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Journal of Vertebrate Paleontology

Volume 38, 2018 - Issue 6
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Articles

A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction

Brandon R. Peecook Integrative Research Center, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, U.S.A., bpeecook@fieldmuseum.org;; Burke Museum and Department of Biology, University of Washington, Seattle, Washington 98195, U.S.A., casidor@uw.edu;ORCID Iconhttp://orcid.org/0000-0002-0566-1487
ORCID Icon,
Roger M. H. Smith Evolutionary Studies Institute, School for Geosciences, University of the Witwatersrand, Johannesburg, Private Bag 3 Wits 2050, South Africa, Roger.Smith4@wits.ac.za;; Karoo Palaeontology, Iziko South African Museum, P.O. Box 61, Cape Town 8000, South Africa
&
Christian A. Sidor Burke Museum and Department of Biology, University of Washington, Seattle, Washington 98195, U.S.A., casidor@uw.edu;ORCID Iconhttp://orcid.org/0000-0003-0742-4829
ORCID Icon
Article: e1536664
Received 19 Sep 2017
Accepted 15 Aug 2018
Published online: 31 Jan 2019

Articles

A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction

Abstract

Abstract

Triassic-aged fossil vertebrates have been sporadically collected from the Fremouw Formation, central Transantarctic Mountains, since their initial discovery in the late 1960s, giving paleontologists insight into high-latitude faunas in the wake of the end-Permian mass extinction event. On a recent expedition (2010–2011), a small reptile skeleton was collected from Graphite Peak, which we present here alongside novel geological data and interpretations taken on site. Antarctanax shackletoni, gen. et sp. nov., is known from a partial postcranial skeleton including cervical and dorsal vertebrae, a humerus, and both pedes. Important morphological information includes well-defined laminae and deep fossae on cervicodorsal vertebrae. The new taxon can be differentiated from previously known Fremouw Formation reptiles (e.g., Prolacerta, Procolophon), as well as those from the Karoo Basin, South Africa (e.g., Mesosuchus, Proterosuchus, Euparkeria). Our inclusion of A. shackletoni in phylogenetic analyses of early amniotes finds it as an archosauriform archosauromorph, increasing known archosauriform diversity in the Early Triassic. The fauna of the lower Fremouw Formation traditionally has been considered to represent a subset of the Lystrosaurus Assemblage Zone of the Karoo Basin, with differences largely a result of pronounced differences in sampling intensity. However, a review of recent changes to the fauna, as well as a reassessment of occurrences based on older literature, indicates that significant discrepancies, including the co-occurrences of taxa known from both earlier and later in time and the presence of endemic forms in Antarctica, exist between the faunas of the Lystrosaurus Assemblage Zone and lower Fremouw Formation.

http://zoobank.org/urn:lsid:zoobank.org:pub:B1DAD1A4-7054-454D-89B2-17CAF2865AD4

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Citation for this article: Peecook, B. R., R. M. H. Smith, and Christian A. Sidor. 2019. A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1536664.

INTRODUCTION

Radiations of new tetrapod clades during the Early and Middle Triassic, particularly of Archosauromorpha, contained the beginnings of vertebrate taxonomic and ecomorphological diversity that would dominate the Mesozoic (Sereno, 1991; Nesbitt, 2011; Benton, 2016; Ezcurra, 2016). In the 5 million years of the Early Triassic following the end-Permian mass extinction, several lineages of sauropsids rapidly diversified on land (Archosauromorpha: Butler et al., 2011; Nesbitt et al., 2011; Gower et al., 2014) and in the sea (Sauropterygia: Jiang et al., 2014; Ichthyopterygia: Scheyer et al., 2014; Motani et al., 2015a, 2015b).

Antarctic vertebrate fossils were first discovered in the Transantarctic Mountains in the late 1960s at Graphite Peak and have been sporadically collected at several localities since (Fig. 1; Barrett et al., 1968; Colbert, 1970; Kitching et al., 1972; Cosgriff et al., 1978; Hammer et al., 1990, 2004; Hammer and Hickerson, 1994). Strata exposed in the Transantarctic Mountains and greater Victoria Land can be assigned to the Lower Triassic, Middle Triassic, Upper Triassic, and Lower Jurassic based on vertebrate biostratigraphic correlations with Karoo-aged basins in Africa, principally South Africa (Elliot et al., 1970; Hammer et al., 1987, 1990, 2004; Hammer, 1995; Sidor et al., 2008, 2013, 2014a, 2014b). Since the discovery of its first vertebrate fossils, the lower Fremouw Formation has been correlated with the Lystrosaurus Assemblage Zone (LAZ) of the Karoo, because several species are shared (e.g., Thrinaxodon liorhinus, Procolophon trigoniceps, Lystrosaurus curvatus; Kitching et al., 1972; Sidor et al., 2008). More recent work has emphasized, however, that the lower Fremouw assemblage also contains a number of taxa not known from the LAZ despite it having had much less collecting effort. For example, the lower Fremouw contains the reptile Palacrodon, the dicynodont Kombuisia, and brachyopoid temnospondyls, which all appear later in the Karoo Basin succession. In addition, Lystrosaurus maccaigi, as well as a large-bodied archosauriform (estimated body length >3 m) and a possible gomphodont cynodont, occur in the lower Fremouw but are generally lacking in the LAZ (Table 1; Colbert and Kitching, 1977; Cosgriff et al., 1982; Gow, 1992, 1999; Sidor et al., 2008; Fröbisch et al., 2010; Smith et al., 2011). Higher in section, close biostratigraphic connections break down as Antarctic assemblages share decreasing numbers of species and genera with age-equivalent strata across southern Pangea, despite having had a continuous continental connection (Hammer, 1995; Sidor et al., 2013, 2014a, 2014b). This trend toward endemism culminates in the Jurassic assemblage of the Hanson Formation that shares no genera with any other basin in Pangea (Hammer and Hickerson, 1994; Smith and Pol, 2007; Hammer and Smith, 2008).

FIGURE 1. A, map of central Transantarctic Mountains highlighting fossil tetrapod localities in the lower Fremouw Formation in the Beardmore and Shackleton glacier regions. B, inset, map showing Shackleton Glacier localities, indicated on the larger map by a rectangle. Members of Synapsida are in black, Temnospondyli in light gray, and Reptilia in dark gray. C, specimen numbers for all lower Fremouw Archosauromorpha. Map modified from Collinson et al. (2006). Silhouettes modified from Wikimedia.

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TABLE 1. Fauna of the lower Fremouw Formation of the Transantarctic Mountains.

In this contribution, we describe a small postcranial skeleton from the lower Fremouw Formation and update the taxonomic composition of the lower Fremouw vertebrate assemblage. When placed in phylogenetic analyses including a broad array of early amniotes, the new specimen is recovered as an early-diverging archosauriform. Enough skeletal material is present to rule out assignment to previously known archosauromorphs from the lower Fremouw, such as Prolacerta broomi and the large humerus and vertebra described by Smith et al. (2011), as well as Karoo archosauromorphs such as Noteosuchus colletti, Mesosuchus browni, Howesia browni, Proterosuchus fergusi, Garjainia madiba, Erythrosuchus afrcianus, and Euparkeria capensis. Accordingly, we describe the specimen as the holotype of a novel archosauriform taxon.

Institutional Abbreviations—AM, Albany Museum, Grahamstown, South Africa; AMNH, American Museum of Natural History, New York, New York, U.S.A.; BP, Evolutionary Studies Institute (formerly Bernard Price Institute), University of the Witswatersrand, Johannesburg, South Africa; GMB, Geological Institute, Beijing, China; MCNAM, Museo de Ciencias Naturales y Anthropológicas de Mendoza, Mendoza, Argentina; NHMUK, Natural History Museum, London, U.K.; NMQR, National Museum, Bloemfontein, South Africa; PIN, Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow, Russia; QM, Queensland Museum, Brisbane, Queensland, Australia; SAM, Iziko South African Museum, Cape Town, South Africa; UTGD, School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia; UWBM, University of Washington Burke Museum, Seattle, Washington, U.S.A.

GEOLOGICAL SETTING

The Karoo-aged Beacon Supergroup sedimentary sequences in the central Transantarctic Mountains fill one of a series of retroarc foreland basins that developed on the continental side of the rising Gondwanide mountains from early Permian through Early Jurassic times (Collinson, 1991; Isbell, 1991; Collinson et al., 1994). Today, remnants of these foreland basin fills occur in Australia, Antarctica, southern Africa, and South America, with the Karoo Supergroup of South Africa being the largest and best studied. Roughly duplicating the Karoo succession, the Beacon Supergroup begins with diamictites (Pagoda Formation) recording several advance/retreat cycles of the Permo-Carboniferous Gondwanan glaciation followed by expansive melt-out black shales (Mackellar Formation) that grade upward into deltaics and fluviodeltaics of early Permian age (Fairchild Formation). With progressive infilling, the deltas coalesced to form expansive alluvial plains drained by large-scale distributary fluvial channel/floodplain systems flowing parallel to the mountain range in a northeasterly direction (Isbell, 1991). The fluvial regimes varied in response to source area tectonism at various times from the middle to late Permian (Buckley Formation) through the Early Triassic (Fremouw Formation) into the Late Triassic (Falla Formation). The base of the Fremouw Formation, which is defined as the first cliff-forming channel sandstone (Barrett, 1969), is now considered diachronous across the Beacon Basin, with some areas suggested to be latest Permian and others earliest Triassic in age (Collinson et al., 2006).

The Buckley Formation is a coal-bearing fluviolacustrine sequence, whereas Fremouw strata are generally, but not exclusively, non-coal-bearing and fluvially dominated (Barrett et al., 1986). The new archosauriform described here was recovered from the lower member of the Fremouw, which consists of equal proportions of coarse- to medium-grained sandstone and green-gray or red rooted siltstone beds arranged in stacked fining-upward fluvial cycles (Fig. 2). Fine-grained green-gray or red beds dominate the middle member. The upper member is predominately fine- to medium-grained sandstone with isolated coal beds (Barrett, 1969). The differences in color between the Buckley and Fremouw formations are a reflection of paleopedogenic modification (Retallack et al., 1996; Retallack and Krull, 1999; Krull and Retallack, 2000). The upper Buckley paleosols are interpreted as having formed in woodlands on swampy floodplain environments in a humid climate possibly with seasonal snowfall, which change abruptly at the Permo-Triassic boundary into paleosols indicative of much warmer paleoclimate conditions and evidence woodland vegetation on seasonally wet, well-drained floodplains (Retallack et al., 2005). On Graphite Peak, this abrupt change in paleosols occurs some 17 m below the base of the first cliff-forming sandstone (i.e., the currently mapped base of the Fremouw Formation). There, the disappearance of carbonaceous shale, and the appearance of white-weathering claystone-lined root molds, is easily recognized and is interpreted as the result of rapid climatic drying associated with the end-Permian mass extinction event (Retallack et al., 2005). In sections that have an arenaceous lithological contact between Buckley and Fremouw formations lacking overbank facies, Retallack and Krull (1999) proposed that the first appearance of distinctive smooth-surfaced berthierine nodules is a reliable indicator of the low-oxygen conditions of the earliest Triassic.

FIGURE 2. Measured section of the main fossil-bearing horizons at Graphite Peak. Vertebrate fossils collected at this locality include (from bottom to top) (1) UWBM 95546, unidentified bone, and UWBM 95526, a possible temnospondyl; (2) UWBM 95523, fragmentary material of Lystrosaurus, and UWBM 95524, a large tetrapod burrow cast; (3) UWBM 95525, articulated skull and anterior skeleton of Lystrosaurus; (4) UWBM 95527 and UWBM 95528, scattered bones of Lystrosaurus, and UWBM 95529, a semi-disarticulated skeleton of Prolacerta; (5) UWBM 95531, Antarctanax shackletoni, gen. et sp. nov., material described herein; (6) UWBM 95530, Thrinaxodon liorhinus dentary and scattered postcranial elements; and (7) UWBM 88572, Procolophon jaws, and UWBM 95522, a partial skull of a small lapillopsid. A legend for the sedimentology is provided as Figure S1 in Supplemental Data 3. Abbreviations: Fe, iron-rich horizon; Fm., Formation; mid., middle; PTB, inferred position of Permo-Triassic boundary.

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The stratigraphic position of the Permo-Triassic boundary (PTB) on Graphite Peak relative to that of the new archosauriform and its associated fauna is important to this study because it has bearing on how high-latitude tetrapods were affected by the mass extinction event and the tempo of subsequent ecosystem recovery. On Graphite Peak, the PTB is placed at the first appearance of claystone-lined rootlets in massive gray siltstones, which also coincides with a pronounced negative stable carbon isotope excursion (Retallack and Krull, 2005; Fig. 2). This sequence of PTB facies transitions associated with a negative stable carbon isotope anomaly is very similar to that seen in the southern half of the South African Karoo Basin, which Smith and Botha-Brink (2014) attribute to rapid climatic drying and general lowering of floodplain water tables. Biostratigraphic comparison of the new archosauriform and associated fauna with that of the main Karoo Basin would make the Graphite Peak locality equivalent to 35 m above the PTB in the Karoo, within the lower LAZ. Biostratigraphically significant common taxa are Thrinaxodon liorhinus, Prolacerta broomi, Procolophon trigoniceps, and Lystrosaurus murrayi, whose ranges are concurrent only in the lower LAZ starting 25 m above the boundary (Smith and Botha-Brink, 2014).

SYSTEMATIC PALEONTOLOGY

REPTILIA Laurenti, 1768, sensu Modesto and Anderson, 2004

SAURIA Gauthier, 1984, sensu Gauthier et al., 1988

ARCHOSAUROMORPHA Huene, 1946, sensu Dilkes, 1998

ARCHOSAURIFORMES Gauthier, Kluge, and Rowe, 1988

ANTARCTANAX SHACKLETONI, gen. et sp. nov.

(Figs. 3–7)

FIGURE 3. Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, line drawings and photographs of sides 1 (A) and 2 (B) of block 1. More detailed photographs are available as Figures S2 and S3 in Supplemental Data 3. An asterisk indicates the vertebrae measured and measurement included in Table 2. Abbreviations: cdv, cervicodorsal vertebra; cv, cervical vertebra; dv, dorsal vertebra; h, humerus; I–V, pedal digits; r, rib. Scale bar equals 5 cm.

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FIGURE 4. Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, photograph (A) and line drawing (B) of block 2. Abbreviations: I–IV, pedal digits; mt, metatarsal; ph, phalanx; r, rib; t, tarsals.

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FIGURE 5. Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, reconstruction of cervicodorsal vertebra in A, anterior, B, left lateral, and C, posterior views. Abbreviations: cpodf, centro-postzygodiapophyseal fossa; dia, diapophysis; iprf, infraprezygapophyseal fossa; pa, parapophysis; podl, postzygodiapophyseal lamina; posf, postspinal fossa; post, postzygapophysis; pre, prezygapophysis; spdf, spino-prezygodiapophyseal fossa. Scale bar equals 1 cm.

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FIGURE 6. Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, photograph of cervicodorsal vertebrae and dorsal vertebra. Abbreviations: acdl, anterior centrodiapophyseal lamina; cpodf, centro-postzygodiapophyseal fossa; dia, diapophysis; iprf, infraprezygapophyseal fossa; pa, parapophysis; podl, postzygodiapophyseal lamina; posf, postspinal fossa; post, postzygapophysis; prdl, prezygodiapophyseal lamina; pre, prezygapophysis; spdf, spino-prezygodiapophyseal fossa. Scale bar equals 1 cm.

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FIGURE 7. Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, illustration of left humerus in ventral view. Abbreviations: dpc, deltopectoral crest; rc, radial condyle; tb, tubercle; uc, ulnar condyle; vf, ventral fossa. Scale bar equals 2 cm.

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Etymology

‘Antarctic king’: ‘Antarct’ for Antarctica (from the Greek ‘ant’ meaning ‘against or opposite’ and ‘arktikos’ meaning ‘near the bear, or north’) and ‘anax’ from the Greek for king, here a reference to the archosaurian (ruling reptile) status of UWBM 95531. ‘Shackletoni’ in reference to British polar explorer Ernest Shackleton, who named the Beardmore Glacier, which runs between lower Fremouw localities such as Graphite Peak.

Material

UWBM 95531, consisting of eight presacral vertebrae, several ribs, a left humerus, left metatarsals I–V, phalanges, unguals, and unidentifiable flat bones on one slab and an almost complete right pes and a dorsal rib on a second, which was adjoining the first but separated during preparation. Specimen UWBM 95531 is considered to represent a somatically mature individual because all neural arches present are completely fused to their centra, with no indications of sutures.

Horizon and Locality

Collected from the lower Fremouw Formation at Graphite Peak, locality UWBM C1591 at 85°3.175′S, 172°22.148′E, central Transantarctic Mountains, Antarctica (Figs. 1, 2).

Differential Diagnosis

Antarctanax shackletoni (UWBM 95531) can be diagnosed by the following unique combination of characters: presence of (1) an anterior centrodiapophyseal lamina, (2) a prezygodiapophyseal lamina, and (3) a postzygodiapophyseal lamina but (4) absence of a posterior centrodiapophyseal lamina in posterior cervical/anterior dorsal vertebrae; (5) unkeeled ventral surface of cervical vertebrae; deep pits lateral to the base of the neural spine in (6) cervical and (7) dorsal vertebrae; and (8) metatarsal IV greater than 1.3 times the length of metatarsal III.

Antarctanax shackletoni can be differentiated from known archosauromorphs from the lower Fremouw Formation and the Lower–Middle Triassic Lystrosaurus Assemblage Zone (AZ) and Cynognathus AZ (subzones A and B) of the Karoo Basin, South Africa. Antarctanax shackletoni differs from Prolacterta broomi (AMNH 9502, AMNH 9573, NMQR 3763) and Proterosuchus fergusi (NMQR 1484) in possessing robust postzygadiapophyseal laminae and deep centro-postdiapophyseal and spino-prezygodiapophyseal fossae and lacking ventrally keeled cervical vertebrae. Antarctanax shackletoni differs from the large lower Fremouw archosauriform (AMNH 24262) in the asymmetrical distal end of the humerus (possessing a larger entepicondyle), circular cross-section at the humeral midshaft, and much smaller size (see Fig. 8). Antarctanax shackletoni differs from the early rhynchosaurs Noteosuchus colletti (AM 3591), Mesosuchus browni (SAM-PK-6046), and Howesia browni (SAM-PK-5886) in possessing prezygodiapophyseal laminae, anterior centrodiapophyseal laminae, and postzygodiapophyseal laminae in the posterior cervical vertebrae; a metatarsal IV greater than 1.3 times longer than metatarsal III; and the expanded entepicondyle of the humerus (Dilkes, 1998; Ezcurra et al., 2014). Antarctanax shackletoni differs from Euparkeria capensis in having deep cervicodorsal fossae and lacking a posterior centrodiapophyseal lamina in posterior cervical and anterior dorsal vertebrae (Nesbitt, 2011; Sookias and Butler, 2013). Antarctanax shackletoni differs from the Karoo erythrosuchids Garjainia madiba (BP/1/7338, BP/1/6233b, BP/1/7135, BP/1/7336) and Erythrosuchus africanus (NHMUK 3762a, NHMUK R3592) in a more gracile humerus (ratio of length to proximal width is approximately 2.5:1, compared with 1.6:1 in E. africanus, NHMUK R3592), a much smaller body size, as well as lacking ventral keels on vertebral centra (Nesbitt, 2011; Ezcurra et al., 2013; Gower et al., 2014).

FIGURE 8. Life size comparative line drawings of archosauromorph left humeri from the lower Fremouw Formation. A, Prolacerta broomi (UWBM 95529); B, Antarctanax shackletoni, gen. et sp. nov. (UWBM 95531); and C, the large unnamed archosauromorph (AMNH 24262) in ventral views.

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Analyses (below) find A. shackletoni as an archosauriform archosauromorph that is phylogenetically positioned between the common ancestor for Archosauriformes and the node uniting Erythrosuchidae and Eucrocopoda. Accordingly, A. shackletoni must be differentiated from other taxa within this grade. Sarmatosuchus otschevi (PIN 2865/68) from the Middle Triassic of Russia possesses keeled cervical centra and a subcentral foramen on the lateral surface of dorsal centra (Gower and Sennikov, 1997; Ezcurra, 2016). Fugusuchus hejiapensis (GMB V313) from the Early–Middle Triassic of China does not share overlapping elements with A. shackletoni, but they are unlikely to be congeneric because the occipital condyle of F. hejiapensis is approximately 20 mm across, approximately twice the width of articular faces of centra in A. shackletoni, and therefore represents a larger animal (Table 2; Gower and Sennikov, 1996). Cuyosuchus huenei (MCNAM 2669) from the Late Triassic of Argentina has posterior centrodiapophyseal laminae in posterior cervical and anterior dorsal vertebrae, and an unexpanded entepicondyle (Ezcurra 2016).

TABLE 2. Linear measurements (in mm) of Antarctanax shackletoni, gen. et sp. nov., UWBM 95531.

Description

Of the eight presacral vertebrae present in UWBM 95531 (Figs. 3, 5, 6), two can be identified as cervicals based on the anteroventral position of the parapophyses and low, elongated morphology of their centra. The two best-preserved vertebrae are cervicodorsals, based on the high position of the parapophyses alongside the diapophyses, whereas another is a dorsal vertebra. The three remaining vertebrae are weathered and difficult to place with confidence along the axial column. All vertebrae share slightly amphicoelous articular faces.

The cervical vertebrae possess long, spool-like centra that are ventrally and laterally rounded and lack a ventral keel. The centra are rectangular in lateral view. The neural arches of the cervicals are badly weathered, but a shallow fossa is visible at the lateral junction of the neural arch and centrum. The most elongated cervical is heavily weathered, but the centrum is measured as approximately 1.8 times longer than tall.

The cervicodorsal vertebrae are much better preserved (Figs. 5, 6). The centra are anteroposteriorly shorter and dorsoventrally taller than those of the cervicals (1.5 times longer than tall), and they retain a rounded, unkeeled morphology. The neural arches are well developed, with strong laminae and fossae. Two of the vertebrae possess prezygodiapophyseal and postzygodiapophyseal laminae, resulting in deep spino-prezygodiapophyseal and spino-postzygodiapohyseal fossae, with the former achieving a pit-like morphology lateral to the base of the neural spine. A robust anterior centrodiapophyseal lamina is also present. The neural spines are rectangular in lateral view and lack mammillary processes. In both cervicodorsal vertebrae, the dorsal ends of the neural spines expand gradually, but not to the point of forming a spine table. In Euparkeria, phytosaurs, proterochampsids, doswellids, and members of Archosauria, spine tables are often associated with the presence of osteoderms (Nesbitt, 2011). Osteoderms were not found with the type of Antarctanax, although its vertebral morphology and phylogenetic placement leave the possibility open that it did possess them. The single unambiguous dorsal vertebra has a short centrum (∼1.2 times longer than tall), with both rib facets close together and well above the centrum. The centrum is smooth and unkeeled. The neural spine is rectangular, with parallel anterior and posterior faces and no dorsal expansion of any kind. A pit is present lateral to the base of the neural spine, but it is not quite as deep as in the cervicodorsal vertebrae.

The humerus of UWBM 95531 (Fig. 7; Table 2) is approximately 60 mm long and well preserved, with only a small break on the surface of the humeral head and deltopectoral crest. The distal humerus is only visible in ventral view. Both the proximal and distal ends of the humerus are flared, with the width of the distal end longer than 0.3 of the total humeral length. The asymmetrical distal end is due to the enlarged entepicondyle relative to the ectepicondyle. Torsion of roughly 90° exists between the proximal and distal ends. An entepicondylar foramen cannot be made out. The distal indentation is positioned at the mid-width of the distal end and therefore offset from the midline of the shaft. Neither a capitellum for articulation with the radius nor a trochlea for articulation with the ulna is present. The midshaft of the humerus is circular in cross-section.

Both pedes are present and mostly articulated in UWBM 95531 (Figs. 3, 4; Table 3). The metatarsals overlap one another proximally before spreading out to form a plesiomorphic saurian foot. Metatarsals I–IV are progressively longer, with metatarsal I being less than one-third the length of metatarsal IV. Metatarsal IV is quite long, measured at over 1.5 times longer than the preserved length of metatarsal III, but this is likely a slight overestimate because the proximal end of metatarsal III is worn. Metatarsal V is short, with a ‘hooked’ medial process flexed toward the other digits proximally, and the proximal lateral process is pointed. This combination of characters in metatarsal V is shared only with erythrosuchids in the Ezcurra (2016) data set (Erythrosuchus africanus, BP/1/2096). Despite the inferred mature ontogenetic stage of UWBM 95531, plantar tubercles cannot be seen on metatarsal V, as they can in other Permo-Triassic saurians. A complete phalangeal formula cannot be identified, but digit I possesses one phalanx and one ungual and digit II possesses two phalanges and an ungual. The number of phalanges in the remaining digits is more difficult to determine: digit III has a minimum of three phalanges (no ungual preserved), digit IV one phalanx, and digit V two phalanges (no ungual preserved).

TABLE 3. Linear measurements (in mm) of Antarctanax shackletoni, gen. et sp. nov., UWBM 95531, pedes.

PHYLOGENETIC ANALYSIS

Methods

Antarctanax (UWBM 95531) was added to the phylogenetic analysis of Ezcurra et al. (2014), which sampled a broad swath of early amniote morphological diversity, including synapsids and sauropsids. The revised matrix contained 41 taxa and 219 characters. UWBM 95531 was scored for 35 of the 219 characters (16%). Antarctanax was also added to the matrix of Ezcurra (2016) to further elucidate its position among Archosauromorpha and to allow comment on the evolution of the clade in the Early Triassic. The revised matrix contained 80 taxa and 600 characters, with UWBM 95531 scored for 59 of the 600 characters (∼10%).

For the first analysis, we followed the methodology of Ezcurra et al. (2014). All characters were equally weighted, and the following characters were ordered: 2, 6, 8, 10, 12, 16–18, 20, 21, 24, 26, 30, 31, 36–38, 40, 45, 49, 51–55, 57, 58, 62, 70, 73, 79, 91, 92, 94, 103, 143, 160, 198, 214, 216, and 219, as in Ezcurra et al. (2014). Several taxa included in the original analysis were unscored for most characters, including UWBM 95531, resulting in a breakdown in topology and rampant polytomies. Therefore, we followed Ezcurra et al. (2014) in removing problematic taxa a posteriori, including Aenigmastropheus parringtoni, Archosaurus rossicus, Eorasaurus olsoni, Noteosuchus colletti, and Paliguana whitei. Specimen UWBM 95531 remained in our analyses, and we report the resulting topology and support values. The diadectomorph stem-amniote Tseajaia was the designated outgroup.

For the second analysis, based on the archosauromorph study of Ezcurra (2016), the matrix included 600 characters, nearly half of which are postcranial, and 82 ordered characters. Ezcurra (2016) scored 102 specimens belonging to 96 taxa, many of which are missing most character scores or represent nomia dubia. Accordingly, we followed the reduced Analysis 3 of Ezcurra (2016) for taxon sampling. The early diapsid Petrolacosaurus kansensis was the designated outgroup.

For both analyses, the matrices were edited in Mesquite 3.02 (Maddison and Maddison, 2008) and the data set was analyzed using TNT 1.1 (Goloboff et al., 2008). We performed a heuristic search using the parsimony criterion, tree bisection and reconnection option, and 10,000 random addition sequence replicates. Bremer support and bootstrap values were calculated in TNT using a heuristic search and 10,000 replicates (Goloboff et al., 2008). Nexus files for both analyses can be found online as Supplemental Data 1 and 2.

Results

The analyses based on Ezcurra et al. (2014) recovered four most parsimonious trees (MPTs) of 858 steps. Specimen UWBM 95531 is recovered as an archosauromorph saurian, and possibly as an archosauriform (Bremer support = 1), but little more can be said about its phylogenetic relationships (Fig. 9A). The strict consensus tree of the four MPTs places it in a polytomy with the archosauriforms Proterosuchus, Erythrosuchus, and Euparkeria. The following are character-state optimizations supporting the position of Antarctanax within successive clades in the MPTs (given as character number:state number): Sauria: possession of a hooked metatarsal V (112:1); Archosauromorpha: moderately long transverse processes in dorsal vertebrae (74:1), subrectangular neural spines in anterior and mid-dorsal vertebrae (179:0), and absence of entepicondylar foramen (90:1); Rhynchosauria + Euparkeria: a dorsally opened pit lateral to the base of the neural spine in dorsal vertebrae (184:1), entepicondyle moderately large (89:0), and distinct capitellum and trochlea absent (198:2); Prolacerta + Euparkeria: paradiapophyseal lamina present (180:1) and zygapophyses mainly oriented parasagittally (185:1); Archosauriformes: ratio of width of diapophyses to centrum length >0.75 in anterior dorsal vertebrae (178:1).

FIGURE 9. Phylogenetic placement of Antarctanax shackletoni as an archosauromorph (A) based on the matrix of Ezcurra et al. (2014), and within Archosauriformes (B) when placed in Ezcurra (2016). Strict consensus of four most parsimonious trees (MPTs) of 858 steps for Ezcurra et al. (2014) and strict consensus of 24 MPTs of 2,649 steps for Ezcurra (2016). Node support indicated by bootstrap values to the left or below and Bremer support to the right or above.

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The analysis based on Ezcurra (2016) recovered 24 MPTs of 2,649 steps (Fig. 9B). Specimen UWBM 95531 is recovered as an archosauriform saurian in a polytomy with Sarmatosuchus otschevi, Fugusuchus hejiapensis, and a clade containing Cuyosuchus huenei as the sister group to Erythrosuchidae + Eucrocopoda. There is little statistical support for the placement of UWBM 95531. The following are character-state optimizations supporting the position of Antarctanax within successive clades in the MPTs (given as character number:state number): Rhynchosauria + Archosauria: no posterior centrodiapophyseal lamina in posterior cervicals and anterior dorsals (316:0); Boreopricea + Archosauria: humeral torsion >45° (415:0); Tasmaniosaurus + Archosauria: lateral fossa at level of neurocentral suture in dorsal vertebrae (354:1). Importantly, the clade Cuyosuchus + Archosauria is supported by the presence of a posterior centrodiapophyseal lamina in posterior cervical and anterior dorsal vertebrae, which Antarctanax lacks.

DISCUSSION

Archosauromorphs of the Lower Fremouw Formation

Antarctanax shackletoni is the third distinct archosauromorph taxon from the lower Fremouw Formation, alongside Prolacerta broomi and the large archosauriform represented by AMNH 24264 (Cosgriff, 1983; Colbert, 1987; Smith et al., 2011). Globally, Lower Triassic strata are expected to contain a diverse archosauromorph fauna given that members of Archosauromorpha are known from across Pangea in the Wuchiapingian Stage of the Permian: Protorosaurus speneri from Germany and the U.K., Eorasaurus olsoni from Russia, and Aenigmastropheus parringtoni from Tanzania (Meyer, 1832; Evans and King, 1993; Sennikov, 1997; Nesbitt, 2011; Ezcurra et al., 2014; Ezcurra, 2016; Pritchard and Nesbitt, 2017). The proterosuchid Archosaurus rossicus comes from the uppermost Permian assemblage in Russia and is sister to the remaining members of Proterosuchidae (Tatarinov, 1960; Ezcurra et al., 2014; Ezcurra, 2016). Given these occurrences and current hypotheses on archosauromorph phylogenetic relationships, it can be inferred that several archosauromorph clades must have diverged in the Permian, crossed the Permo-Triassic boundary, and diversified in the Triassic: Tanystropheidae, Rhynchosauria, Allokotosauria, Prolacertidae (sensu Ezcurra, 2016), later Proterosuchidae, and the lineage leading to Erythrosuchidae + Eucrocopoda (Nesbitt, 2011; Ezcurra et al., 2014; Nesbitt et al., 2015; Ezcurra, 2016; Pritchard and Nesbitt, 2017).

The taxonomic composition of the lower Fremouw archosauromorphs fits well within current global understanding of Early Triassic archosauromorph diversity, consisting of a prolacertid and at least two other more crownward taxa that likely represent non-proterosuchid archosauriforms. Recently, several authors have found Early Triassic archosauromorphs to be more phylogenetically and ecologically diverse than previously appreciated, consisting of more diversity than generalized ‘proterosuchian’ and ‘prolacertiform’ clades (Butler et al., 2011; Nesbitt et al., 2011; Ezcurra and Butler, 2015; Ezcurra, 2016; Foth et al., 2016; Sookias, 2016). In his richly sampled phylogenetic analysis of early archosauromorphs, Ezcurra (2016) found Proterosuchidae to consist of Archosaurus rossicus from Russia, three species of Proterosuchus from the earliest Lystrosaurus Assemblage Zone (LAZ) of South Africa (P. fergusi, P alexanderi, and P. goweri), and one species of Proterosuchus from China (P. yuani [= ‘Chasmatosaurus’ yuani]) and a newly defined monophyletic Prolacertidae to consist of Prolacerta broomi from South Africa and Antarctica and Kadimakara australiensis from Australia (Bartholomai, 1979; Colbert, 1987: Ezcurra and Butler, 2015). Importantly, the taxa remaining in the taxonomically updated Proterosuchidae are from strata associated with the Permo-Triassic boundary in the uppermost Permian (Vyazniki Biotic Assemblage of Russia) and lowermost Triassic (Palingkloof Member of the Balfour Formation of South Africa, Jiucaiyuan Formation of China; Golubev, 2000; Lucas, 2001; Rubidge, 2005; Nesbitt et al., 2015), indicating that the absence of a Proterosuchus-like taxon from well-sampled lower Fremouw localities is unsurprising given its likely correlation with the portion of the LAZ tens of meters above the PTB (see below). Ezcurra (2016) found Early Triassic taxa that had been previously considered ‘proterosuchian’ in different positions relative to the newly defined Proterosuchidae: Tasmaniosaurus triassicus from Australia was found as sister to Archosauriformes, whereas Kalisuchus rewanensis from Australia and Chasmatosuchus (C. rossicus and C. magnus) from Russia were found more crownward than Proterosuchidae (Huene, 1940; Camp and Banks, 1978; Ochev, 1979; Thulborn, 1979; Ezcurra, 2014, 2016).

For the sake of resolution, several formerly ‘proterosuchian’ taxa from the Early Triassic were pruned from our phylogenetic analysis, but it is worth noting how each can or cannot be differentiated from Antarctanax. Although its presence is variable in specimens of Proterosuchidae (sensu Ezcurra, 2016), Tasmaniosaurus triassicus (UTGD 54655) lacks postzygodiapophyseal laminae in posterior cervical and anterior dorsal vertebrae, which are present in Antarctanax. Ezcurra (2016) found Kalisuchus rewanensis and Chasmatosuchus spp. in similar phylogenetic positions as we find Antarctanax (in polytomies with Sarmatosuchus otschevi, Fugusuchus hejiapensis, and [Erythrosuchidae + Eucrocopoda]; also with Proterosuchidae, or more crownward of Proterosuchidae depending on the iteration). After a revision by Ezcurra (2016), Kalisuchus consists of the holotypic maxilla (QM F8998) and a referred pterygoid (QM F9521) that was originally identified as a jugal, whereas other material from the Arcadia Formation could not be confidently assigned (Thulborn, 1979). Therefore, Kalisuchus and Antarctanax do not have overlapping elements that can be compared, which leaves the possibility open that the two taxa from the high latitudes of southern Pangea could be synonymous. Both species of Chasmatosuchus (C. rossicus PIN 2252/381; C. magnus PIN 951/65) possess posterior centrodiapophyseal laminae in cervical and anterior dorsal vertebrae, whereas Antarctanax does not. A final taxon to consider is Vonhuenia friederichi from the Induan Vokhmian Gorizont of Russia, with a hypodigm of two cervicodorsal vertebrae (PIN 1025/11, PIN 1025/419; Sennikov, 1992, 1995; Ezcurra, 2016) that share anatomical details with the cervicodorsal vertebrae of Antarctanax (Ezcurra, 2016:fig. 10). The orientation, presence, and shape of laminae and fossae on the neural arch, particularly the anterior centrodiapophyseal lamina, and form of the neural spine, are strikingly like those of Antarctanax. However, critical differences include a ventral keel and the presence of three foramina on the lateral surface of the centrum in Vonhuenia. Both specimens of the hypodigm have a third accessory rib facet on the anterior centrodiapophyseal lamina between the parapophysis and diapophysis, whereas other referred vertebrae lack this feature and are hypothesized to come from other regions of the vertebral column (Sennikov, 1995; Ezcurra, 2016). The two best-preserved cervicodorsals of Antarctanax do not have an accessory rib facet on their robust anterior centrodiapophyseal laminae, leaving the presence of such a facet on some Antarctanax vertebrae ambiguous.

Prolacerta broomi is the most common and widely distributed archosauromorph in the lower Fremouw assemblage, known from a minimum of 21 specimens and occurring at all six Shackleton Glacier localities, as well as at Graphite Peak (Fig. 1). Smith et al. (2012) reported only 11 specimens of P. broomi from the LAZ, but many more have been collected in recent years (Smith and Botha-Brink, 2014), meaning that future studies of P. broomi should take the Antarctic material into account to fully understand the range of morphology of the taxon. Prolacerta was first mentioned in the lower Fremouw assemblage by Colbert (1978), who noted that Antarctic specimens were smaller than Prolacerta from South Africa and possibly represented a new species. However, in his description of 17 Antarctic specimens, Colbert (1987) referred all of the material to the South African taxon Prolacerta broomi, despite differences in tooth counts and cranial proportions, which he suggested might be due to ontogenetic changes. In 2011, we recovered a Prolacerta specimen (UWBM 95529; Fig. 8; see Spiekman, in press) at Graphite Peak that is roughly equivalent in size (60+ cm body length) to LAZ Prolacerta individuals, suggesting that the specimens from the Shackleton Glacier localities most likely represent subadults. Modesto and Sues (2004) studied the cranial morphology of Karoo specimens of P. broomi and did not include data from Antarctic specimens.

A partial humerus (AMNH 24262) and a pectoral vertebra (AMNH 24264) are tantalizing examples of a larger-bodied archosauromorph in the lower Fremouw assemblage. Cosgriff (1983) recognized the specimens as large ‘thecodontian’ reptiles and suggested that the humerus in particular came from a ‘rauisuchid’ such as the Middle Triassic Stagonosuchus nyassicus or Ticinosuchus ferox, asserting that AMNH 24262 was the earliest Triassic evidence of a ‘non-proterosuchian thecodont.’ Smith et al. (2011) reconsidered the specimens with an apomorphy-based approach and concluded that AMNH 24262 was an indeterminate archosauromorph and AMNH 24264 an indeterminate archosauriform, although like Cosgriff (1983) they considered it possible that both elements came from the same archosauriform species. Specimen AMNH 24262 indicates an archosauromorph with an estimated body length greater than 3 m, which is substantially larger than the 1.5–2 m lengths of Early Triassic forms such as Proterosuchus, Tasmaniosaurus, and Kalisuchus (Camp and Banks, 1978; Thulborn, 1979; Welman, 1998; Smith et al., 2011; Ezcurra, 2014; Ezcurra and Butler, 2015), yet smaller than Middle Triassic taxa such as Erythrosuchus, which has an estimated body length of 4–5 m (Gower, 2003). Intriguingly, fragmentary remains of a similarly large-bodied archosauriform are known from the Katberg Sandstone Formation in the LAZ (NMQR 3570; Modesto and Botha-Brink, 2008). The archosauromorph represented by AMNH 24262 was likely the apex predator of the lower Fremouw assemblage and represents the largest archosauromorph from the earliest Triassic (Smith et al., 2011). At Collinson Ridge, AMNH 24264 comes from the same green-gray mudstone horizon as the dicynodonts Myosaurus gracilis and Lystrosaurus, whereas at Kitching Ridge AMNH 24262 was discovered at the same medium-grained sandstone horizon as Thrinaxodon liorhinus and 5 m above Lystrosaurus maccaigi (Fig. 1; Smith et al., 2011). The close stratigraphic position is notable as the near-coeval occurrence of a large-bodied archosauromorph, usually a member of later Mesozoic faunas, and a typically upper Permian dicynodont (Viglietti et al., 2015).

Faunal Assemblage of the Lower Fremouw Formation

Fossil collecting in the Fremouw Formation has occurred sporadically over the past 50 years or so, which is not surprising given its remote location and the short field work season available. Nonetheless, a number of expeditions have collected a reasonable sample of vertebrate fossils, comparable in taxonomic diversity to that of the Karoo (Colbert, 1970; Kitching et al., 1972; Sidor et al., 2008). Sidor et al. (2008) provided the most recent survey of the Fremouw vertebrate fossil record. An updated version of this survey is given in Table 1, with important changes noted below.

Temnospondyli

There are a minimum of four temnospondyl taxa known from the lower Fremouw Formation. The most recently identified was by Beightol et al. (2013), who published an abstract describing a small new stereospondyl skull (UWBM 95522) collected from Graphite Peak during the 2010–2011 expedition. The relatively complete specimen lacks sensory sulci, indicating a possibly terrestrial lifestyle, and was found to be sister taxon to the semiterrestrial Lapillopsidae in a phylogenetic analysis of Temnospondyli. Lapillopsids are not known from the Karoo and were previously restricted to India and Australia (Warren and Hutchinson, 1990; Yates, 1999; Yates and Sengupta, 2002).

Therapsida

There are 10 species of therapsids currently recognized from the lower Fremouw Formation (Table 1). The dicynodont Lystrosaurus is by far the most common fossil in the lower Fremouw Formation, occurring at most localities and providing the strongest biostratigraphic link to the Karoo LAZ. The occurrence of L. maccaigi in the lower Fremouw Formation (Cosgriff et al., 1982) with several taxa known only from the Triassic LAZ (L. murrayi, Thrinaxodon liorhinus, Prolacerta broomi, Procolophon trigoniceps) differs from its occurrence in the LAZ, where it is a Permian form that co-occurs with Pareiasaurus, Dicynodon, Daptocephalus, Theriognathus, and gorgonopsids (Viglietti et al., 2015).

Kombuisia antarctica is a dicynodont described by Fröbisch et al. (2010) on the basis on two skulls, which had been previously mentioned by Colbert (1975). The genus Kombuisia was previously only known from the Middle Triassic Cynognathus Assemblage Zone, of the main Karoo Basin (Hotton, 1974), and had been hypothesized to be a sister taxon to the well-known kingoriid Dicynodontoides (Angielczyk, 2007; Fröbisch, 2007) from the upper Permian of South Africa, Zambia, and Tanzania. The discovery of a new species of Kombuisia in the Lower Triassic lower Fremouw Formation reduced the ghost lineage between Dicynodontoides and Kombuisia frerensis but also illustrated another incongruity between the LAZ and the lower Fremouw in that Kombuisia appears stratigraphically lower in Antarctica.

Two clades of therocephalian therapsids are present in the lower Fremouw Formation and the LAZ: Baurioidea and Akidnognathidae. Colbert (1975) mentioned ‘Scaloposaurus constrictus’ in arguments for connectivity between Africa and Antarctica before Colbert and Kitching (1981) published on five therocephalian specimens from the lower Fremouw. A pair of pterygoids and a mandible (AMNH 9542) and a pair of hind limbs with the pelvic girdle (AMNH 9550) were referred to Ericiolacerta parva, and two new taxa were erected: Pedaeosaurus parvus, based on a skull and hind limb material (AMNH 9548), and Rhigosaurus glacialis, based on a partial skull and lower jaw (AMNH 9525). Finally, Colbert and Kitching (1981) considered an associated right maxilla, jugal, and dentary (AMNH 9527) to be from an indeterminate theriodont. Huttenlocker and Sidor (2012) reconsidered the therocephalians of the lower Fremouw in a phylogenetic context, acknowledging the poor preservation of the specimens and the advances in therocephalian systematics and character polarity since their initial description. Both Pedaeosaurus parvus and Rhigosaurus glacialis were found to be based on material only referable to Baurioidea, relegating the two genera to nomina dubia. The cranial elements (AMNH 9542) originally referred to Ericiolacerta were still tentatively considered a true occurrence (cf. Ericiolacerta parva). Colbert and Kitching’s (1981) indeterminate theriodont (AMNH 9527) was found to be an indeterminate akidnognathid by Huttenlocker and Sidor (2012), who confirmed the presence of both Baurioidea and Akidnognathidae in Antarctica. Specimen AMNH 9550 was found to be an indeterminate eutherocephalian by Huttenlocker and Sidor (2012; Eutherocephalia contains both Baurioidea and Akidnognathidae).

Cynodont therapsids are most commonly represented by Thrinaxodon liorhinus in the lower Fremouw Formation (Colbert and Kitching, 1977), although two specimens hint at greater diversity: (1) AMNH 9523, an impression of a mandibular ramus with a heavy coronoid process and large, peg-like teeth; and (2) AMNH 9554, a skull and skeleton of a likely galesaurid-grade cynodont twice the linear dimensions of typical Thrinaxodon specimens. Colbert and Kitching (1977) considered AMNH 9523 to have clear gomphodont affinities and stated that it is certainly not referable to Thrinaxodon. Without well-preserved teeth, it is difficult to determine whether AMNH 9554 represents a new species of Thrinaxodon or a new species of large galesaurid cynodont.

Correlation and Inconsistencies with the Lystrosaurus Assemblage Zone

The Lystrosaurus Assemblage Zone (LAZ) has been heavily investigated and densely sampled in order to understand the immediate effects of the end-Permian mass extinction (Smith, 1995; Smith and Ward, 2001; Ward et al., 2005; Nicolas and Rubidge, 2010; Smith et al., 2012; Smith and Botha-Brink, 2014; Gastaldo et al., 2015). However, research conducted near the upper limits of the LAZ indicates that several well-known LAZ taxa are not contemporaneous with one another (see below; Neveling, 2004; Botha and Smith, 2006; Smith and Botha-Brink, 2014).

Since its initial discovery, the lower Fremouw assemblage has been considered to represent a subset of LAZ taxa (Colbert, 1970; Elliot et al., 1970; Kitching et al., 1972). However, the lower Fremouw assemblage contains unique taxa, and co-occurrences of species not seen in the LAZ, despite much less sampling (Colbert and Kitching, 1977; Cosgriff et al., 1982; Cosgriff and Hammer, 1984; Gow, 1999; Sidor et al., 2008; Fröbisch et al., 2010; Smith et al., 2011; Beightol et al., 2013). Both of these observations suggest to us that (1) the lower Fremouw vertebrate assemblage is not a taxonomic subset of the LAZ, and (2) the Antarctic assemblage can be biostratigraphically tied to the lower reaches of the LAZ but is somewhat removed from the Permo-Triassic boundary.

Recently, Smith and Botha-Brink (2014) divided the uppermost Daptocephalus AZ and lowermost LAZ into several phases describing diversity dynamics in the vertebrate fossil record across the end-Permian mass extinction event. Their ‘Recovery Phase 1’ was 5–25 m above the Permo-Triassic boundary (PTB) and saw the first appearance of several Triassic taxa (i.e., Proterosuchus, Lystrosaurus murrayi, L. declivis). Importantly, Thrinaxodon liorhinus, Prolacerta broomi, and Procolophon trigoniceps were not found as part of ‘Recovery Phase 1’ in the LAZ, lending credence to our proposal that the lower Fremouw assemblage is temporally removed from the immediate wake of the PTB. Procolophon trigoniceps in particular is not associated with the PTB, with its first appearance, and high abundance, 116 m above the boundary in the Katberg Sandstone used to sometimes characterize an assemblage after the Lystrosaurus disaster fauna (Botha and Smith, 2006; Procolophon Zone of Broom, 1906, and Watson, 1914; Procolophon Abundance Zone of Neveling, 2004). A further circumstantial line of evidence is the absence of Proterosuchus from the lower Fremouw assemblage, which is only present in the LAZ 5–14 m above the PTB (Botha and Smith, 2006; Smith and Botha-Brink, 2014). Despite sharing several tetrapod taxa (Lystrosaurus murrayi, Lystrosaurus curvatus, Thrinaxodon liorhinus, Prolacerta broomi, Procolophon trigoniceps) with the LAZ of the Karoo Basin, it is clear that the lower Fremouw assemblage is not simply a subset of what is present in South Africa.

The lower Fremouw assemblage contains a combination of taxa with nonoverlapping stratigraphic distributions in the main Karoo Basin (Fig. 10). Both L. maccaigi and L. curvatus are present before Thrinaxodon, Prolacerta, and Procolophon in the LAZ, with L. maccaigi originating in the Permian upper Daptocephalus AZ and extending 10 m above the PTB into the lowermost LAZ. Thus, both L. maccagi and L. curvatus cross the PTB and occur alongside early ‘Recovery Phase 1’ fauna (Botha and Smith, 2006, 2007; Smith and Botha-Brink, 2014; Viglietti et al., 2015). Fröbisch et al. (2010) suggested that perhaps Antarctica served as a high-latitude refuge during the mass extinction, allowing taxa such as L. maccaigi to persist into the Triassic. Conversely, lower Fremouw taxa such as Kombuisia, Palacrodon browni, and brachyopoid temnospondyls do not show up in the Karoo record until the Cynognathus AZ (see Sidor et al., 2008).

FIGURE 10. Biostratigraphic comparison of the South African assemblage zones (Karoo Basin) and the Antarctic Fremouw Formation (Beacon Basin). Stratigraphic ranges of nine species and one genus common to both basins are plotted (ranges apply to the Karoo Basin and are based on the work of Botha and Smith, 2006; Smith and Botha-Brink, 2014; Viglietti et al., 2015). Despite much less sampling, the lower Fremouw assemblage is best correlated with the Lystrosaurus Assemblage Zone, but after Extinction Phase 3 and Recovery Phase 1 of Smith and Botha-Brink (2014). The emydopoid Kombuisia is shared at the genus level. Abbreviations: CAZ, Cynognathus Assemblage Zone; DAZ, Daptocephalus Assemblage Zone; LAZ, Lystrosaurus Assemblage Zone; RP1, Recovery Phase 1; u, upper.

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An additional discrepancy in biostratigraphy between the lower Fremouw and LAZ warrants attention: AMNH 9523, identified as a possible gomphodont cynodont. Gomphodontia is a clade of cynognathian cynodonts, including Diademodon tetragonus, Trirachodontidae, and Traversodontidae. Colbert and Kitching (1977) were able to determine that a large, massive dentary ramus from Antarctica (AMNH 9523) was not Thrinaxodon and indeed resembled South African gomphodont forms from the Cynognathus AZ. In the main Karoo Basin, gomphodonts do not appear until the Cynognathus AZ subzone A (viz., Langbergia modisei; Abdala et al., 2006) before radiating and becoming significant components of Middle and Late Triassic faunas (Ruta et al., 2013). Similarly, in northern Pangea, early gomphodonts are known from upper Lower Triassic rocks, but with less stratigraphic certainty (Beishanodon youngi; Gao et al., 2010). If AMNH 9523 is indeed a gomphodont, it would be the earliest known occurrence not only of Gomphodontia, but of Eucynodontia.

Our interpretation of the biostratigraphic position of the lower Fremouw assemblage revolves around the co-occurrence of common taxa shared with the LAZ well above the PTB (Thrinaxodon liorhinus, Prolacerta broomi, Procolophon trigoniceps, Lystrosaurus murrayi). Other biostratigraphically relevant taxa pose the opposing problems of occurring either earlier (L. maccaigi, L. curvatus) or later in the Karoo (Palacrodon browni, Kombuisia), which reinforces the conservative conclusion that the assemblage is indeed best correlated with the aforementioned biostratigraphic position. However, the lower Fremouw assemblage is not simply a subset of the LAZ, because it possesses both unique forms (Antarctanax shackletoni; the large archosauriform) and lineages not shared with the LAZ (Lapillopsidae; early members of Gomphodontia, Brachyopoidea, Palacrodon, and Kombuisia). Discrepancies in faunal makeup and stratigraphic ranges will hopefully be clarified in the future with continued sampling in the Karoo Basin, and especially in the Transantarctic Mountains.

CONCLUSIONS

Antarctanax shackletoni is a novel archosauriform taxon and the third archosauromorph known from the lower Fremouw Formation, Transantarctic Mountains, reinforcing the rapid diversification of Archosauromorpha during Early Triassic times. Phylogenetically, Antarctanax falls within a grade of archosauriforms more crownward than Proterosuchidae and stemward of Erythrosuchidae and Eucrocopoda, alongside Sarmatosuchus otschevi, Fugusuchus hejiapensis, and Cuyosuchus huenei, and possibly the more fragmentary taxa Kalisuchus rewanensis, Chasmatosuchus spp., and Vonhuenia friederichi. Refined biostratigraphy of the Lystrosaurus Assemblage Zone of the Karoo Basin, South Africa, leads us to hypothesize that the lower Fremouw assemblage is temporally removed from the end-Permian mass extinction event and the immediate disaster faunas found in the LAZ. Continued field work and study of the lower Fremouw vertebrate assemblage reveals increased differences between its fauna and the fauna of the Lystrosaurus Assemblage Zone. Dissimilarities in the two fossil assemblages are manifest in the form of endemic taxa in Antarctica, lineages uniquely shared between Antarctica and Australia, and some biostratigraphic discrepancies between Antarctica and the Karoo. Together, these findings undercut the idea that the Antarctic assemblage is a less-well-sampled subset of the LAZ and instead argue for the lower Fremouw assemblage possessing a unique high-latitude community.

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A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction
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A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction
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ACKNOWLEDGMENTS

Field work in 2010–2011 was supported by NSF-ANT 1146399, with additional research supported by ANT 1341304 to C.A.S. Comparative work was conducted thanks to an American Museum of Natural History Collection Study Grant, NSF Doctoral Dissertation Improvement Grant [Grant 1501097], NSF Graduate Research Fellowship, and Washington Research Foundation Benjamin Hall Fellowship to B.R.P. We especially thank M. Turner (Brown University) for her drawings in Figures 5 and 7, and L. Herzog (North Carolina Museum of Natural Sciences) and B. Crowley (Burke Museum) for their expert preparation work. Comments and insights from A. Pritchard and R. Sookias improved the manuscript, as did help from editor L. Zanno.

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