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30 December 2013 Biodiversity and Evolutionary History of †Lophionotus (Neopterygii: †Semionotiformes) from the Western United States
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Abstract

Two species of the neopterygian genus †Lophionotus Gibson, 2013, are described. Specimens of †Lophionotus chinleana, new species, were previously and recently collected from freshwater deposits in the Upper Triassic Chinle Formation of Lisbon Valley, southeastern Utah. †Semionotus kanabensis Schaeffer and Dunkle, 1950, from lacustrine deposits in the Lower Jurassic Moenave Formation of southwestern Utah, is herein redescribed and attributed to the genus †Lophionotus, based on shared characters, including the infraorbital in the posteroventral corner of the orbit being expanded and contacting the anterior ramus of the preoperculum. Both new species of †Lophionotus are distinct from †L. sanjuanensis Gibson, 2013, in that they lack a postcranial hump, deep body, dense tuberculation, and ventrally expanded preoperculum. The addition of two new species lends to a revised generic description of the genus †Lophionotus. A phylogenetic analysis infers a monophyletic †Lophionotus sister to the genus †Semionotus, and †Lophionotus is placed within the family †Semionotidae within †Semionotiformes.

†SEMIONOTIFORMES are a diverse group of extinct neopterygian fishes with a worldwide distribution in marine and continental deposits that range in age from the Middle Triassic (e.g., Deecke, 1889; Gardiner, 1993) to Cretaceous (e.g., Gardiner, 1993; Gallo and Brito, 2005; López-Arbarello and Codorniú, 2007; Forey et al., 2011). Fossil specimens which have been attributed to the family †Semionotidae have been described from Europe (e.g., Agassiz, 1833–1843; Fraas, 1861; Woodward, 1890; Larsonneur, 1964; Wenz, 1968; Wenz et al., 1994; Lambers, 1999), Asia (e.g., Su, 1996; Cavin and Suteethorn, 2006), Gondwana (e.g., López-Arbarello, 2004; López-Arbarello et al., 2008), and North America (e.g., Newberry, 1888; Schaeffer and Dunkle, 1950; McCune, 2004; López-Arbarello and Alvarado-Ortega, 2011; Gibson, 2013). One of the defining morphological characters of semionotids is the presence of prominent dorsal ridge scales (Olsen and McCune, 1991) and, like extinct and extant Lepisosteiformes (e.g., gars), their scales possess ganoin (Goodrich, 1907). The order †Semionotiformes has a complicated taxonomic history (Olsen and McCune, 1991; López-Arbarello, 2008), and, most recently, semionotid taxa have been included in several taxonomic revisions of holosteans that have focused on extinct and extant taxa in the closely related order Lepisosteiformes (e.g., Grande and Bemis, 1998; Cavin, 2010; Grande, 2010).

The taxonomic composition of the family †Semionotidae has been controversial. In Olsen and McCune's (1991) work on semionotids from the Newark Supergroup rock formations of Eastern North America, the family †Semionotidae was restricted to two genera, †Lepidotes and †Semionotus. Olsen and McCune (1991) proposed two synapormorphies that unite these genera within the family: 1) the presence of dorsal ridge scales and 2) the presence of a large posteriorly directed process on the epiotic. Within †Semionotidae, the number of suborbitals was used as the primary diagnostic character to distinguish between †Semionotus and †Lepidotes (McCune, 1986). Wenz (1999) proposed that the family †Semionotidae include the genera †Semionotus, †Lepidotes,Paralepidotus, †Araripelepidotes, and †Pliodetes. Wenz (1999) reevaluated morphological characters from previous studies (e.g., Patterson, 1975; Thies, 1989; Olsen and McCune, 1991; Gardiner et al., 1996) and included additional semionotiform taxonomic sampling; however, Wenz (1999) did not perform any character-based phylogenetic analysis, nor did she provide a new diagnosis for the family itself. Later parsimony-based studies of †Semionotiformes (Cavin and Suteethorn, 2006; Cavin, 2010) recovered a largely unresolved clade that includes semionotiform and lepisosteiform taxa, though the family †Semionotidae sensu Olsen and McCune (1991) was recovered as monophyletic when taxon sampling was limited to extinct species with the least amount of missing character data (Cavin and Suteethorn, 2006). Grande (2010) recovered the order †Semionotiformes as sister to Lepisosteiformes, but within †Semionotiformes, his taxonomic sampling was restricted to †Semionotus, excluding many other †Semionotiformes taxa, including †Lepidotes. However, Grande (2010) resurrected the term Holostei to include the Ginglymodi and Halecomorphi within Neopterygii. This was also supported by Xu and Wu (2012) in their parsimony-based analysis of neopterygian relationships. Xu and Wu (2012), however, limited their analysis to 15 taxa and did not provide a robust hypothesis of relationships within the order †Semionotiformes.

López-Arbarello (2012) provided the most comprehensive hypothesis of ginglymodian relationships to date. Using a matrix of 37 taxa and 90 morphological characters, López-Arbarello (2012) recovered a monophyletic †Semionotiformes based on five unambiguous synapomorphies, and restricted the family †Semionotidae to a single genus, †Semionotus. Other genera that were placed in †Semionotidae by Wenz (1999) and others, including †Lepidotes, were reexamined and placed in other groups within †Semionotiformes or Lepisosteiformes.

The first semionotiform fossils recorded in the western United States included fragments of scales, teeth, and fully articulated fish from deposits near Kanab, Utah (Eastman, 1917; Schaeffer and Dunkle, 1950). Schaeffer and Dunkle (1950) described and erected †Semionotus kanabensis (holotype AMNH 8870) from three-dimensional specimens collected near Kanab, Utah. These specimens were originally attributed to the Upper Triassic Chinle Formation (Schaeffer and Dunkle, 1950), but further investigation proved that the fully articulated specimens are from the younger, Lower Jurassic Moenave Formation (Schaeffer, 1967; Milner and Kirkland, 2006). In overall body morphology, †S. kanabensis resembles other species of †Semionotus, such as a fusiform body, gently sloping dorsal border, and smooth dorsal ridge scales. However, elements of the skull differ between †S. kanabensis and other species of †Semionotus, such as the ventrally expanded infraorbital series, and new information regarding the biodiversity of western United States semionotiforms (Gibson, 2013) indicates that attributing this species to †Semionotus is in need of reexamination.

Lophionotus sanjuanensis, a hump-backed