Danian Research Articles

Unique C‐Ring Methylated Hopene Biomarkers in a Paleocene Lake

AbstractHopanoids are bacterial lipids that are ubiquitous on the surface of the Earth and in the sedimentary record. Hopanoids with an extra methyl group located in the A‐ring (at the C‐2 or C‐3 position) of the carbon skeleton are well known and these 2‐ or 3‐methylhopanoids have been used to infer specific biogeochemical or environmental conditions in the geological past, including the Paleogene. However, hopanoids with additional methylations outside the A‐ring and/or more than one additional methylation in their pentacyclic ring system are rare. This paper reports the occurrence of hop‐17(21)‐enes methylated in the C‐ring (tentatively assigned to the C‐12 position) and dimethylated hop‐17(21)‐enes with methylations at both C‐3 (A‐ring) and C‐12 (C‐ring) in sediments from the Boltysh Core, Ukraine, that represents an early Paleocene (∼65 Myr) lake system. These (di)methyl hop‐17(21)‐enes are 1–2 orders of magnitude more abundant compared to regular hopanes in some samples. The occurrence of these unique hopanoids suggests abnormal environmental conditions during their deposition, likely attributable to volatile ecosystems in the aftermath of the Cretaceous ‐ Paleogene boundary extinction and/or the environmental stressors associated with the lower C29n hyperthermal (65.3 Myr) that is recorded in this section.

On the genus Vanikoropsis Meek, 1876 (Gastropoda, Caenogastropoda) from the Paleocene of Denmark and West Greenland with descriptions of three new species

The predominantly Cretaceous gastropod genus Vanikoropsis Meek, 1876 is represented in the Paleocene of Denmark and West Greenland by four species, of which three are established herein as new, viz. Vanikoropsis mortenseni n. sp., Vanikoropsis (s.l.) jakobseni n. sp. and Vanikoropsis (s.l.) bashforthi n. sp. The Danish species was found in a boulder of Kerteminde Marl (Selandian, middle Paleocene) from Gundstrup, while the species from West Greenland were found in the localities Sonja Lens and Qaarsutjægerdal on the Nuussuaq peninsula (late Danian, early Paleocene). The Danish species extends the stratigraphic range of the genus into the middle Paleocene and supports the affinities of the Kerteminde Marl fauna to the Paleocene fauna of West Greenland.

Biostratigraphy, paleoenvironment and paleoclimate in northeastern Patagonia during the early Paleocene based on the micropaleontological record

Biostratigraphy, paleoenvironment and paleoclimate in northeastern Patagonia during the early Paleocene based on the micropaleontological record

Palaeoenvironmental interpretation and isotope stratigraphy of the Campanian–Danian successions in the Zagros Foreland Basin (Kabir Kuh, Lurestan, Iran)

The successions of the Gurpi Formation and the lower Pabdeh Formation were examined through analyses of lithology, sequence stratigraphy, stable isotope stratigraphy, and ichnofossils in the Jahangir Abad and Abhar sections of the Kabir Kuh anticline within the Lurestan Zone. Stratigraphic correlation with the well-dated Gandab section, located within the same anticline, indicates a late Campanian to early Danian age for these successions. This correlation further confirms the presence of notable carbon isotope excursions associated with the Campanian-Maastrichtian Boundary Event (CMBE) and Mid-Maastrichtian Event (MME). Field observations, along with sedimentological and paleontological analyses, suggest that the Kabir Kuh area was situated within the fore-bulge of the Zagros foreland basin during the Late Cretaceous. Changes in lithology, fossil content, and ichnofacies indicate a significant decrease in water depth at the end of the Campanian and the beginning of the Maastrichtian. This interval contains shallow marine macrofossils, including oysters (mainly Lopha), benthic foraminifers, and fossil debris traces, indicative of a high-energy environment. In contrast, shallow-water facies are not recorded within the middle and late Maastrichtian sequences. These sequences are composed of pelagic mud-dominated facies containing deep-sea ichnofossils, indicating a rapid increase in water-depth during the middle–late Maastrichtian. The observed late Campanian to early Maastrichtian shallowing is associated with the development of the Zagros fore-bulge, while the subsequent deepening and significant sediment deposition during the mid to late Maastrichtian are attributed to the subsidence of the basin. This research re-evaluates and refines the existing sequence stratigraphic and sedimentary models of the Zagros foreland basin, offering new perspectives on its geological evolution.

Oldest menispermaceous endocarp fossil from the Deccan Intertrappean Beds of Central India and its biogeographic implications

Palaeosinomenium Chandler, a fossil genus of the Menispermaceae, has a characteristic endocarp morphology that is easily identified in the fossil floras. Palaeosinomenium was widely distributed in the Paleogene, based on reports from North America, Europe, and East Asia, but was not known from South Asia. Here, we recognize a new fossil-species, Palaeosinomenium indicum sp. nov., based on an endocarp mold and locule cast from the latest Maastrichtian (late Cretaceous)-earliest Danian (early Paleocene) sediments (c. 66–65 Ma old) of the Deccan Intertrappean beds of the Umariya site in Madhya Pradesh, Central India. This new species is characterized by a horseshoe-shaped endocarp with a broad-rounded dorsal margin, almost equal limbs, and a nearly straight ventral margin. It has one dorsal ridge, shallowly concave lateral faces, and a pair of c-shaped lateral ridges one on each side of the plane of bisymmetry, and about 19 radially aligned dorsal ribs. This new finding is the earliest fossil fruit evidence of Menispermaceae indicating an early occurrence of the family in Gondwana.

Northward drift of the Burma Terrane with India during the Cenozoic and implications for the India–Asia collision

The past location of the Burma Terrane during the convergence of the Indian and Asian tectonic plates is key to unravelling the regional geodynamic, palaeoenvironmental and palaeobiogeographical history of the eastern edge of the Himalayan orogen. Palaeomagnetic data provide the ability to constrain the location of the Burma Terrane, but it has been very difficult to find rocks with palaeomagnetic records of primary characteristic remanent magnetizations. We present here new palaeomagnetic results spanning the Paleocene to late middle Eocene within the Burma Terrane, complementing palaeolatitudes previously established from Late Cretaceous intrusive rocks and late middle Eocene sedimentary rocks. Our palaeomagnetic data indicate that the Burma Terrane remained at equatorial latitudes during the Paleocene and early Eocene, at a considerable distance from the South Asian margin. In addition, palaeomagnetic results from mid- to late Eocene sedimentary rocks yield a predominantly north–south orientation of the Burma Terrane over the past 45 Myr, showing that it was not part of the NW–SE-oriented Sundaland margin before its collision with India. Our results support collision models involving a Trans-Tethyan subduction system during the Late Cretaceous and early Paleocene. We propose that this system incorporated the Burmese volcanic arc and continental fragments of Argoland before drifting north with India towards Asia. The new palaeogeographical model considers a reduced amount of oblique subduction of the Indian plate below Burma during the Cenozoic. A possible source of sediments filling the thick Myanmar basins from the Gangdese belt during the Eocene supports the hypothesis of an India–Asia collision around ∼50 Ma. The new palaeogeography supporting the formation of the Myanmar Cretaceous amber on an isolated Trans-Tethyan Arc is also a key element in discussions of the palaeobiogeographical evolution of the numerous faunas it contains.

Magnetic stratigraphy of Paleogene rocks of West Gobustan (Azerbaijan)

On the territory of Azerbaijan, Paleogene deposits are widespread and often have facies alteration.This article presents the results of joint magnetostratigraphic and biostratigraphic studies of Paleogene deposits of Western Gobustan (Azerbaijan) in order to clarify the boundaries of the Paleocene, Eocene, Oligocene and Miocene. Paleogene deposits of the southern slope of Western Gobustan were chosen as the object of research. Paleomagnetic studies of the Azerbaijan’s Paleogene deposits showed frequent reversals of the geomagnetic field on the period. As a result of research the southern slope of the West Gobustan, the fact (frequent reversals of the geomagnetic field on the period) was confirmed. In the Lower Paleocene the two subzones of direct polarity were distinguished: Danian and Mons. The position of the reversal zone (R-zone) made it possible to draw the boundary between the Danian and Mons stages. The identification of a reversal paleomagnetic zone (reverse polarity) contributed to the determination of the boundary between the Mons and Tenes stages.

A New Genus of Kitefin Sharks (Squaliformes, Dalatiidae) from the Berezovaya Strata (Lower Paleocene) of the Lower Volga Region

A New Genus of Kitefin Sharks (Squaliformes, Dalatiidae) from the Berezovaya Strata (Lower Paleocene) of the Lower Volga Region

The White Hills Gravel of central Victoria: an anomalous Paleogene very coarse-grained fluvial deposit

The White Hills Gravel is the coarsest fluvial gravel within Victoria, southeastern Australia, and outcrops discontinuously across a substantial part (>35 000 km2) of this region. It overlies a major regional unconformity representing >300 Ma, and is dominated by conglomerate (61% of beds), with less abundant sandy lithofacies (36% of beds) and rare mudstones (3% of beds). The sedimentary characteristics of the basal conglomerates (e.g. thick bedding, common boulders, very poor sorting and in places chaotic appearance) indicate deposition by significant paleofloods, including debris floods, within braided streams that had similar flow directions to the modern drainage. Paleohydraulic calculations estimate that the paleofloods had discharges exceeding >7200 m3 s−1, over 25 times the largest recorded floods in the same valleys. These paleofloods were probably caused by exceptionally severe storm systems, consisting of many individual thunderstorms. Deposition of the White Hills Gravel has previously been attributed to the mid-Cretaceous uplift that affected southeastern Australia, but the maximum possible age of the formation (early Paleocene) is too young for this to be a viable explanation. Instead, the White Hills Gravel may have been deposited during a period of increased precipitation at the Paleocene–Eocene Thermal Maximum (ca 56 Ma), when an intensification of the hydrological system generated significant sediment responses within fluvial systems around the world. Modelling indicates that this climatic event had a substantial impact in Victoria, where increases in mean annual precipitation (50–60%) and the most extreme rainfall rates achieved by the strongest storms (60–70%) were some of the largest globally, and frequent and intense Atmospheric Rivers were an important part of the hydrological system. These factors, together with a period of disturbed vegetation cover, resulted in major paleofloods that could have deposited the coarse-grained conglomerates of the White Hills Gravel.

Remagnetization of Pre‐Fan Sediments Offshore Sumatra: Alteration Associated With Seismogenic Diagenetic Strengthening

AbstractIncreases in temperature and pressure caused by rapid burial of sediments seaward of the Sumatra subduction zone have been hypothesized to trigger dehydration reactions that diagenetically strengthen sediments and contribute to the formation of an over‐pressured pre‐décollement, which together facilitate the occurrence of large shallow earthquakes. We present paleomagnetic, rock magnetic, and electron microscopic analyses from drill cores collected offshore Sumatra at Site U1480 during IODP Expedition 362 that support this hypothesis. The older pre‐fan units (Late Cretaceous to early Paleocene) were deposited when Site U1480 was moving rapidly northward with the Indian plate from a paleolatitude of 50° to 30°S, which would equate to expected absolute paleomagnetic inclinations of 70°–43°. Most of the older pre‐fan sediments, however, have shallow observed inclinations (shallower than ±20°), indicating that the sediments were overprinted when Site U1480 was located near the paleoequator, as it has been since the early Oligocene. Electron microscopic observations reveal that the pre‐existing detrital magnetite grains have undergone pervasive dissolution and alteration by hydrothermal fluids. The diagenesis observed is consistent with mineral dehydration, possibly driven by rapid burial of pelagic sediments by the ∼1250 m thick Nicobar Fan sequence. In addition, the elevated burial temperature also facilitated the smectite to illite conversion reaction. We hypothesize that chemical reactions resulted in the formation of fine‐grained magnetite that records a chemical remanent magnetization overprint. This overprint is consistent with the alteration occurring after burial by the thick Nicobar Fan sequence sometime in the past few million years.

Unlocking the geochemical features of the Paleocene southern Pacific Ocean using carbon isotopes and biolipids

Little is known about the Paleocene in the southern Pacific Ocean due to scarce marine records. Here, we present a systematic geochemical investigation using biomarkers, carbonate content, and carbon isotopes of a set of early Paleocene deep-sea cores from International Ocean Discovery Program Expedition 378 Site U1553. The results provide a new Paleocene biomarker profile for the Campbell Plateau, New Zealand. The occurrence and distribution of a series of hopenes, ββ hopanes, sterenes and biomarker-based maturity parameters indicate that these organically-lean sediments are in an early diagenetic stage, with an equivalent vitrinite reflectance of approximately 0.4%. Biomarker data (e.g., gammacerane index and homohopane index), carbonate carbon isotopes (−27.9 ‰ to +2.0 ‰) combined with onboard data (sulphur and methane contents) indicate that the redox properties of the Paleocene southern Pacific Ocean changed from an oxidising-anoxic transition zone to a more reducing marine environment (water depth > 1000 m). The sources of the organic matter are diverse, including algae, bacteria, diatoms, dinoflagellates, and higher plants, but are dominated by aquatic organisms. Our new biomarker record bridges a Paleocene biosphere knowledge gap in the high latitude South Pacific.

Early Paleogene plant biomes of the Pacific side of Eurasia

Major plant biome changes in East Asia during the early Paleogene are studied using the Integrated Plant Record vegetation analysis for the first time based on paleobotanical records. The paleobotanical record employed for the reconstruction comprises 110 paleofloras covering the early Paleocene to early Eocene, a time-span of ca. 17 Myr. Our data indicate the presence of more forested conditions over East Asia during the early Paleogene than at present. The early Paleogene vegetation cover in the study area fundamentally differed from modern and was much more homogeneous compared to the modern one. Mixed mesophytic zonal vegetation dominated on the Pacific side of Eurasia during the early Paleogene. Thermophilous mixed mesophytic forests were distributed even at very high latitudes, but at the same time, ecotone between the mixed mesophytic / broadleaved evergreen forests and broadleaved evergreen forest were confined to regions south of 50°N. The proportions of sclerophyllous plants are low in general, even in the mid-latitudes. The localities with open woodlands are all located south of ca. 45°N. The observed vegetation patterns and their changes throughout the early Paleogene in many cases can be correlated with climate patterns and the overall continental paleoclimate evolution as reconstructed from the paleobotanical record. The higher-than-present spatial homogeneity of early Paleogene vegetation coincides with shallow temperature gradients and a significantly more humid regional rainfall pattern over East Asia during the early Paleogene.

Biomagnetostratigraphy of the Eastern Gobustan’s paleogene rocks

On the territory of Azerbaijan, Paleogene deposits are widespread and often have facies alteration. To address the issues of stratigraphic, paleogeography, facies and paleoecological directions, a detailed stratigraphic basis of both the region as a whole and its individual sections is required. This article presents the results of joint magnetobiostratigraphic studies of the Paleogene deposits of the Greater Caucasus (Azerbaijan) in order to clarify the boundaries of the Paleocene, Eocene, Oligocene and Miocene. Biostratigraphically dated Paleogene deposits of the Khilmili, Pirakushkul, Dzhangi sections (southern slope of the Greater Caucasus) were chosen as the object of study. Paleomagnetic studies of the Paleogene deposits of Azerbaijan showed frequent reversals of the geomagnetic field of this period. The results of scientific studies of the southern slope of the Greater Caucasus confirmed this fact. We have identified two paleomagnetic zones: 1) Khorem (Maastrichtian 70 Ma - Eocene 45 Ma) (zones of reverse polarity prevail in it, but the upper part of the Paleocene has a direct polarity) and 2) Sogdiana (RN-Oligocene and Miocene - characterized by alternating direct and reverse polarity paleomagnetic field). In the Lower Paleocene, 2 subzones of direct polarity were distinguished: Danian and Selandian. The position of the R-zone made it possible to draw the boundary between the Danian and Selandian stages. The identification of a paleomagnetic zone of reverse polarity contributed to the determination of the boundary between the Selandian and Thanetian stages. This result was confirmed by identifying changes in the micro-faunistic composition of deposits and observing the nature of variations in micro-floristic complexes. Such a paleomagnetic zone of reverse polarity is the benchmark (plug) for establishing the boundary between the Mons and Thanetian stages. Keywords: paleomagnetism; southern slope of the Greater Caucasus; Paleogene; magnetostratigraphic boundaries.

Cretaceous–Paleocene extension at the southwestern continental margin of India and opening of the Laccadive basin: constraints from geophysical data

Abstract. Previous geophysical investigations of the western continental margin of India (WCMI) confirm the two-phase breakup history of the margin with the first breakup taking place between India and Madagascar that created the Mascarene Basin in the Late Cretaceous and the second breakup event in Early Paleocene with Seychelles separating from India. Despite numerous geoscientific studies along the WCMI, the opening of the Laccadive basin, situated along the southern part of the margin, remains poorly constrained. In this study, we evaluate the multi-channel seismic reflection and gravity anomalies at the margin to identify the early rift signatures in conjunction with the magnetic anomaly identifications in the Mascarene Basin. The analysis led to the identification of two trends of extensional structures, a NNW–SSE-oriented structure over the Laccadive Ridge north of Tellicherry Arch, interpreted to result from ENE–WSW extension, and a SSW–NNE-oriented structure in the Laccadive basin region towards the south, interpreted to result from NW–SE extension. Previous plate reconstruction models of the Mascarene Basin using marine magnetic lineations suggest that the ENE–WSW extension observed over the Laccadive Ridge could be related to the India–Madagascar separation. We associate the pattern of sediment deposition and the presence of a Paleocene trap volcanics, linked with the NW–SE grabens observed in the Laccadive basin region, to the extension between the Laccadive Ridge and the western coast of India after the separation of Madagascar from India. We further propose that the anticlockwise rotation of India and the passage of the Réunion plume have facilitated the opening of the Laccadive basin.

End of the Cretaceous

Abstract The Cretaceous ended when the Chicxulub asteroid struck the Yucatán platform, and all deposits and landforms generated are earliest Paleocene in age. Drill core from the Cretaceous–Paleogene (K–Pg) boundary sequence deposited on top of the Chicxulub impact structure's peak ring paired with geophysical data within the crater and other well-preserved marine and terrestrial K–Pg boundary sites provide insight into impact processes and effects on Earth's ecosystems, in terms of extinction and recovery. Key findings as reviewed here are: (1) the confirmation of acoustic fluidization as a physical principle to explain impact structure formation; (2) the importance of the target rock properties and marine setting for impactite depositional processes and drivers of climate change; (3) the state of understanding of extinction mechanisms and ecological recovery; and (4) implications for impact-generated hydrothermal habitat creation. Areas highlighted for the next stages of discovery include: (1) physics and chemistry of impact vapour plumes and ejecta processes; (2) sampling of the K–Pg boundary deposit and impact melt sheet within the central basin of the impact structure; (3) examination of ecosystem recovery in an ‘Early Ocean’ scenario at a range of K–Pg boundary sites; and (4) experiments and observations of impact hydrothermal systems as habitats on Earth and other worlds.

Development of major unconformities in the forearc regions: A signal of west Myanmar−Asia assemblage before the late Paleocene

It has long been debated whether the India-Eurasia collision was a single-stage event that began 60-55 million years ago, or whether it was a two-stage process that involved a collision between India and the Trans-Tethyan Arc before the early Paleocene, and the collision of India with Eurasia during the middle Eocene. Here, we report a late Paleogene angular unconformity (ca. 40-28 Ma) in western Myanmar. This angular unconformity developed around the same time as the Assam unconformity (NE India) but is younger than those found in northern Myanmar. Development of these unconformities indicates that an oblique convergence margin in western Myanmar formed before the middle Eocene, with a major dextral strike-slip fault (proto-Sagaing/Shan Scarp Fault) in the backarc. We interpret this oblique convergence margin to be partial continental collision between the West Myanmar Terrane (WMT) and NE India. In backarc regions, syn-rift successions of the Shwebo sub-basin have formed as a consequence of transtensional tectonics along the proto-Sagaing/Shan Scarp Fault since at least the late Paleocene. The syn-rift successions consist of Asian-derived materials that were not identified in the forearc because of the Wuntho-Popa Arc served as a geographical barrier. The presence of the unconformities and tectonic configuration of the Myanmar backarc sub-basins are inconsistent with the scenario inferred from paleomagnetic data, in which the WMT was part of an intra-oceanic arc at near-equatorial latitudes before the late Oligocene. Instead, we propose that the WMT has been part of continental SE Asia since at least the Paleocene (ca. 60-58 Ma). We reconsider the paleomagnetic data and suggest that the Mawgyi Arc, rather than the WMT, is the oceanic fragment that rifted from the northern Gondwana margin during the Late Jurassic. The Mawgyi Arc collided with continental SE Asia (WMT) during the Late Cretaceous, and then with India during the early Eocene (ca. 51-49 Ma). Our results support the collision between India and Eurasia is a multistage event.

New records of early Paleocene (earliest Torrejonian) plesiadapiforms from northeastern Montana, USA, provide a window into the diversification of stem primates

Plesiadapiforms (putative stem primates) appear in the fossil record shortly after the Cretaceous/Paleogene boundary and subsequently radiated throughout the Paleocene into a taxonomically and ecomorphologically diverse group. The oldest known plesiadapiforms come from early Puercan (the oldest North American Land Mammal ‘age’ [NALMA] of the Cenozoic) deposits in northeastern Montana, and all records of Puercan plesiadapiforms are taxonomically restricted to members of the Purgatoriidae and the enigmatic genus Pandemonium. Plesiadapiform diversity substantially increased in the following Torrejonian NALMA, but the sparse record of faunas between the Puercan and the well-known middle and late Torrejonian has hampered our understanding of this important interval in early primate evolution. Here we report new plesiadapiform dental fossils from early Torrejonian (To1) deposits from the Tullock Member of the Fort Union Formation in northeastern Montana that record several poorly known taxa including members of the Purgatoriidae, Paromomyidae and Pandemonium, and that document the largest and most diverse assemblage of To1 plesiadapiforms known. We describe a new species of the purgatoriid Ursolestes (Ursolestes blissorum, sp. nov.) that represents the largest plesiadapiform known from the early Paleocene and, among other taxa, provides additional evidence that the temporal range of purgatoriids extended into the Torrejonian. Large sample sizes of the oldest known paromomyid, Paromomys farrandi, allowed us to document intraspecific variability and one undescribed tooth locus. Our observations illuminate changes in dental morphology of some taxa that occurred in To1 and may inform the acquisition of certain diagnostic plesiadapiform dental characters. We evaluate plesiadapiform species richness, mean body mass and body-mass disparity through the Paleocene and reveal unrecognized levels of richness in To1 and a general trend of stable body mass and body-mass disparity. Our findings contribute to documented patterns of plesiadapiform provincialism in the early Paleocene and shed light on the early stages of their Torrejonian radiation.

First record of Borealosuchus sternbergii from the lower Paleocene Denver Formation (lower Danian), Colorado (Denver Basin)

ABSTRACT The Late Cretaceous through Paleogene fossil record for North American eusuchians is remarkable in that it indicates high levels of survivorship across the Cretaceous–Paleogene (K–Pg) boundary, as well as an increase in diversity through the early Eocene. Despite the clade’s continued existence, gaps remain in its fossil record, particularly near the K–Pg boundary and in Colorado. A eusuchian from the lower Paleocene Denver Formation of Corral Bluffs is represented by specimens identified as Borealosuchus sternbergii. Here, we provide anatomical descriptions and comparisons supported by micro-CT scan data of three partial crania. We conduct a phylogenetic analysis, the results of which are influenced by the potential sub-adult ontogenetic stage of the specimens and species-level assignment is preliminary. The specimens fill both geographic and temporal gaps in the record of the Borealosuchus species complex as the first specimens from the earliest Paleocene of Colorado. These additions highlight the uncertainty in phylogenetic relationships among the Borealosuchus species complex, the importance of taking ontogenetic stage into account when assessing such relationships, and help constrain biogeographic dispersal patterns and ecological niche occupation of the clade.

Earliest megafossils of scandent calamoid palms from the Deccan Intertrappean Beds of Central India and their paleobiogeographic implications

Two well-preserved petrified palm stems from the latest Maastrichtian (Late Cretaceous) to earliest Danian (Early Paleocene) sediments of the Deccan Intertrappean Beds of Madhya Pradesh, Central India are described. Their significant anatomical characteristics include a Calamus-type general stem pattern, the presence of well-preserved fibrovascular bundles (fvbs) with two wide metaxylem vessel elements (230 μm–250 μm) and one phloem strand, uniform density of fvbs, lack of continuity between protoxylem and metaxylem vessel elements, and an absence of centrifugal differentiation of sclerenchymatous fibrous parts. These features reveal a close resemblance to those of extant genera of scandent Calamoideae. The permineralized stems are described as a new species namely, Palmoxylon calamoides Kumar, Roy et Khan sp. nov. The fossils represent the oldest reliable fossil records of this family, supporting their Gondwanan origin, their importance in tracing their migration pathways from India to Europe and other continents after the docking of the Indian subcontinent with Eurasia during the Paleocene, and an “Out-of-India” dispersal hypothesis. Today the subfamily Calamoideae is disjunctly occurred in Africa, Asia, Australia, Europe, and South America, but the poor deep-time fossil record of this subfamily with a small number of Cenozoic fossils makes hypotheses concerning its origin and dispersal difficult to evaluate. The present study has significant implications for the origin and migration of this subfamily and the paleoclimate.

Skull of a new periptychid mammal from the lower Paleocene Denver Formation of Colorado (Corral Bluffs, El Paso County)

The Periptychidae, an extinct group of archaic ungulates (‘condylarths’), were the most speciose eutherian mammals in the earliest Paleocene of North America, epitomizing mammalian ascendency after the Cretaceous–Paleogene (K–Pg) mass extinction. Although periptychids are mostly known from fragmentary gnathic remains, the Corral Bluffs area within the Denver Basin, Colorado, has yielded numerous exceptionally well-preserved mammalian fossils, including periptychids, from the earliest Paleocene. Here we describe a partial cranium and articulated dentaries plus an additional unassociated dentary fragment of a small-bodied (~273–455 g) periptychid from ca. 610 thousand years after the K–Pg mass extinction (Puercan 2 North American Land Mammal ‘age’) at Corral Bluffs. Based on these new fossils we erect Militocodon lydae gen. et sp. nov. The dentition of M. lydae exhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative to Oxyacodon, resembling putatively basal periptychids like Mimatuta and Maiorana in several dental traits. As such, we interpret M. lydae as a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians. M. lydae falls along a dental morphocline from basal periptychids to derived conacodontines, which we hypothesize reflects a progressive, novel modification of the hypocone to enhance orthal shearing and crushing rather than grinding mastication. The discovery and thorough descriptions and comparisons of the partial M. lydae skull represent an important step toward unraveling the complex evolutionary history of periptychid mammals.