Late Ordovician vertebrates from the Bighorn Mountains of Wyoming, USA

Abstract

Late Ordovician vertebrate faunas occur in clastic sedimentary units along the length of the Rocky Mountains from Colorado to Montana, and across the border into Canada. Most research has, however, been conducted on localities in the southern part of the outcrop belt, particularly the Harding Sandstone Formation of Colorado. Micropalaeontological sampling of the coeval South Piney Member (Winnipeg Formation) in the Bighorn Mountains of Wyoming has revealed an abundant vertebrate fauna. The vertebrate assemblage includes a low-abundance fauna of 13 conodont taxa that together indicate an undatus Chronozone age (midMohawkian; mid-Caradoc; Late Ordovician). The pteraspidomorphs Astraspis desiderata Walcott and Eriptychius americanus Walcott are also present together with one new taxon, Eleochera glossa gen. et sp. nov., which is interpreted as a derived stem-gnathostome on the basis of its scale histology and morphology. The fauna bears a strong similarity to that of the Harding Sandstone but is of lower diversity. In particular, it lacks the fine-grained, deeper water component of the Harding Sandstone that contains, inter alia, thelodonts and stem-chondrichthyans.

Principal aspects of the Ordovician biotic radiation

“…Some broken pieces, commonly scales or sclerites, have been reported from isolated sites in the United States, Canada, Australia, Norway, Bolivia, and Hubei, China [57][58][59][60]. Their feeding habits are unknown, but may have been deposit feeders or filter feeders [61].…”

A Phylogeny for Heterostraci (stem-gnathostomes)

“…Sacabambaspis and Arandaspis have large singular dorsal and ventral headshields and a trunk region covered in elongate rod shaped scales (Ritchie & Gilbert-tomlinson, 1977; Gagnier, Blieck, & Rodrigos, 1986; Gagnier, 1995). Astraspis and Eriptychius are less completely preserved then Arandaspida taxa due to their headshields being composed of non-fused platelets or tesserae (Denison, 1967; Elliott, 1987; Sansom et al ., 1995; Sansom & Smith, 2005). Unlike the heterostracans, the Ordovician pteraspidimorphs appear to have more than one gill or branchial opening, but histologically are very similar to heterostracans, possessing a three-layered dermal skeleton comprising a superficial layer of dentine tubercles capped with enameloid, a middle vascular layer composed of tissue similar to aspidin and a compact lamellar basal layer (Denison 1967, Sansom, Smith, & Smith, 1997; Sansom, Donoghue, & Albanesi, 2005, Lemierre & Germain 2019).…”

“…Kallostrakon and Oniscolepis are only known from disarticulated fragments of bone and are placed in the Heterostraci mostly due to similarities in histology(Märss & Karatajūtė-Talimaa, 2009). Astraspis and Eriptychius are less completely preserved then Arandaspida taxa due to their headshields being composed of non-fused platelets or tesserae(Denison, 1967;Elliott, 1987;Sansom et al, 1995;Sansom & Smith, 2005). Unlike the heterostracans, the Ordovician pteraspidimorphs appear to have more than one gill or branchial opening, but…”

Ichnofacies of the Stairway Sandstone fish-fossil beds (Middle Ordovician, Northern Territory, Australia)

“…The Stairway Sandstone is one of several similar clastic shallow marine ichnofossil-dominated Ordovician successions from both Gondwana and Laurentia that host pteraspidomorph fossils (Davies & Sansom 2009). These include the Anzaldo Formation (?Sandbian) of Bolivia (Gagnier et al 1986, Davies et al 2007, the Amdeh Formation (Dapingian/Darriwilian) of Oman (Sansom et al 2009), the Sepulturas Formation (Darriwilian) of Argentina (Albanesi & Astini 2002, the Harding Sandstone (Sandbian) of Colorado, USA (Walcott 1892, Denison 1967, Lehtola 1983, Elliott 1987, Sansom et al 1997 and the South Piney Member of the Winnipeg Formation (Sandbian) of Wyoming, USA (Darton 1906, Sansom & Smith 2005. Of these, the Stairway Sandstone is best compared with the Anzaldo Formation and Harding Sandstone, these being the only formations from which articulated pteraspidomorph fossils have been recovered; the other successions contain fragmentary and microremains that have the potential for transportation away from their original habitat (e.g.…”