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Protungulatum

Protungulatum is an extinct genus of small, primitive eutherian mammals known from dental and cranial remains dating to the latest Cretaceous (Maastrichtian) and earliest Paleocene (Puercan) of North America, representing one of the earliest known members of the placental mammal radiation.^[1] The genus is classified within the extinct family Protungulatidae and is often regarded as an archaic ungulate or "condylarth," though its exact phylogenetic position remains debated, with some analyses placing it as a stem eutherian or basal to crown Placentalia.^[2] Named "first ungulate" for its early occurrence and primitive ungulate-like dental features, Protungulatum provides key evidence for the pre-K-Pg boundary origins of placental mammals.^[3] Several species have been described, including the type species Protungulatum donnae, originally from the Bug Creek Anthills locality in northeastern Montana, based on isolated teeth including premolars and molars that exhibit low-crowned (bunodont) morphology with rounded cusps, small conules lacking well-developed crests, and a trigon basin suited for an omnivorous or insectivorous diet (see "Described species" for details).^[3] These dental traits include a p4 lacking a deep talonid basin and posterior transverse crest, with the protoconid only slightly larger than the paraconid and metaconid, while molars feature convex cristids, labially inclined protocones, and procumbent hypoconulids on m3.^[2] One such species, Protungulatum coombsi, was identified from an isolated upper premolar in the Hell Creek Formation of southeastern Montana, approximately 300,000 years before the K-Pg boundary, measuring 21% longer and 18% wider than comparable teeth of P. donnae, yet morphologically similar.^[1] Fossils of Protungulatum are significant for challenging earlier views that placental mammals diversified only after the end-Cretaceous mass extinction, instead supporting a Cretaceous origin for Eutheria around 108–72 million years ago.^[4] Micro-CT studies of its inner ear reveal primitive therian features, such as low cochlear coiling and a low aspect ratio, specialized for high-frequency hearing, further highlighting its basal position among eutherians and potential affinities with early Pan-Euungulata.^[5] Overall, Protungulatum underscores the rapid evolutionary diversification of mammals across the K-Pg transition.^[1]

Taxonomy and nomenclature

Etymology

The genus name Protungulatum derives from the Greek prōtos ("first") and the Latin ungulatum ("hoofed animal"), emphasizing its recognition as an early representative of ungulate-like mammals in Paleocene faunas. This etymological choice underscores the taxon's primitive dental and skeletal traits suggestive of basal ungulate affinities, as interpreted in its initial description. Species epithets within Protungulatum predominantly honor individuals associated with Paleocene fossil discoveries. The type species P. donnae commemorates Donna Sloan Gillette for her discovery of the holotype specimen. P. coombsi recognizes Walter Coombs, who identified the Spigot-Bottle locality yielding key material.^[1] P. gorgun is named after "gorgûn," the Woses' name for orcs in J.R.R. Tolkien's The Lord of the Rings.^[6]^[7] P. mckeeveri pays tribute to a field collector instrumental in early excavations (now considered a synonym of P. donnae).^[8] P. sloani acknowledges Robert E. Sloan, a prominent contributor to studies of early Cenozoic mammals.^[9] In the context of Paleocene mammal taxonomy from the 1960s to 1980s, such eponymous naming was common practice among researchers like Sloan, Van Valen, and Archibald, who described numerous taxa from North American sites to credit collaborators and advance systematic frameworks for post-Cretaceous faunal radiations.^[10]

Classification

Protungulatum is an extinct genus of eutherian mammal assigned to the family Protungulatidae, a group of early ungulate-like forms erected by Chatterjee, Scotese, and Bajpai in 2017 to accommodate basal taxa exhibiting primitive dental and postcranial features suggestive of early placental evolution.^[11] This family distinguishes Protungulatum from earlier classifications and highlights its role as a transitional form near the base of placental mammal diversification. No synonyms are recognized for the genus itself.^[12] Upon its initial description in 1965, Protungulatum was placed by Sloan and Van Valen within the subfamily Oxyclaeninae of Arctocyonidae, a family of Paleocene "condylarths" characterized by bunodont dentition and inferred omnivorous habits.^[3] Subsequent taxonomic revisions, informed by additional fossil material and phylogenetic analyses, have removed it from Arctocyonidae, emphasizing instead its distinct morphology and stratigraphic position as a Cretaceous-Paleogene boundary taxon.^[1] In higher-level classification, Protungulatum falls within the subclass Eutheria and is frequently interpreted as a stem placental mammal or basal crown placental, potentially allied with the earliest members of Placentalia based on shared derived traits such as tribosphenic molars adapted for grinding. Phylogenetic studies have variably positioned it within Pan-Euungulata, a clade encompassing true ungulates (Euungulata) and their stem relatives, underscoring its affinities to the radiation of hoofed mammals following the Cretaceous-Paleogene extinction.^[13] The genus comprises four valid species: the type species P. donnae (Sloan & Van Valen, 1965; including junior synonym P. mckeeveri Archibald, 1982), P. sloani and P. gorgun (Van Valen, 1978), and P. coombsi (Archibald et al., 2011).^[3]^[12]^[1]

Physical description

Cranial and dental features

The holotype of Protungulatum donnae consists of a partial left dentary preserving the last lower premolar and first two molars, characterized by bunodont cusps and low crowns typical of early eutherian mammals.^[3] These molars exhibit rounded, bulbous cusps arranged in a quadrate pattern, with minimal shearing crests and a broad talonid basin suited for processing varied food items. The dental formula of Protungulatum is inferred to be 3.1.4.3/3.1.4.3, reflecting a primitive eutherian condition with four premolars and three molars in each jaw quadrant. The premolars are simple and conical, transitioning to more complex molars with small stylar shelves on the uppers and expanded talonids on the lowers, indicating adaptations for grinding and crushing rather than slicing. Limited cranial fragments, including petrosal elements, suggest a small braincase relative to body size, with a compact bony labyrinth and encephalization quotient exceeding 0.25, consistent with early placental morphology.^[5] The jaw appears robust, with a straight horizontal ramus and evidence of strong masseteric attachment, supporting an inferred insectivorous or omnivorous diet based on dental wear patterns and cusp morphology. Among described species, P. coombsi exhibits dental variation, with upper premolars and molars approximately 20% larger in linear dimensions than those of P. donnae, based on comparative measurements of multiple specimens (e.g., mean upper premolar length 3.85 mm in P. coombsi versus 3.06 mm in P. donnae).^[1] This size difference may reflect ecological or stratigraphic distinctions, though cusp and crown proportions remain similar across species.^[1]

Postcranial elements

The postcranial skeleton of Protungulatum is known from scattered, isolated elements recovered primarily from Bug Creek Anthills localities in the Hell Creek Formation of eastern Montana, including humeri and tarsal bones attributed to the genus.^[14]^[15] These remains indicate a small-bodied, quadrupedal eutherian with terrestrial adaptations, lacking specialized features of later ungulates.^[14] Humeri tentatively referred to Protungulatum (morphotype EuC) are characterized by a robust entepicondyle, deep olecranon fossa, and a spindle-shaped capitulum approximately three times the width of the trochlea, with a total distal width of 9.52 mm.^[14] These features suggest semifossorial capabilities, including powerful wrist and digit flexors for grasping and potential scratch-digging behaviors, consistent with a terrestrial lifestyle.^[14] Tarsal elements, such as the astragalus and calcaneus, exhibit restricted ankle joint mobility due to fibula-calcaneus contact and an ovoid, oblique calcaneocuboid facet, further supporting quadrupedal locomotion on the ground rather than arboreal habits.^[15] No complete vertebrae or phalanges have been described, but the available limb bones imply short limbs with claw-like terminal phalanges, indicative of burrowing or scratching tendencies without evidence of hoof development.^[14]^[15] Body size estimates for P. donnae, the type species, range from 6 to 245 g based on skeletal reconstructions and character scoring in phylogenetic analyses.^[16] P. coombsi is estimated to be larger, approximately 20% bigger than P. donnae based on comparative dental dimensions that correlate with overall size, potentially reaching up to around 120 g, though direct postcranial measurements are limited.^[1] These sizes place Protungulatum among the smaller early Paleocene eutherians, emphasizing its primitive build. The overall body plan of Protungulatum reflects a generalized eutherian morphology, with robust proximal limb elements suited for weight-bearing in a quadrupedal gait but without elongated metapodials or other cursorial specializations seen in derived ungulates.^[14]^[15] This primitive configuration aligns with basal placental ancestors, prioritizing versatility for terrestrial environments over specialized locomotion.^[16]

Discovery and species

Initial discovery

The genus Protungulatum was first identified from fossils recovered at the Bug Creek Anthills locality in the Hell Creek Formation, northeastern Montana, USA. These early Paleocene deposits, dated to approximately 66 million years ago, yielded the initial specimens during fieldwork conducted primarily by Robert E. Sloan in the early 1960s.^[3] The site, part of a series of channel-fill deposits in the valley of Bug Creek, produced a diverse assemblage of microvertebrate remains, including mammals, that were initially interpreted as representing a Late Cretaceous (Lancian) fauna due to the presence of reworked dinosaur bones and sediments. In 1965, Sloan and Leigh Van Valen formally described and named the genus Protungulatum based on a fragmentary lower jaw (dentary) preserving the posterior three premolars (p2–p4) and three molars (m1–m3), designated as the holotype specimen SPSM 62-2028 and representing the type species P. donnae. This description appeared in a seminal paper published in Science, where the authors highlighted the primitive ungulate-like dental features and positioned the taxon within the broader context of post-Cretaceous-Paleogene (K-Pg) boundary survivor mammal assemblages, akin to those in the southern "Alamosaurus fauna" of the Lancian stage. The Bug Creek Anthills material suggested a transitional fauna bridging Cretaceous and Paleocene mammal radiations, with Protungulatum emerging as an index fossil for this interval.^[3] Challenges arose from the site's complex geology, characterized by reworked sediments that mixed latest Cretaceous vertebrate remains—such as dinosaur fragments—with unambiguously Paleocene mammals, leading to debates over the precise age of the Protungulatum fossils. Initial assignments to the Late Cretaceous were based on stratigraphic position below the Hell Creek-Tullock formational boundary and associations with Lancian taxa.^[3] These uncertainties were resolved in the 1980s through integrated biostratigraphic and magnetostratigraphic analyses, which placed the Bug Creek Anthills in magnetochron C29r and confirmed an early Puercan (early Paleocene) age for the primary depositional event, with Cretaceous elements attributed to erosion and redeposition from underlying strata.

Described species

In addition to the type species Protungulatum donnae, four other species have been described within the genus, primarily based on dental remains from early Paleogene and latest Cretaceous deposits in North America.^[1] Protungulatum sloani was named by Van Valen in 1978 based on lower dentition from the Hell Creek Formation at Harbicht Hill, Montana, and is distinguished from P. donnae by subtle differences in molar cusp patterns, such as a more pronounced paracristid; it is similar in overall size to the type species.^[1] Protungulatum gorgun, also erected by Van Valen in 1978 from the same Montana locality, is larger than P. sloani and P. donnae, with upper molars approximately 8–17% larger and featuring more robust ectoloph crests on molars.^[1] Protungulatum mckeeveri was proposed by Archibald in 1982 for additional isolated teeth from the Bug Creek Anthills locality in the Hell Creek Formation, Montana, noted for minor size variations and enamel features relative to P. donnae, though later studies have suggested it falls within the morphological variation of the type species.^[17]^[1]^[18] The most recent species, Protungulatum coombsi, was described by Archibald et al. in 2011 from an isolated upper fourth premolar (holotype OMNH 64985, housed at the Oklahoma Museum of Natural History) collected at the Spigot-Bottle locality in the Hell Creek Formation, Carter County, Montana; this specimen is notably larger (21% longer and 18% wider) than comparable teeth of P. donnae, with a more inflated buccal surface.^[1] All described species of Protungulatum are known exclusively from the Puercan North American Land Mammal "Age" (early Paleocene) in Montana, with P. coombsi extending the record into the latest Maastrichtian (Late Cretaceous) approximately 300,000 years before the K-Pg boundary; no post-Puercan occurrences have been documented, and the genus is restricted to western North America, including referred material from the Ravenscrag Formation in Saskatchewan.^[1]^[19]

Paleoecology

Geological settings

The fossils of Protungulatum are primarily known from the Hell Creek Formation in eastern Montana, United States, which dates to the Puercan North American land-mammal age spanning approximately 66 to 63.8 million years ago (Ma). This formation consists predominantly of fluvial and floodplain deposits, including fine-grained sandstones, siltstones, and mudstones that represent ancient river channels, crevasse splays, and overbank environments in a coastal plain setting. The type locality for P. donnae, the Bug Creek Anthills in McCone County, was initially interpreted as Late Cretaceous (Maastrichtian) based on associated dinosaur remains and palynological data, but subsequent discovery of an iridium enrichment layer—the geochemical signature of the Cretaceous-Paleogene (K-Pg) boundary—immediately below the fossil-bearing paleochannel confirmed its placement in the early Paleocene.^[20]^[21]^[22] Secondary occurrences of Protungulatum have been reported from the Ravenscrag Formation in southwestern Saskatchewan, Canada, which is also of early Puercan age and overlies the Late Cretaceous Frenchman Formation. The Ravenscrag Formation comprises nonmarine clastic sediments, including claystones, siltstones, and sandstones interbedded with coal-bearing layers that indicate deposition in a low-energy floodplain and swamp environment under a mesothermal, humid subtropical climate with year-round precipitation. These coal seams reflect lush, vegetated wetlands that supported early post-extinction recovery ecosystems.^[23] In these formations, Protungulatum co-occurs with other early eutherian mammals such as Purgatorius—a plesiadapiform often considered part of the post-K-Pg recovery biota—alongside multituberculates, metatherians, and rare reptilian holdovers, reflecting the rapid diversification of small mammals in the wake of the end-Cretaceous mass extinction. The Puercan faunas from both the Hell Creek and Ravenscrag formations document transitional ecosystems dominated by opportunistic, insectivorous taxa adapting to disturbed, vegetated floodplains shortly after the K-Pg boundary.^[24]^[1]

Inferred biology

Protungulatum, as one of the earliest known eutherian mammals, is inferred to have had an insectivorous diet based on its dental morphology, which includes low-crowned, bunodont molars suited for crushing and grinding small, hard foods such as insects and possibly seeds.^[16]^[25] The absence of specialized shearing crests or high-crowned teeth indicates no adaptations for herbivory, aligning with the opportunistic feeding strategies of small Paleocene mammals in resource-scarce post-extinction environments.^[25] Locomotion in Protungulatum is inferred as scansorial (capable of climbing), consistent with the reconstructed lifestyle of early placental mammals. Its body size, estimated at 6–245 grams, and limb proportions indicate a versatile, possibly nocturnal lifestyle, enabling evasion of predators and exploitation of burrows or ground cover in early Paleocene forests.^[16] As a eutherian mammal, Protungulatum is inferred to have been viviparous, with reproductive traits including a hemochorial placenta, a two-horned uterus, and production of single young born hairless with closed eyes, consistent with the early radiation of placental mammals shortly after the Cretaceous-Paleogene extinction.^[16] Direct fossil evidence for reproduction is lacking, but these features position it within the foundational diversification of viviparous eutherians adapting to vacated ecological niches.^[16] Ecologically, Protungulatum served as an opportunistic survivor in the disrupted post-K-Pg ecosystems, contributing low biomass as a small insectivore that likely filled insect-based trophic roles left open by the mass extinction of non-avian dinosaurs and other vertebrates.^[16] Its persistence across the boundary highlights adaptability in a landscape with reduced competition and abundant invertebrate resources, facilitating the initial placental radiation.^[16]

Phylogeny

Evolutionary relationships

Protungulatum occupies a basal position within the clade Pan-Euungulata, serving as the sister taxon to crown Euungulata, which comprises perissodactyls and artiodactyls, based on a comprehensive analysis integrating morphological and molecular data from fossil and extant mammals.^[26] This placement highlights its role as an early diverging member of ungulate-like eutherians, assigned to the monotypic family Protungulatidae. In cladograms derived from such studies, Protungulatum forms the most basal branch of Pan-Euungulata, preceding a polytomy of other extinct taxa like Apheliscus and Hyopsodus, thereby underscoring its primitive status relative to more derived ungulates.^[26] Supporting this phylogenetic hypothesis are shared derived traits, including aspects of molar structure such as the postparaconule crista on M1, which align Protungulatum with synapomorphies of Pan-Euungulata and early ungulates. Although it exhibits dental similarities to Arctocyonidae—such as relatively simple, bunodont molars with a widened talonid—these features do not warrant inclusion in that family, distinguishing Protungulatum as a separate lineage near the base of ungulate evolution.^[26]^[27] As one of the earliest documented eutherians postdating the Cretaceous-Paleogene (K-Pg) boundary extinction event, dating to approximately 66 million years ago in the Puercan North American Land Mammal Age, Protungulatum bridges the evolutionary transition from Mesozoic eutherians to Paleogene placental diversification. Its temporal position emphasizes the rapid radiation of eutherian mammals in the wake of the K-Pg mass extinction, providing critical evidence for post-boundary faunal recovery. While its eutherian affinities are well-supported, ongoing debates center on whether Protungulatum represents a crown placental or a stem eutherian outside Placentalia proper.^[27]^[26]

Debates on affinities

The classification of Protungulatum as either a crown placental mammal or a stem eutherian has been debated since its initial description, with early interpretations favoring its placement within Placentalia based on primitive ungulate-like dental and skeletal features suggestive of basal condylarths. Sloan and Van Valen (1965) described P. donnae from Paleocene deposits in Montana as an archaic ungulate, aligning it with early placentals due to its small size, sectorial carnassial teeth, and simple molars that hinted at herbivorous adaptations emerging from insectivoran ancestors. However, subsequent analyses highlighted its primitive traits, such as unreduced dentition and lack of specialized placental synapomorphies, leading to views of it as a stem eutherian outside the crown group.^[1] Archibald et al. (2011) confirmed a genuine Late Cretaceous occurrence of Protungulatum in Montana, arguing that while it exhibits eutherian characteristics, its mosaic morphology—combining insectivore-like shearing teeth with early ungulate bunodonty—precludes definitive crown placental status and suggests a transitional form predating the K-Pg boundary.^[1] Central to the debate is the absence of direct fossil evidence for placentation, a key placental trait, which cannot be preserved in the skeleton and must be inferred from correlated hard-tissue features like pelvic morphology or cranial specializations; Protungulatum lacks unambiguous markers of these, retaining primitive eutherian conditions such as a simple basicranium and non-derived auditory bullae. Its dental mosaic further fuels uncertainty, with upper molars showing small, unspecialized conules and lower molars featuring a protocone that blends carnivorous shearing (reminiscent of Cretaceous insectivorans like Cimolestes) and incipient grinding surfaces akin to Paleocene ungulates, positioning it as a potential bridge between generalized eutherians and derived herbivores.^[25] This ambiguity has led to conflicting phylogenetic placements, with some analyses rooting it near the base of Euungulata (a laurasiatherian clade including artiodactyls and perissodactyls) as a stem placental, while others exclude it from Placentalia altogether. More recent studies have refined but not resolved these tensions. O'Leary et al. (2013) incorporated Protungulatum into a comprehensive morphological dataset, recognizing it as the earliest crown placental.^[28] In contrast, Halliday et al. (2016) reanalyzed Paleocene taxa with expanded sampling and found Protungulatum as a stem eutherian outside Placentalia, emphasizing its primitive dentition and arguing against Cretaceous crown origins.^[25] A 2022 study combining morphological and molecular data resolved Protungulatum within crown Placentalia as the basalmost member of Pan-Euungulata.^[26] Molecular clock estimates complicate this further, often dating placental origins to the Late Cretaceous (around 80–100 Ma) using genomic data from modern taxa, yet these conflict with the fossil record where Protungulatum represents the sole pre-K-Pg candidate; if classified as stem, it implies no crown placentals crossed the boundary, questioning the accuracy of relaxed-clock models that assume hidden early diversification. A 2023 Bayesian analysis supported Late Cretaceous origins for Placentalia but with ordinal crown groups originating at or after the K-Pg boundary.^[29]^[30] These debates carry broader implications for understanding mammalian evolution across the K-Pg extinction. Affirming Protungulatum as a stem eutherian supports the "explosive model" of Paleocene radiation, where crown placentals underwent rapid adaptive diversification post-extinction to fill niches vacated by non-avian dinosaurs, rather than a gradual Cretaceous buildup.^[1] Additionally, concerns over potential reworking of Cretaceous specimens—where Paleocene fossils might erode into older strata—have been addressed by stratigraphic and taphonomic evidence confirming in situ deposition, but lingering doubts affect confidence in pre-K-Pg timings and reinforce the view of Protungulatum as a pivotal, yet enigmatic, taxon in eutherian origins.^[1]

References

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