Frederickson and Davis—First actinopterygian from Lower Jurassic Navajo Sandstone


551


Figure 2. Semionotiform specimens recovered from OMNH V1700. (1, 2) OMNH 77070, a specimen preserving the rostrodorsal squamation, including the characteristic dorsal ridge scales (black arrow); (3, 4) OMNH 77069, mid-ventral scale pattern and possible pelvic fin rays; (5, 6) OMNH 77069, dorsal ridge scales likely from a second individual on the same slab as Figure 2.3, 2.4; (7, 8) OMNH 69349, isolated ganoid scale preserving the peg and socket joint; (9, 10) OMNH 69350, isolated ganoid scale without the peg and socket joint.


analysis as an autapomorphy for the genus Semionotus (or a synapomorphy for Semionotidae, including both Lophionotus and Semionotus [Gibson, 2013b]). This split largely agrees with Grande’s (2010) phylogeny, where Semionotiformes and Lepi- sosteiformes form sister groups within the broader Ginglymodii. Without a more complete dorsal series, it is nearly impossible for us to assign the Navajo specimens to either the more Lepidotes-like Lepisosteiformes or the Semionotus-like Semi- onotiformes using this scheme. Given that the aforementioned taxonomic issues require a more comprehensive revaluation of


the entire clade, we hesitate to delve into this debate. Here we stick to a more classic definition of Semionotiformes (sensu Olsen and McCune, 1991), with the understanding that more complete material will require a thorough incorporation of this species into the current ginglymodian phylogeny.


Discussion


Paleoenvironmental significance.—Due to an increased risk of predation by wading predators at shallow water depths, it has


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