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. 2023 Apr 12;18(4):e0283505.
doi: 10.1371/journal.pone.0283505. eCollection 2023.

The oldest known bat skeletons and their implications for Eocene chiropteran diversification

Tim B Rietbergen  1 , Lars W van den Hoek Ostende  1 , Arvid Aase  2 , Matthew F Jones  3 , Edward D Medeiros  4   5 , Nancy B Simmons  6
Affiliations

The oldest known bat skeletons and their implications for Eocene chiropteran diversification

Tim B Rietbergen et al. PLoS One. .

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.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Skeleton of Holotype of Icaronycteris gunnelli (FM.145747A) A) Dorsal view; B) Counterpart (FM.145747B).
Fig 2
Fig 2. Illustrated dentitions of Green River bats.
A) Onychonycteris finneyi (ROM 55351A-B) upper P1-M3; B) Icaronycteris index (YPM-PU 18150) upper P1-M3; C) Icaronycteris gunnelli (FM.145747A) upper P1-M3; D) Icaronycteris gunnelli (FM.145747A) lower c1-m3, with reconstructed i1-i3, in labial view. Abbreviations of features discussed in the text: dc—distal cingulid; ec—ectocingulum; ef—ectoflexus; hs—hypocone shelf; lc—lingual cingulum; pbc—posterobasal cusp; prm—premetacrista; prp—preparacrista; ps—parastyle; psm—postmetacrista; psp—postparacrista; tal—talonid.
Fig 3
Fig 3. Skeleton of paratype of Icaronycteris gunnelli (ROM:Palaeobiology-Vertebrate Fossils:52666).
Fig 4
Fig 4. Remaining deposits of Fossil Lake sediments of the Green River Formation are illustrated in gray with highly-fossiliferous, deep-water, laminated limestone deposits of Eocene Fossil Lake in southwest Wyoming and poorly-fossiliferous, shallow-water, non-laminated limestones in northeast Utah and southeast Idaho.
Eleven of more than 20 fossil quarries are still active today. Three active quarries (A, B and D) and one inactive quarry (C) have produced fossil bats currently held in public institutions. All other historic and active quarry locations are marked by small black dots. American Fossil Quarry (A) yielded I. gunnelli (AMNH.FM.145747 and ROM.52666), I. index (AMNH.FM.125000 and AMNH.FM.144215), I. cf index (WDC-CGR-115) and O. finneyi (AMNH.FM.142467 and ROM.55351). Thompson Ranch north quarry (B) yielded I. index (FMNH.PM.62096 and HMNS.PV.001468). The Holland brother’s quarry (C) was active for only one season in the mid-1930s and yielded I. index (YPM-PU.18150). The Smith Hollow quarry (D) yielded a poorly preserved and unidentified specimen (FOBU13777). Most bat specimens were recovered from two quarries operating continuously since the 1980s in nearshore deposits. A combination of proximity to the eastern shore and a large volume of rock excavated is the most probable explanation for the greater number of bat fossils being discovered in those locations.
Fig 5
Fig 5. A composite stratigraphic column of the Green River Formation showing the relative position of the Fossil Butte Member and stratigraphic levels where various bat fossils have been found.
The Fossil Butte Member is the thinnest of the members in the Green River Formation, but was deposited when the lake was deepest, providing conditions that were optimal for exceptional preservation. Five units in the Fossil Butte Member (Sandwich Beds, 18-inch Layer, Minifish Bed, Gastropod Beds, and Upper Split-fish Bed) are actively quarried on private and state-owned land by companies collecting the abundant fossil fish to sell. Among the fish, one bat is found on average every two years. To date, all bats have come from the 18-inch Layer and Sandwich Beds. Bat specimens are coded by color: Icaronycteris gunnelli in red, I. index in orange, Onychonycteris finneyi in blue, and unidentified or indeterminate are black. More than 10 bat specimens are held in private collections (not shown here), but none are confirmed lower in the section than the two specimens of I. gunnelli. Left two columns modified after Buchheim et al, 2011 [40].
Fig 6
Fig 6. Phylogentic position of Icaronycteris gunnelli and other Eocene bat fossils with respect to extant bat lineages.
A) Strict consensus of three most parsimonious trees of 1511 steps resulting from analyses including all taxa; B) Strict consensus of two most parsimonious trees of 1455 steps for analyses excluding Icaronycteris? menui and Icaronycteris sigei. Bootstrap values from 10,000 bootstrap replicates are shown above and to the left of nodes. Fossil taxa are represented by a dagger (†).
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