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| Oncorhynchus rastrosus (Cavender & Miller, 1972) in Claeson, Sidlauskas, Troll, Prescott et Davis, 2024. |
Abstract
The impressive †Oncorhynchus rastrosus of the Pacific Northwest’s Miocene and Pliocene eras was the largest salmonid ever to live. It sported a hypertrophied premaxilla with a pair of enlarged teeth which the original describers reconstructed as projecting ventrally into the mouth, leading them to assign the species to “Smilodonichthys,” a genus now in synonymy. Through CT reconstruction of the holotype and newly collected specimens, we demonstrate that the famed teeth projected laterally like tusks, not ventrally like sabers or fangs. We also expand the original description to characterize sexual dimorphism in mature, breeding individuals. Male and female †Oncorhynchus rastrosus differ in the form of the vomer, rostro-dermethmoid-supraethmoid, and dentary, much as do other extant species of Oncorhynchus. Male specimens possess a more elongate vomer than do females, and female vomers have concave ventral surfaces and prominent median dorsal keels. The dentary of females has no evidence of a kype, though some specimens of †O. rastrosus have a non-uniform density mesial to the tooth bed, which we interpret as a male kype. Unlike extant Oncorhynchus, male and female †O. rastrosus do not differ in premaxilla shape. Because male and females possess hypertrophied premaxillae and lateral premaxillary spikes, the former common name “Sabertoothed Salmon” no longer reflects our understanding of the species’ morphology. Accordingly, we redub †O. rastrosus the Spike-Toothed Salmon and postulate that its spikes were multifunctional, serving as defense against predators, in agonism against conspecifics, and as a practical aid to nest construction.
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| Oncorhynchus rastrosus. (A) CT model of Holotype, UO F-26799, skull in right lateral view with a stylized drawing of the originally proposed “sabertoothed” position of the isolated premaxilla; (B) UO_A in anterior view of skull, prior to complete preparation and CT scan; (C) Artist’s rendering skull of male iconic fish with accurate spike-tooth configuration; (D) Artist’s rendering of complete female iconic fish with accurate spike-tooth configuration. Scale bar blocks = 1 cm each. |
Conclusion:
Extant Pacific salmon are sexually dimorphic, anadromous fishes navigating complex terrains to successfully breed. They undergo impressive physiological and morphological transformations prior to and during migration inland to spawn. †Oncorhynchus rastrosus was no exception, despite its huge size and unusual filter feeding ecology. The newly recovered specimens from the Gateway Locality of Oregon represent mature individuals with subtle dimorphism in the vomer, rostro-dermethmoid-supraethmoid, and dentary. Male specimens possess a more elongate vomer than do females and do not have a dorsal keel as in females. The female dentary has no evidence of a kype and possesses two mesial teeth. Male specimens of †O. rastrosus have a non-uniform density deep to the tooth bed and edentulous portion of the dentary, which we interpret as evidence of a kype. Finally, the rds is blunt in females and pointed in males. Unlike extant Oncorhynchus, male and female specimens of †O. rastrosus do not differ in premaxilla shape and both sexes possess prominent, laterally-directed premaxillary teeth. These spikes on the hypertrophied premaxilla could have defended against predators, enhanced agonism against conspecific individuals, and/or aided nest construction.
Kerin M. Claeson, Brian L. Sidlauskas, Ray Troll, Zabrina M. Prescott and Edward B. Davis. 2024. From Sabers to Spikes: A newfangled Reconstruction of the Ancient, Giant, sexually dimorphic Pacific Salmon, †Oncorhynchus rastrosus (SALMONINAE: SALMONINI). PLoS ONE. 19(4): e0300252. DOI: 10.1371/journal.pone.0300252
