Yalkaparidontia, Archer et al., 1988

Beck, Robin M. D., Voss, Robert S. & Jansa, Sharon A., 2022, Craniodental Morphology And Phylogeny Of Marsupials, Bulletin of the American Museum of Natural History 2022 (457), pp. 1-353 : 201

†Yalkaparidontia Archer et al., 1988

CONTENTS: † Yalkaparidon .

STEM AGE: 39.9 Mya (95% HPD: 30.7–49.7 Mya).

CROWN AGE: Not applicable.

UNAMBIGUOUS CRANIODENTAL AUTAPOMORPHIES: Postglenoid process greatly reduced or absent (char. 75: 0→1; ci = 0.200); mandible usually with one mental foramen (char. 98: 1→0; ci = 0.063); second and third upper incisors entirely lacking enamel (char. 105: 0→1; ci = 1.000); first upper premolar absent (char. 114: 0→1; ci = 0.200); protocone absent (char. 141: 0→1; ci = 1.000); talonid greatly reduced or absent (char. 166: 0→1; ci = 0.500).

COMMENTS: The peculiar † Yalkaparidon is known only from two described species († Y. coheni and † Y. jonesi ) from late Oligocene to middle Miocene sites at Riversleigh World Heritage Area in northern Australia ( Archer et al., 1988; Beck et al., 2014). † Yalkaparidon coheni is by far the better known of the two species, being represented by partial cranial material, whereas † Y. jonesi is known only from a fragmentary right mandible ( Archer et al., 1988; Beck et al., 2014). † Yalkaparidon combines some relatively plesiomorphic features of the skull (particularly of the basicranium) with a highly derived dentition that includes an enlarged “gliriform” anteriormost lower incisor and zalambdodont molars ( Archer et al., 1988; Beck, 2009; Beck et al., 2014). Of the other unambiguous craniodental apomorphies identified here, extreme reduction of the postglenoid process likely reflects extensive anteroposterior movement of the lower jaw (a trait also seen, for example, in members of the placental clade Glires; Cope, 1888; Druzinsky, 2015), whereas absence of enamel from I2 and I3 is an autapomorphy that we have not seen in any other metatherian.

Based on its unusual combination of features, we agree with Archer et al. (1988) and Beck et al. (2014) that † Yalkaparidon warrants classification within its own family and order. † Yalkaparidon is the only definitive member of this order currently known, although the enigmatic † Yingabalanara richardsoni (known from two lower molars from the early Miocene of Riversleigh; Archer et al., 1990) may be a dentally plesiomorphic yalkaparidontian (see Beck et al., 2014: 155).

Our undated (fig. 32) and dated ( fig. 33) total-evidence analyses both recover a clade that unites † Yalkaparidon with paucituberculatans. However, isolated tarsals tentatively referred to † Yalkaparidon by Beck et al. (2014) show greater derived similarities to australidelphians than to paucituberculatans and Beck et al.’s (2014) accompanying phylogenetic analyses also supported australidelphian affinities for this taxon (see also Beck et al., 2016). Nevertheless, the subsequent phylogenetic analyses of Beck (2017a) and Zimicz and Goin (2020), which included † Yalkaparidon and used character scores from these referred tarsals also placed this taxon in a clade with paucituberculatans (including argyrolagids, a fossil group not included in our study; but see Abello and Candela, 2019). Although this clade is worthy of further investigation, we suspect that placement of † Yalkaparidon with paucituberculatans largely reflects convergent acquisitions of a gliriform lower incisor. Current evidence (including the analyses presented here) indicates that diprotodontians evolved a similar lower incisor independently of paucituberculatans, so it is plausible that † Yalkaparidon represents a third origin of this tooth type. Based on our results and those of other recent studies ( Beck et al., 2014, 2016; Beck, 2017a; Abello and Candela, 2019; Zimicz and Goin, 2020), we consider the higher-level affinities of † Yalkaparidon to be uncertain, beyond its being a member of Marsupialia .