Scientific research of Fossil Basin began in the mid-1800s when the first article regarding a fossil was published and continues to this day. See below for a list of articles to date.
Paleontology Publications
Hymenoptera
Eocene giant ants, Arctic intercontinental dispersal, and hyperthermals revisited: discovery of fossil Titanomyrma (Hymenoptera: Formicidae: Formiciinae) in the cool uplands of British Columbia, Canada (Open Access)
S. Bruce Archibald, Rolf W. Mathewes, and Arvid Aase
Published February 6, 2023
Abstract: We examine the implications for intercontinental dispersal of the extinct ant genus, Titanomyrma Archibald et al. (Hymenoptera: Formicidae: Formiciinae), following the discovery of its first fossil in Eocene temperate upland Canada. Modern Holarctic distributions of plants and animals were in part formed by dispersals across Late Cretaceous through early Eocene Arctic land bridges. Mild winters in a microthermal Arctic would allow taxa today restricted to the tropics by cold intolerance to cross, with episodic hyperthermal events allowing tropical taxa requiring hot climates to cross. Modern ants with the largest queens inhabit low latitudes of high temperature and mild coldest months, whereas those with smaller queens inhabit a wide variety of latitudes and climates. Gigantic and smaller formiciine ants (Titanomyrma and Formicium Westwood) are known from Europe and North America in the Eocene. The new Canadian Titanomyrma inhabited a cooler upland. It is incomplete, indistinctly preserved, and distorted in fossilisation, and so we do not assign it to a species or erect a new one for it. The true size of this fossil is unclear by this distortion: small size would support gigantism in Titanomyrma requiring hot climates and dispersal during hyperthermals; if it was large, it may have been cold-winter intolerant and able to have crossed during any time when the land bridge was present.
Past ecosystems drive the evolution of the early diverged Symphyta (Hymenoptera: Xyelidae) since the earliest Eocene
Corentin Jouault, Arvid Aase, and André Nel
Published: December 13, 2021
Abstract: Paleoxyela nearctica gen. et sp. nov., is described from the upper Eocene of Florissant Formation in Colorado. We placed Paleoxyela gen. nov. in the subfamily Macroxyelinae and the tribe Macroxyelini based on the numerous wing venation characters visible on the specimen. Proxyelia pankowskii gen. et sp. nov. is described from the lower Eocene Fossil Lake deposits of the Green River Formation in Wyoming. We placed Proxyelia gen. nov. in the subfamily Macroxyelinae and the tribe Xyeleciini based on the numerous wing venation characters visible on the specimen. These new records of the family Xyelidae are of particular importance to better understand the past diversity of the clade and propose hypotheses about their diversification. Extant Xyelidae inhabit temperate Northern Hemisphere forests, and most of their larvae feed on conifers, which may explain why they are relatively poorly diversified compared to the other symphytan families. We suggest that the global decline in conifers and the reduced diversity of extant host trees partly explain the diversity of extant Xyelidae. We correlate the biome repartition during the Eocene to that of the extant xyelid.
The second North American fossil horntail wood-wasp (Hymenoptera: Siricidae), from the early Eocene Green River Formation (Open Access)
Corentin Jouault, Arvid Aase, and André Nel
Published: July 13, 2021
Abstract: Eoteredon lacoi gen. et sp. nov., is described from the early Eocene Green River Formation in Wyoming, the second fossil siricid genus and species described from North America. We propose Eoteredon as sister to the genus Teredon, whose sole species is one of the rarest of extant Siricidae. The majority of siricids today inhabit temperate Northern Hemisphere forests; Teredon cubensis, however, is one of its few species that live in megathermal tropical lowlands. The Eocene forest that Eoteredon inhabited had a mesothermal to megathermal climate. We place Eoteredon in the context of broad Cenozoic climate change.
Odonata
The damselfly palaeofauna from the Eocene of Wyoming and Colorado, USA (Insecta, Odonata, Zygoptera)
Günter Bechly, Romain Garrouste, Arvid Aase, Jered A. Karr, Lance Grande, and André Nel
Published: August 2021
Abstact: A new family, five new genera, and nine new species of fossil damselflies (Insecta, Odonata, Zygoptera, Calopterygida) from the USA are described, seven from the Eocene Fossil Lake deposits and one from Lake Uinta deposits, both from the Green River Formation, and an additional specimen from the Wind River Formation of Wyoming and Colorado. Namely, Carlea eocenica gen. et sp. nov. (in Carleidae fam. nov.), Labandeiraia riveri sp. nov., Labandeiraia browni sp. nov., Eodysphaea magnifica gen. et sp. nov., Litheuphaea sp. cf. coloradensis Petrulevičius et al., 2007, Zacallites cockerelli sp. nov., Dysagrion integrum sp. nov., Tenebragrion shermani gen. et sp. nov., Tynskysagrion brookeae gen. et sp. nov., and Oreodysagrion tenebris gen. et sp. nov. Epallagoidea and Amphipterygoidea are most common while Calopterygoidea, Coenagrionoidae and Lestoidea damselflies are less diverse. Genera of zygopteran Dysagrionidae are known from Europe and North America, further supporting the hypothesis of Palaeogene terrestrial interchange. Representatives of Epallagoidea and Amphipterygoidea in the Green River Formation confirm that warm conditions occurred at the time of deposition.
Birds
An Exceptionally Preserved Specimen From the Green River Formation Elucidates Complex Phenotypic Evolution in Gruiformes and Charadriiformes (Open Access)
Grace Musser and Julia A. Clarke
Published: October 26, 2020
Abstract: The stem lineage relationships and early phenotypic evolution of Charadriiformes (shorebirds) and Gruiformes (rails, cranes, and allies) remain unresolved. It is still debated whether these clades are sister-taxa. New phylogenetic analyses incorporating Paleogene fossils have the potential to reveal the evolutionary connections of these two speciose and evolutionarily critical neoavian subclades. Although Gruiformes have a rich Paleogene fossil record, most of these fossils have not been robustly placed. The Paleogene fossil record of Charadriiformes is scarce and largely consists of fragmentary single elements. Only one proposed Eocene charadriiform-like taxon, Scandiavis mikkelseni of Denmark, is represented by a partial skeleton. Here, we describe a new species from the early Eocene Green River Formation of North America comprising a partial skeleton and feather remains. Because the skeleton lacks the pectoral girdle and forelimbs as in S. mikkelseni, only features of the skull, axial skeleton, and hind limb are available to resolve the phylogenetic placement of this taxon. These anatomical subregions initially showed features seen in Charadriiformes and Gruiformes. To assess placement of this taxon, we use a matrix consisting of 693 morphological characters and 60 taxa, including S. mikkelseni and the oldest known charadriiform taxa represented by single elements. These more fragmentary records comprise two distal humeri from the earliest Eocene Naranbulag Formation of Mongolia and the early Eocene Nanjemoy Formation of Virginia. Our phylogenetic analyses recover the new taxon and S. mikkelseni alternatively as a charadriiform or as a stem-gruiform; placement is contingent upon enforced relationships for major neoavian subclades recovered by recent molecular-based phylogenies. Specifically, when constraint trees based on results that do not recover Charadriiformes and Gruiformes as sister-taxa are used, the new taxon and S. mikkelseni are recovered within stem Gruiformes. Both Paleogene fossil humeri are consistently recovered within crown Charadriiformes. If placement of these humeri or the new taxon as charadriiforms are correct, this may indicate that recent divergence time analyses have underestimated the crown age of another major crown avian subclade; however, more complete sampling of these taxa is necessary, especially of more complete specimens with pectoral elements.
The anatomy and taxonomy of the exquisitely preserved Green River Formation (early Eocene) lithornithids (Aves) and the relationships of Lithornithidae. (Bulletin of the American Museum of Natural History, no. 406)
Sterling J. Nesbitt; Julia A. Clarke
Published: June 30, 2016
Abstract: Fossil remains of Paleogene Palaeognathae are poorly documented and are exceedingly rare. One group of palaeognaths, the lithornithids, is well represented in the Paleogene of North America. Nevertheless, few specimens of the same species are known from each of those Paleogene geologic units. Here, we report five new partial skeletons of lithornithids from the Fossil Butte Member of the Green River Formation (early Eocene) of Wyoming. One spectacularly preserved specimen is identified as the holotype of a new species, Calciavis grandei, gen. et sp. nov., and fully described. Preserved soft tissues (e.g., feathers, pes scales) surround the nearly articulated and complete skeleton. A second well-preserved but disarticulated skeleton is referred to this new taxon. We conclude that there are only two lithornithid taxa in the Green River Formation after careful comparisons with the other known taxon from the same geological unit, Pseudocrypturus cercanaxius. Morphological data generated from the new taxon and other Green River Formation lithornithid specimens were integrated into a osteology-only phylogenetic data set containing stem avians as outgroups and extinct and extant members of Palaeognathae (Tinamidae, ratites) and Neognathaes (Anseriformes, Galliformes, Neoaves), unnamed lithornithid specimens, and the following named lithornithid taxa: Lithornis plebius, Lithornis promiscuus, Lithornis celetius, Paracathartes howardae. We find a monophyletic Lithornithidae (containing Calciavis grandei, Pseudocrypturus cercanaxius, Lithornis plebius, Lithornis promiscuus, Lithornis celetius, Paracathartes howardae) as the sister taxon of Tinamidae at the base of Palaeognathae and also recover a monophyletic Ratitae in the morphology-only analysis. A Lithornithidae-Tinamidae relationship, which could imply a broad Northern Hemisphere distribution in the Paleogene for this total group retracted to the present day Neotropical distribution after the Eocene, is weakly supported in our analysis and is also supported by other lines of evidence such as eggshell morphology. Relationships among flightless palaeognaths and assessment of character homology in this group remain problematic. Indeed, when the morphological analyses were constrained to enforce topologies recovered from all recent analyses of molecular sequence data and retroelement insertions, Lithornithidae is no longer recovered with Tinamidae, which is nested within the now paraphyletic ratites, but remains at the base of Palaeognathae. Thus, regardless of the position of Tinamidae, Lithornithidae is recovered at the base of the clade. However, evidence that many, if not all, of these "ratite" lineages independently evolved similar morphologies related to large size and flight loss suggests that the proposed position of the Lithornithidae remains tentative. Significant morphological variation within Lithornithidae should be captured in inclusive future analyses through use of species terminals.
A new species of Threskiornithidae-like bird (Aves, Ciconiiformes) from the Green River Formation (Eocene) of Wyoming
Nathan D. Smith, Lance Grande, Julia A. Clarke
Published: March 5, 2013
Abstract: A new avian species from the late early Eocene Fossil Butte Member of the Green River Formation is described based on a nearly complete postcranial skeleton. The new species, Vadaravis brownae, gen. et sp. nov., can be diagnosed by a unique combination of characters, including the following autapomorphies, which are unique among Aves: two (cranial and caudal) small and discrete pneumatopores on the lateral sides of the caudal-most thoracic centra; and a caudoventrally located pisiform process of the carpometacarpus that projects only weakly cranially. Phylogenetic analyses recover Vadaravis as a member of the waterbird assemblage (including, e.g., penguins, storks, pelicans), closely related to taxa traditionally placed within the avian order Ciconiiformes (storks, flamingos, herons, the hamerkop, ibises, and spoonbills). Additional morphological features and a phylogenetic analysis constrained by a recently recovered waterbird topology suggest close affinities between Vadaravis and Threskiornithidae. This new species represents the first proposed part of Ciconiiformes (and possibly stem-Threskiornithidae) in the Green River Formation of North America. Its discovery increases the known taxonomic and ecological diversity of this diverse fossil avifauna. Vadaravis also represents one of the oldest members of Ciconiiformes (and possibly stem-Threskiornithidae), and implies that additional lineages within the waterbird assemblage had diverged by the late early Eocene.
Combined phylogenetic analysis of a new North American fossil species confirms widespread Eocene distribution for stem rollers (Aves, Coracii) (Open Access)
Julia A. Clarke, Daniel T. Ksepka, N. Adam Smith, Mark A. Norell
Published: November 2009
Abstract: We report a nearly complete skeleton of a new species of stem roller (Aves, Coracii) from the early Eocene Green River Formation of North America. The new species is most closely related to two species-depauperate lineages, Coraciidae (rollers) and Brachypteraciidae (ground rollers), that form a monophyletic crown clade (Coracioidea) with an exclusively Old World extant distribution. Phylogenetic analysis utilizing a matrix of 133 morphological characters and sequence data from three genes (RAG-1, c-myc, and ND2) identifies the new species as a stem member of the Coracii more closely related to the crown clade than the only previously identified New World taxon, Primobucco mcgrewi. The phylogenetic placement of the new species and Primobucco mcgrewi indicates a widespread northern hemisphere distribution in the Eocene with subsequent restriction to Africa, Madagascar, Australia, and temperate to tropical parts of Europe and Asia. It provides evidence of further ecological diversity in early stem Coracii and convergence on crown morphologies. The new species contributes to mounting evidence that extant distributions for major avian subclades may be of comparatively recent origin. Further late Palaeogene sampling is needed to elucidate potential drivers for shifting avian distributions and disappearance of Coracii from North America.
Mammals
The oldest known bat skeletons and their implications for Eocene chiropteran diversification (Open Access)
Tim B. Rietbergen, Lars W. van den Hoek Ostende, Arvid Aase, Matthew F. Jones, Edward D. Medeiros, Nancy B. Simmons
Published: April 12, 2023
Abstract: The Fossil Lake deposits of the Green River Formation of Wyoming, a remarkable early Eocene Lagerstätte (51.98 ±0.35 Ma), have produced nearly 30 bat fossils over the last 50 years. However, diversity has thus far been limited to only two bat species. Here, we describe a new species of Icaronycteris based on two articulated skeletons discovered in the American Fossil Quarry northwest of Kemmerer, Wyoming. The relative stratigraphic position of these fossils indicates that they are the oldest bat skeletons recovered to date anywhere in the world. Phylogenetic analysis of Eocene fossil bats and living taxa places the new species within the family Icaronycteridae as sister to Icaronycteris index, and additionally indicates that the two Green River archaic bat families (Icaronycteridae and Onychonycteridae) form a clade distinct from known Old World lineages of archaic bats. Our analyses found no evidence that Icaronycteris? menui (France) nor I. sigei (India) belong to this clade; accordingly, we therefore remove them from Icaronycteridae. Taken in sum, our results indicate that Green River bats represent a separate chiropteran radiation of basal bats, and provide additional support for the hypothesis of a rapid radiation of bats on multiple continents during the early Eocene.
Phylogenetic relationships of Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments on the evolution of echolocation and foraging strategies in Microchiroptera. Bulletin of the AMNH ; no. 235
Simmons, Nancy B.; Geisler, Jonathan H.
Published: March 9, 1998
Abstract: "The Eocene fossil record of bats (Chiroptera) includes four genera known from relatively complete skeletons: Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx. Phylogenetic relationships of these taxa to each other and to extant lineages of bats were investigated in a parsimony analysis of 195 morphological characters, 12 rDNA restriction site characters, and one character based on the number of R-1 tandem repeats in the mtDNA d-loop region. Results indicate that Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx represent a series of consecutive sister-taxa to extant microchiropteran bats. This conclusion stands in contrast to previous suggestions that these fossil forms represent either a primitive grade ancestral to both Megachiroptera and Microchiroptera (e.g., Eochiroptera) or a separate clade within Microchiroptera (e.g., Palaeochiropterygoidea). A new higher-level classification is proposed to better reflect hypothesized relationships among Eocene fossil bats and extant taxa. Critical features of this classification include restriction of Microchiroptera to the smallest clade that includes all extant bats that use sophisticated echolocation (Emballonuridae + Yinochiroptera + Yangochiroptera), and formal recognition of two more inclusive clades that encompass Microchiroptera plus the four fossil genera. Comparisons of results of separate phylogenetic analyses including and subsequently excluding the fossil taxa indicate that inclusion of the fossils changes the results in two ways: (1) altering perceived relationships among extant forms at a few poorly supported nodes; and (2) reducing perceived support for some nodes near the base of the tree. Inclusion of the fossils affects some character polarities (hence slightly changing tree topology), and also changes the levels at which transformations appear to apply (hence altering perceived support for some clades). Results of an additional phylogenetic analysis in which soft-tissue and molecular characters were excluded from consideration indicate that these characters are critical for determination of relationships among extant lineages. Our phylogeny provides a basis for evaluating previous hypotheses on the evolution of flight, echolocation, and foraging strategies. We propose that flight evolved before echolocation, and that the first bats used vision for orientation in their arboreal/aerial environment. The evolution of flight was followed by the origin of low-duty-cycle laryngeal echolocation in early members of the microchiropteran lineage. This system was most likely simple at first, permitting orientation and obstacle detection but not detection or tracking of airborne prey. Owing to the mechanical coupling of ventilation and flight, the energy costs of echolocation to flying bats were relatively low. In contrast, the benefits of aerial insectivory were substantial, and a more sophisticated low-duty-cycle echolocation system capable of detecting, tracking, and assessing airborne prey subsequently evolved rapidly. The need for an increasingly derived auditory system, together with limits on body size imposed by the mechanics of flight, echolocation, and prey capture, may have resulted in reduction and simplification of the visual system as echolocation became increasingly important. Our analysis confirms previous suggestions that Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx used echolocation. Foraging strategies of these forms were reconstructed based on postcranial osteology and wing form, cochlear size, and stomach contents. In the context of our phylogeny, we suggest that foraging behavior in the microchiropteran lineage evolved in a series of steps: (1) gleaning food objects during short flights from a perch using vision for orientation and obstacle detection; prey detection by passive means, including vision and/or listening for prey-generated sounds (no known examples in fossil record); (2) gleaning stationary prey from a perch using echolocation and vision for orientation and obstacle detection; prey detection by passive means (Icaronycteris, Archaeonycteris); (3) perch hunting for both stationary and flying prey using echolocation and vision for orientation and obstacle detection; prey detection and tracking using echolocation for flying prey and passive means for stationary prey (no known example, although Icaronycteris and/or Archaeonycteris may have done this at times); (4) combined perch hunting and continuous aerial hawking using echolocation and vision for orientation and obstacle detection; prey detection and tracking using echolocation for flying prey and passive means for stationary prey; calcar-supported uropatagium used for prey capture (common ancestor of Hassianycteris and Palaeochiropteryx; retained in Palaeochiropteryx); (5) exclusive reliance on continuous aerial hawking using echolocation and vision for orientation and obstacle detection; prey detection and tracking using echolocation (Hassianycteris; common ancestor of Microchiroptera). The transition to using echolocation to detect and track prey would have been difficult in cluttered environments owing to interference produced by multiple returning echoes. We therefore propose that this transition occurred in bats that foraged in forest gaps and along the edges of lakes and rivers in situations where potential perch sites were adjacent to relatively clutter-free open spaces. Aerial hawking using echolocation to detect, track, and evaluate prey was apparently the primitive foraging strategy for Microchiroptera. This implies that gleaning, passive prey detection, and perch hunting among extant microchiropterans are secondarily derived specializations rather than retentions of primitive habits. Each of these habits has apparently evolved multiple times. The evolution of continuous aerial hawking may have been the 'key innovation' responsible for the burst of diversification in microchiropteran bats that occurred during the Eocene. Fossils referable to six major extant lineages are known from middle-late Eocene deposits, and reconstruction of ghost lineages leads to the conclusion that at least seven more extant lineages were minimally present by the end of the Eocene"--P. 4-5.
Reptiles
Complete specimens of the Eocene testudinoid turtles Echmatemys and Hadrianus and the North American origin of tortoises (Open Access)
Asher J. Lichtig, Spencer G. Lucas, and Steven E. Jasinski
Published: 2021
Abstract: Newly described specimens of North American Eocene turtles provide valuable information on their morphology and, more specifically, variation, both intraspecific and ontogenetic. We describe several complete and nearly complete testudinoid (Testudinoidea) specimens, including juvenile specimens of Hadrianus corsoni, H. majusculus, Echmatemys haydeni and E. naomi. These specimens help us determine that the oldest and most basal tortoises are from the western United States, suggesting Testudinidae evolved in North America from one of the geoemydid-like forms in the genus Echmatemys, which have their lowest stratigraphic occurrence in the earliest Wasatchian North American land-mammal “age” (early Eocene, Ypresian, ~53 Ma).
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