We aim to provide a synthesised review of the current state of knowledge of the pre-Pleistocene fossil Thylacinidae, including biological and ecological implications of their evolutionary history and gaps in our knowledge. It is hoped that by highlighting the history of the thylacine, future studies may benefit from contextualising their research within this framework. This evolutionary history underscores that the thylacine was not a unique organism, but like all organisms part of an evolutionary radiation with clear trends towards what would ultimately be represented in Thylacinus cynocephalus .

Our understanding of the thylacinid phylogeny has been greatly aided by dramatic shifts in our knowledge of fossil thylacinids ( Wroe, 2003 ; Yates, 2015 ). The first pre-Pleistocene fossil thylacinid (i.e., not Thylacinus cynocephalus ) was not described until the end of the 1960s ( Woodburne, 1967 ). It would be over two decades until a second fossil thylacinid was described ( Muirhead & Archer, 1990 ). From that point no fewer than ten additional taxa have been described, with new species described as recently as 2015 ( Yates, 2015 ). This had greatly increased our ability to place the recently extinct thylacine into an evolutionary context, and has underscored the deep divergence and potential evolutionary distance between the thylacine and its closest extant relatives.

While these studies have brought about intriguing results, they are hampered by the fact that the thylacine was the only member of the family Thylacinidae to persist until modern times. In effect this presents the thylacine as a one-off, singular organism unique in its evolution and traits, a concept highlighted in many studies ( Miller et al., 2009 ; Menzies et al., 2012 ; Feigin et al., 2017 ; Newton et al., 2018 ). Although this seeming uniqueness has prompted much research into the thylacine, of which the examples given here are only a small portion, it can potentially hinder efforts or present false signals when attempting to understand its ecology or behaviour.

The dating of Australian fossil localities has been a slow and difficult process, with the majority of sites having no associated absolute ages. Dating the Australian fossil sites has historically been based on biochronology, with many papers seeking to stratify, order, and correlate the localities ( Archer et al., 1989 ; Murray & Megirian, 1992 ; Travouillon et al., 2006 ; Megirian et al., 2010 ; Arena et al., 2015 ). Recent work has begun to provide radiometric dates for some sites ( Woodhead et al., 2016 ). We have attempted to correlate sites and specimens with dates wherever possible, using biocorrelation seriations correlated with sites that now have been quantitatively dated. However, the quantitative dating of Australian fossil localities is very much in its infancy, so we stress that the site dates and taxa age ranges given here are estimates.

Body mass estimates for all published pre-Pleistocene thylacinid specimens known from craniodental material were calculated from the dasyuromorphian-only regression equations developed by Myers (2001) . Body mass estimates for several of the pre-Pleistocene thylacinids have been published ( Wroe, 2001 ; Travouillon et al., 2009 ; Yates, 2014 , 2015 ), but a comprehensive set of estimates using all applicable published specimens has yet to be performed. The highest ranked variable possible was chosen for each specimen, and as each specimen was therefore potentially subject to a different calculation, individual values are given in lieu of mean values for species. Regarding the large Thylacinus megiriani and Thylacinus potens , dental metrics exceed that covered by the equations of Myers (2001) . Wroe (2001) discussed this and noted that both taxa appear to vary in their dental proportions from the extant species, and chose to use geometric similitude with Thylacinus cynocephalus to estimate the mass of these large thylacinids. We here instead choose to employ the equations of Myers (2001) , firstly as there is no a priori reason to view geometric similitude as necessarily a more accurate estimate, given the noted proportional difference between the taxa, and secondly to give a comparative set of estimates to be complementary to those of Wroe (2001) .

A tip-dating analysis was performed on the morphological matrix with the addition of ribosomal 12S RNA molecular data. The single gene was chosen as we had complete coverage of our extant taxa, and as to limit the potential of overwhelming the small set of morphological data, especially considering that our study clade is near-entirely devoid of genomic data. The molecular data and partitioning scheme was taken from Kealy & Beck (2017) , and Genbank accession numbers provided in Table S3 . A single independent gamma rates clock model was used and a fossilised birth–death prior assumed. Extant sampling was set to ‘random’ as we had an unequal sampling of extant dasyurids, with a sample probability of 0.0833 for the extant taxa. Default speciation, extinction, and fossilisation priors were used, and extant terminal taxa were assigned an age of zero Ma. Age range justification was the same as that used in the parsimony tree time-scaling above. The resulting ‘allcompat’ consensus tree was then also time-scaled with the R package ‘strap’ as a separate methodological comparison.

The Bayesian analyses were performed in MrBayes v. 3.2.6 on the CIPRES Science Gateway ( Miller, Pfeiffer & Schwartz, 2010 ; Ronquist et al., 2012 ). An undated Mkv + G model analysis was performed on the morphological matrix, with a root, ingroup, and Phascogalini constraint topology to serve as an analogue to the skeleton tree used in the TNT analysis above. This analysis was comprised of four runs of four chains, sampling every 5,000 generations, for 20 million generations, generating a post-burn-in 50% majority rules consensus tree.

Adult thylacinid dental formula is 4.1.3.4/3.1.3.4 following the premolar/molar boundary of Flower (1867) and Luckett (1993) . Terminology of molar morphology is presented in Fig. 2 , largely following Kielan-Jaworowska, Cifelli & Luo (2004 ; pp. 412, 434). Our use of the terms faunivory and hypercarnivory refer to specific derivations within general carnivore ecology. Faunivory here refers to dental morphology suggesting a subequal consumption of invertebrate and vertebrate prey, without any specialised trends towards either insectivory or the heavy consumption of vertebrate flesh. Hypercarnivory is generally defined as a diet consisting of >70% vertebrate flesh ( Van Valkenburgh, 2007 ), but as the dietary habits of fossil taxa are unobservable, is here used to refer to any dental trend towards the simplification of the carnassial and molar teeth and lengthening of their cutting blades, effectively increasing their shearing and reducing their grinding capabilities.

The members of the genus Thylacinus are larger as a whole than the other taxa within the family. The small and early-diverging Thylacinus macknessi is estimated at 6.7–9.0 kg. The large-bodied thylacinids Thylacinus megiriani and Thylacinus potens are estimated to be 49.1 and 28.3–55.0 kg, respectively, and Thylacinus yorkellus at 14.5–17.8 kg. Recorded masses for the recent Thylacinus cynocephalus are scant, but the estimated range of 15–35 kg by Moeller (1970) is likely accurate.

Body mass estimates for the 35 applicable specimens are given in Table S2 . Badjcinus turnbulli is estimated to be between 1.7 and 3.1 kg, roughly comparable to the modern Dasyurus maculatus . Both specimens of Muribacinus gadiyuli are small, at 1.6 and 1.7 kg, respectively, while Maximucinus muirheadae is surprisingly large (18.4 kg) as stated in Wroe (2001) . Both Ngamalacinus timmulvaneyi and Nimbacinus dicksoni are slightly larger than extant quolls, at 5.7–8.4 and 2.9–6.8 kg, respectively. Tyarrpecinus rothi and Wabulacinus ridei are also estimated to be of similar size at 5.4 and 5.3–7.8 kg, respectively.

The analyses all support placing Mutpuracinus archibaldi outside of Thylacinidae as potentially sister to Dasyuridae or as a stem dasyuromorph, similar to placements recovered by Kealy & Beck (2017) and Travouillon & Phillips (2018) . Badjcinus turnbulli is generally recovered as sister to the remainder of Thylacinidae, with a topology further separated into two internal clades—one comprised of plesiomorphic, small-bodied thylacinids ( Nimbacinus, Muribacinus, Ngamalacinus ) and one consisting of Tyarrpecinus, Wabulacinus , and Thylacinus . This is similar to topologies recovered in previous work ( Yates, 2015 ; Archer et al., 2016 ; Kealy & Beck, 2017 ).

The split of Thylacinidae from the rest of Dasyuromorphia was estimated to be 41.8–35.7 Ma, comparable to that of past studies (range of ~43.0–30.7 Ma; Dos Reis et al., 2012 ; Mitchell et al., 2014 ; Westerman et al., 2016 ; Kealy & Beck, 2017 ; see Fig. S1 ). The internal structure of Thylacinidae is poorly resolved, but some timing can be hypothesised. Badjcinus appears to have split from the rest of Thylacinidae approximately 34.8–32.5 Ma, with the Ngamalacinus + Nimbacinus + Muribacinus clade again splitting 29.0–25.9 Ma. The clade including Tyarrpecinus + Thylacinus cynocephalus originated 24.1–25.4 Ma, and the clade including Thylacinus megiriani , Thylacinus yorkellus , and Thylacinus cynocephalus approximately 13.5–8.0 Ma.

Majority rule consensus of post-burn-in trees showing all compatible clades (‘allcompat’ function in MrBayes) after removal of Maximucinus muirheadae from the matrix. Circles at nodes are BPP, only values ≥50% are shown. Thylacinidae in magenta.

The molecular + morphology tip-dated Bayesian analysis is presented in Fig. 4 . Again, a monophyletic Dasyuridae is supported, but less strongly (BPP = 0.73). A clade consisting of Mutpuracinus, Barinya , and Dasyuridae is relatively well supported (BPP = 0.84) although the placement of these two taxa within this clade is uncertain, with little support for their sister relationship (BPP = 0.34). Within Dasyuridae, Sminthopsis floravillensis and Dasyurius dunmalli again cluster with Dasyurini, although with minimal support (BPP = 0.55). A surprising result is the clade comprised of Sminthopsis murina and Dasyuroides achilpatna (BPP = 0.73), a topology not recovered by Kealy & Beck (2017) or Archer et al. (2016) . Thylacinidae here is unsupported (BPP = 0.29), with only three internal nodes receiving even marginal support: a clade containing Ngamalacinus, Nimbacinus , and Muribacinus (BPP = 0.80), a clade containing Thylacinus megiriani, Thylacinus yorkellus , and Thylacinus cynocephalus (BPP = 0.80), and the sister relationship of Thylacinus yorkellus and Thylacinus cynocephalus (BPP = 0.95). A very interesting result is the non-monophyly of Thylacinus , caused by the exclusion of Thylacinus potens . This result is not recovered in the other analyses and is not well supported here.

Bayesian analysis of the morphological data recovers a very similar, though less resolved topology ( Fig. 3B ). Monophyly of Dasyuridae received higher support here (Bayesian posterior probability (BPP) = 0.83). As with the parsimony analysis, placement of the fossil dasyurids did not support their generic attributions, though with the necessary caveat that the current study was not designed to uncover the relationships within Dasyuridae. Thylacinidae receives moderate supports (BPP = 0.84), though the intrafamilial relationships are highly unresolved and only weakly supported, comprised of a basal polytomy, a polytomy containing Tyarrpecinus, Wabulacinus , and Thylacinus macknessi (BPP = 0.67), and a clade containing the remainder of Thylacinus spp. (BPP = 0.66). The only thylacinid clade here with a relatively strong support is that containing Thylacinus megiriani , Thylacinus yorkellus , and Thylacinus cynocephalus (BPP = 0.91).

(A) Majority rules 50% consensus of 30 most parsimonious trees under implicit enumeration (length 171, CI: 0.520, RI: 0.645) after removal of Maximucinus muirheadae from the matrix. (B) Bayesian analysis majority rules 50% consensus of post-burn-in trees. Branch lengths are arbitrary. Circles at nodes are bootstrap GC values in (A), BPP in (B), only values ≥50% are shown. Thylacinidae in magenta.

The undated parsimony and Bayesian phylogenetic results are presented in Fig. 3 . The parsimony analysis using implicit enumeration recovered 30 most parsimonious trees (length 171, CI: 0.520, RI: 0.645), after a posteriori removal of Maximucinus muirheadae (85% missing characters; Fig. 3A ). Bremer support values higher than 1.0 were found for all nodes, though bootstrap values equal to or higher than 50% were only recovered for Perameles + Dasyuromorphia (100), extant Dasyurus (excluding Dasyurus dunmalli ; 78), a clade comprising Thylacinus spp. excluding Thylacinus macknessi (51), and a polytomy containing Thylacinus megiriani , Thylacinus yorkellus , and Thylacinus cynocephalus (60). The forcing of Mutpuracinus archibaldi into Thylacinidae resulted in a slightly longer tree (length = 174, CI: 0.511, RI: 0.6432), indicating less support for that hypothesis, although both Templeton tests found the difference to be not significant.

Systematic palaeontology

Holotype: NTM P907-3, partial left maxilla.

Type Locality: Bullock Creek Local Fauna (LF), Blast Site, Northern Territory, Australia.

Referred Specimens: see Table S1

Distribution and Age: The Bullock Creek LF in Northern Territory is currently undated, but has been estimated to be Camfieldian (~17–12 Ma) sensu Megirian et al. (2010).

Diagnosis: Following the amended diagnosis by Murray & Megirian (2006a), Mutpuracinus archibaldi differs from Dasyuridae in: elongate snout; retention of three premolars; overall reduction of stylar crest and stylar cusps; reduction of paracones; widened angle of the centrocrista; reduction of protocones, conules, and distal cingulae; presence of a carnassial notch in the cristid obliqua (referred to as the prehypocristid by Murray & Megirian (2006a); uniform reduction of the metaconids; reduction of the entoconid. Differs from Badjcinus turnbulli, Nimbacinus dicksoni, and Thylacinus cynocephalus in: full enclosure of the petrosal and alisphenoid hypotympanic sinuses by the alisphenoid tympanic wing; posterior process of the maxillae narrow unlike Thylacinus cynocephalus in which the posterior maxillary processes flare widely, interposing the lacrimals, and nasals.

Remarks: The taxon is represented by several cranial specimens, including a near-complete cranium and partial mandible (Murray & Megirian, 2000, 2006a). Our findings agree with those of Archer et al. (2016), Kealy & Beck (2017), and Travouillon & Phillips (2018) in consistently falling outside Thylacinidae. The placement of Mutpuracinus archibaldi is a currently unresolved position within, sister, or stem to either Myrmecobiidae or Dasyuridae. We therefore refer Mutpuracinus archibaldi to Dasyuromorphia incertae sedis pending more conclusive results.

Family Thylacinidae Bonaparte, 1838

Genus Badjcinus Muirhead & Wroe, 1998

Badjcinus turnbulli Muirhead & Wroe, 1998

Holotype: QM F30408, partial skull (Fig. 5D)

Type Locality: White Hunter Site, D-Site Plateau, Riversleigh World Heritage Area, Queensland, Australia

Referred Specimens: see Table S1

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, Australia. The White Hunter Site is currently undated, but biocorrelation with the central Australian Ngama LF of South Australia suggests a late Oligocene date of ~26.0–24.0 Ma (Woodburne et al., 1994; Travouillon et al., 2006, 2011). However, fossil bats recovered from the site (Brachipposideros nooraleebus) have been noted to be similar to those found in the 17.4–16.8 Ma Bitesantennary Site as well as 20.4–16.0 Ma Burdigalian sites in France (Sigé, Hand & Archer, 1982; Archer et al., 1989; Hand & Archer, 2005). As such, the date is relatively uncertain, but the White Hunter site is likely to be latest Oligocene/earliest Miocene in age.

Diagnosis: Following Muirhead & Wroe (1998): very small thylacinid; lacking a squamosal epitympanic sinus; tympanic bulla lacks contribution by petrosal part of periotic; M1 preparacrista subparallel to the long axis of the tooth row; M 1 and M 2–4 metaconids differentially reduced. Distinguished from Nimbacinus dicksoni (Muirhead & Archer, 1990) by: stylar shelf, protocone, and conules reduced; more elongate postmetacristae; posterior cingulid joining hypocristid at base of hypoconid. Further differentiated from other similar-sized thylacinids by: protoconules and metaconules less reduced; entoconids and M 2–4 metaconids relatively unreduced; hypocristid transversely oriented; cristid obliqua lacking carnassial notch and anterior termination less lingually shifted than in later occurring thylacinids.

Remarks: Badjcinus turnbulli is the earliest named thylacinid currently known, and is established from relatively complete cranial material. The known elements consist of a partial skull, including the left premaxilla, partial left maxilla, nasals, frontals, zygomatic arches, parietals, and a well-preserved basicranium, as well as partial left and right dentaries and a dentary fragment consisting mostly of dentition and alveolar bone (Muirhead & Wroe, 1998). The entire postcanine mandibular dentition excepting P 1 are represented within the collected specimens, while the postcanine maxillary row is present excepting P3.

The rather plesiomorphic state of many of the characters of Badjcinus turnbulli has resulted in some uncertainty regarding its phylogenetic placement. It has occasionally been found to fall outside the clade as a potential dasyurid (Wroe et al., 2000) or stem-dasyuromorphian (see discussion in Kealy & Beck, 2017). Most phylogenetic analyses, however, recover it as sister to the remainder of Thylacinidae (Muirhead & Wroe, 1998; Wroe & Musser, 2001; Murray & Megirian, 2006a; Kealy & Beck, 2017).

Genus Maximucinus Wroe, 2001

Maximucinus muirheadae Wroe, 2001

Holotype: QM F30331, right M2 (Fig. 5F).

Type Locality: Ringtail Site, Riversleigh World Heritage Area, Queensland, Australia.

Referred Specimens: none

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, Australia. The middle Miocene Ringtail Site has been radiometrically dated to ~14.2–12.9 Ma (Woodhead et al., 2016).

Diagnosis: Modified from Wroe (2001): mid-sized thylacinid; all following features referring to M2: stylar cusps B and D well-developed and laterally compressed; anterior cingulum is continuous with the preparacrista; very small protoconule and metaconule.

Remarks: Maximucinus muirheadae is represented by a single M2 (Wroe, 2001). The tooth is interestingly large given the middle Miocene age of the specimen. The molar shows a large reduction in size of the protoconule and metaconule, but considering the well-developed stylar cusps is clearly less specialised towards hypercarnivory than that of derived members of Thylacinidae.

Genus Muribacinus Wroe, 1996

Muribacinus gadiyuli Wroe, 1996

Holotype: QM F30386, partial right maxilla and jugal (Fig. 5A).

Type Locality: Dwornamor L, Gag Site, Riversleigh World Heritage Area, Queensland, Australia.

Referred Specimens: see Table S1

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, Australia. Muribacinus gadiyuli is known from both Gag Site and Henk’s Hollow Site, Riversleigh. Neither localities have direct dates available, however, Gag Site and Henk’s Hollow have been found to be biostratigraphically correlated with the middle Miocene AL90 and Ringtail Sites (Travouillon et al., 2006, 2011; Arena et al., 2015). These correlated sites give a date range of ~15.1–12.9 Ma (Woodhead et al., 2016).

Diagnosis: Following Wroe (1996): very small thylacinid; differs from other thylacinids by: greater separation between paracones and metacones; large protocones; unreduced stylar shelf; preparacrista long relative to postmetacrista on M1–3; P 3 smaller than P 2 ; M 1–4 metaconids less reduced; relatively large talonids.

Remarks: The holotype and paratype consist of a right maxillary and jugal fragment, respectively, and an unassociated, mostly complete right dentary (Wroe, 1996). Both the maxillary and mandibular specimens retain the last premolar and full molar row. Muribacinus has plesiomorphically unreduced metaconids on all four lower molars, indicating a possible early reversal in dental trends towards hypercarnivory in the clade.

Genus Nimbacinus Muirhead & Archer, 1990

Nimbacinus dicksoni Muirhead & Archer, 1990

Syn. Nimbacinus richi Murray & Megirian, 2000

Holotype: QM F16802, left M 1 .

Type Locality: Henk’s Hollow Site, Gag Plateau, Riversleigh World Heritage Area, Queensland, Australia.

Referred Specimens: see Table S1

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, and Bullock Creek, Northern Territory, Australia; all middle Miocene. Nimbacinus dicksoni has been recovered from the Riversleigh Henk’s Hollow and AL90 Sites. Henk’s Hollow has not been directly dated, but the two sites have been biostratigraphically correlated with each other and AL90 has been radiometrically dated to 15.1–14.2 Ma (Arena et al., 2015; Woodhead et al., 2016). Bullock Creek in Northern Territory is currently undated, but has been biostratigraphically allied with the Camfieldian Land Mammal Age (17–12 Ma; Arena et al., 2015).

Diagnosis: From Muirhead & Archer (1990): small thylacinid; unreduced stylar shelf with prominent stylar cusps B and D in addition to small stylar cusps C and E on M1–2; retention of prominent protoconules and metaconules on M1–3; prominent protocristae; retention of small metaconids on all lower molars.

Remarks: Unlike the majority of fossil thylacinids, Nimbacinus dicksoni is represented by multiple specimens, including a beautifully preserved, near-complete skull and mandible and a near-complete but to date undescribed skeleton (Muirhead & Archer, 1990; Wroe & Musser, 2001; Fig. 5B). The rostrum exhibits little of the mediolateral pinching characteristic of Thylacinus and is relatively short, with only modest diastemata present in the premolar rows, although there is a diastema between the mandibular canine and the P 1 in Nimbacinus dicksoni contra Thylacinus cynocephalus. The skull is more robustly constructed than that of Thylacinus cynocephalus (Attard et al., 2014). The dentition of Nimbacinus dicksoni is relatively plesiomorphic for the family, with a reduction in stylar cusps, stylar shelf, and paracone that is greater than that of dasyurids, but an unelongated postmetacrista. The mandibular dentition is similarly plesiomorphic; the metaconids are reduced but present, and the talonid basin and protocone are reduced over that of dasyurids but not derived to the level seen in Thylacinus.

The validity of Nimbacinus richi has been questioned on the grounds of potential intraspecific variation and ambiguous fossil preservation (Wroe & Musser, 2001). As per Murray & Megirian (2000), Nimbacinus richi differs from Nimbacinus dicksoni in the differential expression of metaconids and entoconids, with Nimbacinus richi displaying a reduced metaconid on M 1 , large, well-developed metaconids on M 2–4 , and large conical entoconids on M 1–3 . The recovery of additional Nimbacinus dicksoni material (QM F36357) led Wroe & Musser (2001) to conclude that the differential metaconid expression shown between the putative specimens of Nimbacinus richi and Nimbacinus dicksoni falls within the range of variation exhibited by other known thylacinids. Furthermore, the authors note that the difference in entoconid size alone is likely to not be a viable character to differentiate species, as shown by variable entoconid expression (including presence/absence) in dasyurids (Dickman et al., 1998; Crowther, Dickman & Lynam, 1999).

Along with these arguments explicitly provided in Wroe & Musser (2001), the specimens of Nimbacinus richi (NTM P9612-4, P98695-92, and P904-7) are of a similar estimated body size to Nimbacinus dicksoni (range of 2.9–6.6 vs. 3.9–6.8 kg, respectively; see Table S2). We consider the likelihood of two near-identical species occurring in a temporally and spatially restricted space to be small. We find it more parsimonious to refer the Nimbacinus ‘richi’ specimens (NTM P9612-4, P98695-92, and P904-7) to Nimbacinus dicksoni rather than split the genus into two species.

Genus Ngamalacinus Muirhead, 1997

Ngamalacinus timmulvaneyi Muirhead, 1997

Holotype: QM F16853, partial right dentary

Type Locality: Inabeyance Site, Godthelp Hill, Riversleigh World Heritage Area, Queensland, Australia

Referred Specimens: see Table S1

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, Australia. Ngamalacinus timmulvaneyi is found at both the Inabeyance and Camel Sputum sites at Riversleigh (Muirhead, 1997). The Camel Sputum Site has been radiometrically dated to ~18.5–17.0 Ma (Woodhead et al., 2016). The age of the Inabeyance Site has not been directly dated, but biocorrelation with the RSO and Neville’s Garden Sites suggest an age of ~18.5–16.2 Ma (Arena et al., 2015; Woodhead et al., 2016).

Diagnosis: Modified from Muirhead (1997): small-sized thylacinid; relatively reduced conules and stylar shelf; retention of small stylar cusps B and D; narrower angle of maxillary molar cristae, less anteroposterior molar elongation, and less reduced talon basin than more derived thylacinids; retention of hypoconulid, entoconid, and relatively large metaconid (larger than paraconid) with distinct metacristid.

Remarks: The small thylacinid Ngamalacinus timmulvaneyi is represented by unassociated partial right dentary preserving M 1–4 , left maxilla preserving P2–M3, and an isolated M2 (Muirhead, 1997; Fig. 5C). The mandibular M 4 is only partially erupted, indicating that this dentary belongs to a juvenile. Nimbacinus timmulvaneyi displays a mixture of plesiomorphic and derived dental characters without obvious specialisations towards hypercarnivory. There are, however, a handful of interesting characters to note. The maxillary dentition is moderately specialised, with relatively reduced stylar cusps and a reduction of the stylar shelf. The mandibular dentition is relatively plesiomorphic, with a relatively large talonid and possessing tall, distinct metaconids. The lower carnassial (M 4 ), however, has a relatively reduced talonid basin with a present but small entoconid and hypoconid.

Genus Thylacinus Temminck, 1824

Thylacinus macknessi Muirhead, 1992

Holotype: QM F16848, right dentary (Fig. 6A).

Figure 6: Non-Pleistocene fossil Thylacinus spp. (A) Thylacinus macknessi QMF 16848 right dentary. (B) Thylacinus megiriani NTM P9618 partial left maxilla. (C) Thylacinus yorkellus SAM P29807 partial left dentary. (D) Thylacinus potens CPC 6746 palatal view. (E) Modern Thylacinus cynocephalus WAM M195 3D surface scan for comparison (image reversed due to damage). Image credits: (A) © Queensland; (B) and (D): Museum and Art Gallery Northern Territory, Adam Yates; (C): © The Museum Board of South Australia, Mary-Anne Binnie; (E) DS Rovinsky. All photos reproduced with permission.

Type Locality: Neville’s Garden Site, Riversleigh World Heritage Area, Queensland, Australia.

Referred Specimens: see Table S1

Distribution and Age: Thylacinus macknessi has been recovered from the early Miocene Neville’s Garden and Mike’s Menagerie sites, Riversleigh (Muirhead, 1992). Neville’s Garden has been dated to ~18.5–17.7 Ma, and Mike’s Menagerie estimated to ~18.5–16.2 via biostratigraphic correlation (Arena et al., 2015; Woodhead et al., 2016).

Diagnosis: Modified from the amended diagnosis by Muirhead & Gillespie (1995): a mid-sized thylacinid; M1 anterior cingulum well developed, continuous with protocrista, and lacking sulcus for preceding premolar; retention of small metaconule and lack of stylar shelf on M1; M1 with relatively unreduced paracone; retention of entoconid on all mandibular molars and a small metaconid on M 3–4 ; M 1 protoconid centrally located on crown, with the preprotocristid, postprotocristid, and cristid obliqua in line anteroposteriorly; reduction of anterior cingulum present on M 1 ; main cusps of P 1–2 anteriorly inclined and vertical on P 3 ; anterior cuspule retained on P 1–3 ; M 4 anteroposteriorly shorter in length than preceding molar unlike other species of Thylacinus; coronoid process of the mandibular ramus departs from the corpus at a more acute angle than Thylacinus cynocephalus (~120° vs. ~130°).

Remarks: Thylacinus macknessi is known from a near-complete dentary and scattered mandibular dentition. The taxon shows a degree of the facial elongation characteristic of the genus, as well as dental reduction and cristae alignment indicative of the dental trending towards hypercarnivory.

There is an additional specimen (QM F16850; partial right M1) ascribed to the species by Muirhead (1992). This specimen is recovered from the Dwornamor LF of Gag Site, Riversleigh, long noted to be middle Miocene in age based on biocorrelation with the ~15.1–14.2 Ma AL90 site (Archer et al., 1989; Arena et al., 2015; Woodhead et al., 2016). The specimen is currently the only maxillary specimen attributed to this species, thus not directly referable to the holotype, and it is not clear precisely why Muirhead (1992) attributed the tooth to Thylacinus macknessi. At the time of this initial publication, there were no lower dentitions known that would have occluded with an upper first molar (i.e., M 1–2 ; of which M 1 was unknown and M 2 missing anterior to the protoconid). Without a mandibular occlusal surface to match, it is difficult to have confidence in the attribution of the maxillary specimen. In a subsequent publication, Muirhead & Gillespie (1995) provide an amended description of the holotype QM F16848 after the anterior section of the dentary was discovered, but they do not supply any additional commentary regarding the M1 (QM F16850). We feel that pending the recovery of additional specimens supporting the alignment of QM F16850 with Thylacinus macknessi, it conservatively should be removed and placed within Thylacinidae incertae sedis.

Thylacinus megiriani Murray, 1997

Holotype: NTM P9618, partial left maxilla (Fig. 6C).

Type Locality: Ongeva LF, Alcoota Station, Northern Territory, Australia.

Referred Specimens: see Table S1

Distribution and Age: The Ongeva LF has not been directly dated, but various biostratigraphic studies have shown correlations with the nearby Alcoota LF as well as the Beaumaris LF, Victoria (Murray, Megirian & Wells, 1993; Megirian, Murray & Wells, 1996; Megirian et al., 2010; Rich, Darragh & Vickers-rich, 2003; Black et al., 2012). Strontium dating of the Beaumaris LF has provided an age of ~6.2–4.5 Ma for that formation (Dickinson & Wallace, 2009). The Ongeva LF is at least slightly younger than the stratigraphically lower Alcoota LF (Woodburne, 1967; Murray, Megirian & Wells, 1993). The zygomaturine Kolopsis torus is present at both the Alcoota and Ongeva LFs but not Beaumaris LF, and it has been noted that K. yperus, present in the Ongeva LF, shares close similarities with and may be synonymous with the Beaumaris LF K. gilli (Megirian, Murray & Wells, 1996). This suggests that the three deposits all may have formed within a span of a few million years at most, with the Alcoota LF the earliest and Beaumaris LF the latest (Murray, Megirian & Wells, 1993; Megirian, Murray & Wells, 1996; Rich, Darragh & Vickers-rich, 2003; Megirian et al., 2010). As a conservative estimate, we consider the Alcoota LF to likely span 8.5–5.5 Ma, and the Ongeva LF 7.5–4.5 Ma.

Diagnosis: Following the revised diagnosis by Yates (2015): very large thylacinid; small postcingulum between the metastyle and protocone of M2; M3 mesiodistally longer than wide; absence of a precingulum on M1 and M3; absence of a metaconule on all maxillary molars; M3 much longer than M2; reduction of the paracone; relative elongation of the postmetacrista; hypertrophied torus along the ventrobuccal margin of the dentary; diastema between P 3 and M 1 . Further differs from Thylacinus potens in: P1 in line with P2–3 instead of obliquely oriented.

Remarks: Thylacinus megiriani is only known from a partial maxillary fragment and recently described partial dentary fragments (Murray, 1997; Yates, 2015; Fig. 6C). The specimens suggest at an animal larger than the modern species, and of roughly similar rostral length but greater posterior palatal width than that of Thylacinus potens. Body mass based on geometric similitude with Thylacinus cynocephalus has estimated Thylacinus megiriani to be approximately 57.3 kg (Wroe, 2001), though see discussion below. As with Thylacinus macknessi, the general dental complexity reduction and elongation of shearing cristae indicate an increasing trend towards hypercarnivory.

Thylacinus potens Woodburne, 1967

Holotype: CPC 6746, partial palate (Fig. 6B).

Type Locality: Alcoota LF, Alcoota Station, Northern Territory, Australia.

Referred Specimens: see Table S1

Distribution and Age: The Alcoota LF is likely to have spanned from 8.5 to 5.5 Ma; see the above discussion regarding Thylacinus megiriani for details.

Diagnosis: Following the amended diagnosis by Yates (2014): large thylacinid; mesiodistal axis of P1 mesiobuccally oriented; M1 mesiodistally longer than wide; palatal fenestrae greatly reduced; absence of a diastema between P 1–2 ; P 2 longer than P 3 and M 1 . Yates (2014) additionally notes that Thylacinus potens may be further distinguished from Thylacinus cynocephalus by: ventrally facing sulcus forming the ventral border of the root of the zygomatic arch on the maxilla; P2 longer than M1.

Remarks: Thylacinus potens was the first pre-Pleistocene thylacinid to be discovered. The taxon is known from craniodental material including a large palatal fragment, maxillary and dentary fragments, and scant postcrania (Woodburne, 1967; Yates, 2014). A striking aspect of Thylacinus potens is the size of taxon, which has been estimated at 38.7 kg (via geometric similitude with Thylacinus cynocephalus; Wroe, 2001), and 40.9–120.6 kg (Yates, 2014). While the upper estimate is clearly an outlier probably caused by the relative robusticity of the dentition (see comments in Yates, 2014), it is clearly still substantially larger than the average body mass commonly cited for Tasmanian Thylacinus cynocephalus (29.5 kg; Paddle, 2000).

Thylacinus yorkellus Yates, 2015

Holotype: SAM P29807, partial left dentary (Fig. 6D).

Type Locality: Curramulka LF, Cora-Lynn Cave, South Australia, Australia.

Referred Specimens: see Table S1

Distribution and Age: The Curramulka LF of Cora-Lynn Cave has yet to be directly dated. Megirian et al. (2010) suggest it is within the Tirarian Australian Land Mammal Age (defined therein at ~5.0–3.0 Ma). They note, however, that there may be limited temporal and taxonomic separation between deposits of the oldest Tirarian and youngest Waitean (the Waitean is defined as 12.0–~5.0 Ma), with some taxa sharing close phyletic ties across the two ages. Several taxa present in the Curramulka LF support the younger age, such as the Zanclean and younger Baringa cf. nelsonensis, Tropsodon cf. bowensis, and Protemnodon cf. chinchillaensis (Megirian et al., 2010). Despite intensive excavation no rodent fossils have been recovered from the Curramulka LF, suggesting that the fauna predates the expansion of rodents into Australia (Pledge, 1992; Yates, 2015). The oldest definitive fossil evidence of rodents, at both Bluff Downs and the Chinchilla LF, occurs by ~4.5–3.6 Ma (Hand, 1984; Pledge, 1992; Tedford, Wells & Barghoorn, 1992; Mackness, Whitehead & McNamara, 2000; Turnbull, Lundelius & Archer, 2003; Ogg, 2012). This date is supported by molecular evidence of the radiation of Sahul rodents (5.5–5.1 Ma) and their estimated expansion into Australia (3.7–3.4 Ma, CI: 4.5–2.4 Ma; (Rowe et al., 2008). This suggests a range of ~5.3–3.6 Ma for the Curramulka LF.

Diagnosis: Following Yates (2015): large thylacinid; strongly developed precingulid terminating in a cuspidule on the mesiobuccal face of the paraconid of M 1–3 ; small basal mesial cuspidule on P 2–3 ; absence of metaconids on M 2 , M 3 , and presumably M 4 ; diastemata between C 1 –M 1 ; mesiodistal lengths of P 2 and P 3 both exceeding that of M 1 .

Remarks: Thylacinus yorkellus was originally described by Pledge (1992) as Thylacinus sp. in the early 1990s, with a subsequent find (SAM P38799 right M 3 ) prompting specific designation (Yates, 2015). The taxon is represented by an incomplete dentary fragment and isolated lower molar, and appears to have been rather longirostral, with diastemata between the canine—P 3 , as well as larger premolars than Thylacinus cynocephalus.

Thylacinus sp. indet.

Referred Specimens: see Table S1

Distribution and Age: Big Sink LF (New South Wales), Chinchilla LF (Queensland), Awe LF (Papua New Guinea). Neither Big Sink LF nor Chinchilla LF have been directly dated. However, the biostratigraphic correlation of Chinchilla LF with the geomagnetically dated Kanunka LF of the Tirarian Fm suggests a date of ~4.2–3.6 Ma for the Chinchilla LF (Tedford, Wells & Barghoorn, 1992; Ogg, 2012). The Big Sink LF has been noted to be Tirarian in age and correlate based on biostratigraphy with the Chinchilla LF (Dawson, Muirhead & Wroe, 1999; Mackness et al., 2000; Megirian et al., 2010), giving a similar age. The Awe LF of Papua New Guinea is younger, with a K/Ar radiometric date of 3.3–2.4 Ma (Hoch & Holm, 1986).

Remarks: There are a small number of isolated specimens that have been referred to Thylacinus sp. or the modern Thylacinus cynocephalus and are purportedly recovered from Pliocene sediments of Australia and New Guinea. The New Guinea specimen consists of a partial dental fragment and is thus otherwise uninformative (UCMP 107737 partial P 2 ; Awe LF, Papua New Guinea; Plane, 1976). Thylacinus sp. has also been recovered from the Big Sink LF of NSW (Dawson, Muirhead & Wroe, 1999). This specimen (AM F69875; partial left dentary with M 4 preserved) is dentally similar in size to both the modern species and Thylacinus potens UCMP 66206, and unfortunately too incomplete for an assessment of the mandibular corpus regarding relative size/robusticity. Furthermore, the size of the M 4 of Thylacinus yorkellus is unknown, and only a highly incomplete M 4 of Thylacinus megiriani is currently known, both of which also may temporally overlap with AM F69875 (Yates, 2015).

The specimens from the Chinchilla LF have had a rather more interesting history. The attribution both of the taxa and the locality of these specimens has been contentious, as many of the specimens have been noted to have poor collection data (Mackness et al., 2002). The fragmentary nature of the specimens further precludes easy comparison, and therefore it has been difficult to confidently ascribe the specimens to a specific taxon. In a review of the 4.2–3.6 Ma Chinchilla Downs LF specimens, Mackness et al. (2002) note that the Chinchilla Downs locality attribution of several specimens were due to curatorial errors, and the Thylacinus cynocephalus specimens were likely recovered from the middle Pleistocene (144–73 ka; Price et al., 2011) Darling Downs. Furthermore, they note that the Thylacinus specimen that does actually originate from the Chinchilla LF (WPC 4506) lack specific diagnosable characters, which is especially noteworthy as the similar-sized Thylacinus yorkellus has been described from late Miocene/early Pliocene deposits in South Australia (Pledge, 1992; Yates, 2015). Louys & Price (2015) offer a dissenting view and note that at least two specimens (QM F3741 and F9476) definitively originate from the Chinchilla LF and are directly attributable to Thylacinus cynocephalus. However, Louys & Price (2015) do not offer any justification for ascribing the specimens to the modern taxon, and the single figure presented (QM F9476 dentary fragment) is ambiguous regarding attributes that would enable a specific designation, especially considering that the poorly temporally-constrained Thylacinus yorkellus, Thylacinus megiriani and potentially Thylacinus potens may also have been present within this general time period. Additionally, while certainly not impossible, it is perhaps unlikely that the modern species had persisted over a ~4 million year span. Without specific evidence to align the specimens in question to Thylacinus cynocephalus, we feel it is most conservative to defer specific attribution of the Pliocene material until an in depth analysis can be made of these specimens, their provenance, and their curatorial history.

Genus Tyarrpecinus Murray & Megirian, 2000

Tyarrpecinus rothi Murray & Megirian, 2000

Holotype: NTM P98211, partial left maxilla (Fig. 5E).

Type Locality: Alcoota LF, Alcoota Station, Northern Territory, Australia.

Referred Specimens: none

Distribution and Age: The Alcoota LF is likely to have spanned from 8.5 to 5.5 Ma; see the above discussion regarding Thylacinus potens for details.

Diagnosis: After Murray & Megirian (2000): M1 narrow and elongate, with centrocrista relatively straight in relation to similar sized thylacinids excepting W. ridei Muirhead, 1997; strong ectoflexus and relatively elongate metastylar wing on M3; paracone closer to and smaller than metacone than in similar sized thylacinids; retention of stylar cusp B and D, with stylar cusp B relatively reduced on M3; conules reduced in size and number.

Remarks: Tyarrpecinus rothi is known only from a highly fragmented partial maxilla and associated dental fragments; nevertheless, it has been noted from the reconstructed fragments that the taxon seems to exhibit greater expression of dental characteristics related to hypercarnivory than in more basal forms (Murray & Megirian, 2000). The maxillary molars are more elongate, with a slight reduction in the complexity and size of cusps and increase in the elongation of shearing crests. That said, the poor preservation and extremely fragmentary nature precludes confidence in any firm comparisons. Interestingly, it has been suggested that the specimen derives from a crocodile coprolite due to its fragmented condition, coating of calcite, and evidence of potential acid etching (Murray & Megirian, 2000).

Genus Wabulacinus Muirhead, 1997

Wabulacinus ridei Muirhead, 1997

Holotype: QM F16851, partial right maxilla (Fig. 5G)

Type Locality: Camel Sputum Site, Godthelp Hill, Riversleigh World Heritage Area, Queensland, Australia

Referred Specimens: see Table S1

Distribution and Age: Riversleigh World Heritage Area, northwestern Queensland, Australia. The Camel Sputum Site has a radiometric date of ~18.5–17.0 Ma (Woodhead et al., 2016).

Diagnosis: From Muirhead (1997): small thylacinid; infraorbital foramen wholly enclosed by the maxilla and positioned anterior to M1; preparacrista and centrocrista of M1 subparallel; entoconid absent and hypoconulid enlarged. Further differs from similar-sized thylacinids by: lack of stylar cusps B, D, and extreme reduction of talon and protocone on M1; M1 lacks sulcus for preceding premolar; lack of stylar cusp B and reduced stylar cusp D on M2; M 3 metaconid reduced, lack of diastemata between mandibular dentition.

Remarks: The taxon is represented by a right maxillary fragment containing M1–2 and a partial left dentary preserving the alveoli for P 1 –M 4 but containing only a broken M 3 (Muirhead, 1997). The teeth show a marked reduction in the robusticity or the presence of many cusps and styles, a reduced protocone, especially on M1, and a reorientation of the major maxillary cristae of both preserved molars closer to parallel to the long axis of the tooth row. The dentary is relatively short, lacking diastemata between the premolars. While the mandibular dentition is unfortunately only represented by a partially preserved M 3 , the relatively reduced talonid basin and conids suggest the lower dentition is similarly reduced.

Thylacinidae incertae sedis Muirhead & Archer, 1990

Syn. Nimbacinus dicksoni

Specimen: QM F16809 partial right M 2 .

Type Locality: D-Site, Riversleigh World Heritage Area, Queensland, Australia.

Distribution and Age: D-site at Riversleigh is currently undated, but has been suggested to be late Oligocene via biostratigraphic correlation (Travouillon et al., 2006; Arena et al., 2015).

Remarks: Nimbacinus dicksoni has purportedly been recovered from the late Oligocene D-Site, Riversleigh (Muirhead & Archer, 1990). The attribution of this specimen to Nimbacinus dicksoni has been contentious, as both the late Oligocene age of the deposit and the state of metaconid reduction differs from that of the holotype, and the fragmentary nature of the specimen precludes further comparison (see discussion in Murray & Megirian, 2000; Wroe & Musser, 2001). We agree with the arguments presented by Murray & Megirian (2000) and Wroe & Musser (2001) and the conservative placement of the specimen QM F16809 as a thylacinid of uncertain position.

Thylacinidae incertae sedis Murray & Megirian, 2006b

Specimen: NTM P2815-10, fragmentary right M2.

Type Locality: Pwerte Marnte Marnte LF, Northern Territory, Australia.

Distribution and Age: The Pwerte Marnte Marnte LF is currently undated, but biostratigraphic correlation with the Etadunna Formation B & C suggests a late Oligocene age of ~25 Ma or older (Woodburne et al., 1994; Murray & Megirian, 2006b).

Diagnosis: Murray & Megirian (2006b) note that the fragmentary tooth is likely to be thylacinid due to the low stylar cusp D, relatively reduced stylar shelf, elongated metastylar wing, and narrow talon. It is comparable in size and gross morphology to the smallest known thylacinid, Muribacinus, differing, however in possessing a broader metastylar wing and larger protoconule.

Remarks: The fragmentary nature of the tooth prohibits any pointed discussion regarding the attribution or implications of the specimen.