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| Opalios splendens gen. et sp. nov. Dharragarra aurora gen. et sp. nov. Parvopalus clytiei gen. et sp. nov. Flannery, McCurry, Rich, Vickers-Rich, Smith & Helgen, 2024 artwork: Peter Schouten. |
Abstract
Six species of monotremes, three newly described here, occur in the Cenomanian fossil fauna from Lightning Ridge in northeastern New South Wales, Australia, making it the most diverse monotreme assemblage on record. Four species are known from a single specimen, suggesting that diversity remains underrepresented. No other mammal lineages are known from the deposit, although the absence of smaller taxa is likely due to sampling biases introduced by the opal mining process. Early-Late Cretaceous monotremes thus clearly diversified in Australia during the absence of other large-bodied mammalian competitors; and subsequently occupied a wider range of ecological niches than at any other time in their evolutionary history. One new taxon described herein represents a previously unknown monotreme family that combines marked elongation and torsion of the dentary with teinolophid character states, including the retention of five molars. Another shares dental features with ornithorhynchids, while the third is a possible diminutive steropodontid and simultaneously represents the smallest-bodied post-Barremian monotreme. Additional material of Steropodon galmani is also documented, confirming that a Meckelian groove is rudimentary or absent in this taxon, thus adding to the morphological understanding of this unusual monotreme. Lastly, we posit that the loss of teeth in ornithorhynchids may have occurred during the Pleistocene as a result of competition with aquatic hydromyin rodents dispersing to Australia from New Guinea.
Keywords: Mammal, opalized fossils, Late Cretaceous, Mesozoic, Eastern Gondwana
Systematic palaeontology
MAMMALIA Linnaeus, 1758
MONOTREMATA Bonaparte, 1837 or 1838
ORNITHORHYNCHOIDEA superfam. nov.
Diagnosis: Distinguished amongst other Monotremata by torsion of the dentary, whereby the lingual surface of the distal portions of the dentaries faces dorsally and are dorsoventrally flattened (except Tachyglossidae); and the distal portion of the masseteric canal being divided into three wide moieties (except Tachyglossidae).
OPALIONIDAE fam. nov.
Opalios splendens gen. et sp. nov.
Diagnosis: Differs from all monotremes except Teinolophos trusleri in possessing five molars rather than four or fewer (Fig. 2A, B). Differs from T. trusleri in being much larger and possessing: a lower ascending ramus; an articular facet below the level of the toothrow (Fig. 2C, D); a hemispherical depression for insertion of the masseteric musculature on the buccal side of the dentary (Fig. 2C, D); the anterior portion of the dentary twisted (torsion) such that the lingual surface faces dorsally and the anterior dentary is dorsoventrally flattened (Fig. 2A, B, E, F); a much reduced or absent Meckelian groove (Fig. 2E, F); an extremely dorsoventrally shallow mandibular symphysis (Fig. 2F). Differs from all other monotremes except stem and crown ornithorhynchids by exhibiting torsion of the horizontal ramus of the dentary. We consider Opalios splendens to be the basal-most diverging taxon within Ornithorhynchoidea.
Etymology: The genus name refers to ‘opal’ in Greek. The species name refers to the large size and spectacular translucency of the holotype (AM F132596–AM F132599), which provides views of its internal structure.
Family ?ORNITHORHYNCHIDAE Gray, Citation1825
Dharragarra aurora gen. et sp. nov.
Diagnosis: Differs from teinolophids in lacking a Meckelian groove, possessing dentary torsion and three molars, and in lacking diastemata between the premolars. Differs from kollikodontids in possessing an enlarged mandibular canal, in possessing three rather than four molars and in possessing dentary torsion. Differs from Steropodontids in possessing dentary torsion, having a dorsoventrally flattened dentary, and a more reduced posterior molar. Differs from Opalios splendens in having three molars rather than five, and in lacking diastemata between p1 and p2. Amongst ornithorhynchoids, Dharragarra aurora differs from the species of Obdurodon and Ornithorhynchus in having molars with two roots rather than more than two; from Patagorhynchus pascuali in being larger; from Monotrematum sudamericanum in being smaller.
Etymology: The genus name derives from ‘Dharragarra’, meaning platypus in the Gamilaraay, Yuwaalaraay and Yuwaalayaay languages (Ash et al. Citation2003). The species name is Latin for ‘dawn’.
STEROPODONTIDAE Archer, Flannery, Ritchie & Molnar, Citation1985
Steropodon galmani Archer, Flannery, Ritchie & Molnar, Citation1985
?STEROPODONTIDAE
Parvopalus clytiei gen. et sp. nov.
Etymology: Genus name is Latin for ‘small opal’. Species is named for Clytie Smith (Lightning Ridge), who has recovered many opal fossils at Lightning Ridge.
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| Six monotremes [100 million years ago] at Lightning Ridge, New South Wales, Australia; clockwise from lower left: Opalios splendens; Stirtodon elizabethae, the largest monotreme of the time; Kollikodon ritchiei, with hot-cross-bun shaped molars; Steropodon galmani, now known from additional opalized fossils; Parvopalus clytiei, the smallest monotreme of the time; and Dharragarra aurora, the earliest known species of platypus. artwork: Peter Schouten. |
Conclusions:
The co-occurrence of six morphologically distinct fossil mammal taxa (three named herein) in the Lightning Ridge faunal assemblage from the Finch Clay facies of the Griman Creek Formation provides the first indication that Australia was home to a hitherto cryptic diversity of monotremes during the mid-Cretaceous (as predicted by Darlington Citation1957).
• The newly described Opalios splendens is annectant between a teinolophid-like ancestral monotreme morphotype and more derived ornithorhynchoids.
• Dharragarra aurora demonstrates that the lower molar formula typical of Cenozoic ornithorhynchids was established by the Cenomanian.
• New specimens of Steropodon galmani, along with the new small-bodied taxon, Parvopalus clytiei, expand current knowledge of morphological diversity in Cenomanian monotremes.
• The loss of teeth in ornithorhynchids may have been related to ecological displacement caused by the arrival of aquatic rodents in Australia during the Pleistocene.
Timothy F. Flannery, Matthew R. McCurry, Thomas H. Rich, Patricia Vickers-Rich, Elizabeth T. Smith and Kristofer M. Helgen. 2024. A Diverse Assemblage of Monotremes (Monotremata) from the Cenomanian Lightning Ridge Fauna of New South Wales, Australia. Alcheringa: An Australasian Journal of Palaeontology. DOI: 10.1080/03115518.2024.2348753
