are being the most distinct. Between these two cristids cantilevered well beyond the underlying roots on both extending in parallel forward from the entoconid is a the labial and lingual sides. small basin partially cut off from the rest of the talonid basin. The same pattern may have been present on M,, P4. On the one specimen where this tooth is preserved but was obliterated by wear in both available specimens. [MSC 145], the break in the rock that exposed the The hypocristid is preserved on the M:, being much lower specimen passed through the tooth. Among other things, than the hypoconulid and convex posteriorly in occlusal the break destroyed the metaconid (if it was present) and view. Extending forward into the talonid basin from the damaged the paraconid. But, what remains is quite similar hypoconulid is an arcuate cristid («cJ, fig. 6a) that paral¬ in form to the better known P5, the only difference being lels the base of the entoconid, continuing to a point where that the P4 is slightly narrower. it joins the posterior cristid from the metaconid and the entocristid from the entoconid. The cristid uc3 appears to Py This tooth consists primarily of a trigonid with the Figure 12 (Opposite). Stereophotographs of referred jaw of protoconid much higher than the metaconid and the Ausktribosphenos nyktos. MSC 145, [= NMV P208228]. (a) paraconid being slightly lower and smaller. A distinct Lingual view, (b) Ventral view. X9.06. 10