550
Journal of Paleontology 91(3):548–553
Materials.—At least two partially articulated individuals com- posed of scales, scale impressions, and possible teeth and fin rays (OMNH 77069–77072). Isolated scales were also recov- ered from the same site, within a few meters laterally of the more complete material.
Occurrence.—OMNH V1700, Navajo Sandstone (Lower Jurassic), ~38km southeast of Cannonville, Kane County, Utah, USA.
Description.—The material described here represents several individuals belonging to one or more indeterminate, medium- sized semionotiform species. Although we cannot exclude the possibility that these specimens represent different taxa, no obvious features (other than size) serve to differentiate them; thus, we will herein refer to them under a single moniker as the Navajo fish. Both partially articulated specimens possess an incomplete squamation composed of rhomboidal ganoid scales (Fig. 2.1–2.4). Preserved scales vary from relatively long and caudally pointed in apparent dorsal ridge scales (Fig. 2.5, 2.6), to taller than long and more rectangular in the scales of the mid- body. Isolated scales indicate that this specimen lacks a large peg and socket articulation, although a small dorsal projection and correspondingmedial groove can be seen on one scale impression (Fig. 2.7, 2.8). In addition, some isolated scales possess both the rostral and rostroventral projection used by Cavin et al. (2009) to diagnose isolated semionotid scales from Thailand. The rostroventral projection is nearly a third the size of the rostrodorsal process in this specimen (OMNH 69349), but other scales appear to lack this feature altogether (Fig. 2.9, 2.10). Given the lack of comprehensive scale studies for semionotid fishes, we tentatively attribute these differences to intraspecific variation between scales from varying parts of the body. The two most complete specimens are composed of
articulated scales preserved mostly as impressions with incon- sistent, small occurrences of heavily mineralized scale or bone. The first specimen (OMNH 77070, Fig. 2.1, 2.2) consists of dorsal and rostral scales abutted to impressions that compare favorably to the triangular posttemporal and rounded supra-
cleithrum of other semionotids (Olsen and McCune, 1991, fig. 4A).As preserved, this specimen is at least seven scales long rostrocaudally, measuring ~30mm in length. The dorsal margin is also preserved in this specimen, typified by scales with a highly modified elongate and caudally directed spine. Moving ventrally, the scales become more poorly preserved, but appear to elongate closer to the hypothesized midline of the fish. The second specimen (OMNH 77069) is larger, with an unbroken series at least eight scales tall by seven scales long, ~50mm in total length as preserved (Fig. 2.3, 2.4). This specimen is composed largely of tall mid-ventral scales, as well as remnants of one of the ventral fins (likely the pelvic fin). Multiple scales on the underside of the block containing this specimen belong to an additional individual. Of interest are two scales preserved with their elongate caudal processes aligned into a single ridge, representing an additional specimen bearing the characteristic semionotid dorsal ridge crest (Fig. 2.5, 2.6). The scales of all of the specimens are relatively smooth, lacking obvious large tubercles. A third block preserves what appear to be cross- sections of small, circular teeth packed in close proximity to one
another. They appear to be of a crushing-style tooth morpho- logy, which McCune (1986) attributed mostly to Lepidotes species, but acknowledged that this is likely size-related and exceptions do occur. The Navajo fish does not have the large humped back seen in Lophionotus sanjuanensis Gibson, 2013a, nor is its body greatly thickened dorsoventrally as in many other semionotiform species (e.g., Jain, 1984; McCune, 1986; Wenz, 2003). In life, this species would have been a medium-sized semionotiform, eclipsed by some of the Late Jurassic and Cretaceous species (e.g., Jain, 1984), but substantially larger than the diminutive Lophionotus kanabensis Schaeffer and Dunkle, 1950 (Gibson, 2013b), one of the few described species of semionotids from the Western United States, which is not known to exceed 74mm in length.
Remarks.—The Navajo fish material is decidedly similar to other North American Jurassic fish, mainly of the genera Lepidotes and Semionotus (the latter of which likely represents two genera, including Lophionotus of Gibson, 2013a, 2013b, but which will be treated here as single genus for historical context); however, difficulties differentiating these genera are well documented (Schaeffer, 1967; McCune, 1986). Precise determination of the Navajo fish species is hindered in part by the state of preservation of the few known specimens. No skull was recovered with any of the specimens, making genus- and species-level identification impossible. Nonetheless, the elongate dorsal ridge scales and scale morphology provide enough comparative material to confidently assign the order Semionotiformes. Further, given the numeric abundance of Semionotus and Lepidotes during the Early to Middle Jurassic, it would be reasonable to hypothesize that this material represents one of these taxa. Olsen and McCune (1991, p. 270) condensed both genera into a restricted definition of the family Semionotidae, based on two synapomorphies: the presence of dorsal ridge scales; and a large, posteriorly directed “epiotic.” More recent phylogenetic analyses of semionotiform relation- ships, however, demonstrated that even this reduction is para- phyletic. Cavin (2010) recovered Lepidotes and Semionotus as consecutive branches on an unnamed node nested within a monophyletic Semionotiformes. Like previous analyses, Semionotiformes has been recovered as a monophyletic group, containing Macrosemiidae, Semionotidae, and Lepisosteidae (Olsen and McCune, 1991; Brito, 1997; Cavin et al., 2003). Cavin’s (2010) phylogeny further recovers an unnamed clade within Semionotiformes to the exception of the Macrosemiidae, which can be diagnosed in part by the presence of dorsal ridge scales. This character, which is unambiguously present on the Navajo material, is highly conspicuous in most Semionotus species, but often more discreet in Lepidotes (McCune, 1986). However, a more recent phylogenetic analysis by López- Arbarello (2012; corroborated by Gibson, 2013a, 2013b, 2016) brings into question the close relationship between Semionotus and Lepidotes. In this analysis, Lepidotes was recovered in a separate clade (order Lepisosteiformes) more similar to the modern gar (Lepisosteus Lacepède, 1803 and Atractosteus Rafinesque, 1820) than to Semionotus. Semionotus was found to be monophyletic (Semionotidae), nested within a broader order Semionotiformes. The large, conspicuous dorsal ridge, used by previous authors to diagnose semionotids, was recovered in this
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