Saws, Scissors, and Sharks: Late Paleozoic Experimentation with Symphyseal Dentition

Abstract

Sharks of Late Paleozoic oceans evolved unique dentitions for catching and eating soft bodied prey. A diverse but poorly preserved clade, edestoids are noted for developing biting teeth at the midline of their jaws. Helicoprion has a continuously growing root to accommodate >100 crowns that spiraled on top of one another to form a symphyseal whorl supported and laterally braced within the lower jaw. Reconstruction of jaw mechanics shows that individual serrated crowns grasped, sliced, and pulled prey items into the esophagus. A new description and interpretation of Edestus provides insight into the anatomy and functional morphology of another specialized edestoid. Edestus has opposing curved blades of teeth that are segmented and shed with growth of the animal. Set on a long jaw the lower blade closes with a posterior motion, effectively slicing prey across multiple opposing serrated crowns. Further examples of symphyseal whorls among Edestoidae are provided from previously undescribed North American examples of Toxoprion, Campyloprion, Agassizodus, and Sinohelicoprion. The symphyseal dentition in edestoids is associated with a rigid jaw suspension and may have arisen in response to an increase in pelagic cephalopod prey during the Late Paleozoic.

Redefining species concepts for the Pennsylvanian scissor tooth shark, Edestus

“…Teeth are preserved either as single ejected elements, or they are stacked en echelon together as part of a whorl containing up to a dozen crowns. According to Zangerl [1] and supported by Tapanila et al [2], lower whorls generally have a greater curvature than upper whorls, and they both form a spiral that, each, could have produced in excess of 40 teeth during the lifetime of an animal, based on measurements presented here.…”

“…Chondrichthyans (Euchondrocephali) belonging to the Edestoidea evolved some of the most unusual dental arrangements among vertebrates [1, 2]. The most conspicuous teeth are located in the symphysis of the lower jaw, forming a whorl of sharp, cutting crowns, best exemplified by Helicoprion Karpinsky, 1899 [3, 4].…”

“…Nearly a century later, Zangerl and Jeremiah [6] documented at least three more specimens indicating dimorphism between the upper and lower whorls, but they stopped short of quantifying those differences or placing them in context of the cranium. CT scanning and 2D x-rays of Zangerl and Jeremiah’s specimens confirm a dimorphic upper and lower whorl and reveal their function as opposing cutting surfaces in the mandibular arch [2]. Although much has been gleaned about the form and function of Edestus from these specimens, the systematics of Edestus remain complicated by the fact that nearly all specimens of Edestus whorls lack cranial context or are recovered as individual teeth disaggregated from the whorl.…”

Developmental bias, macroevolution, and the fossil record

“…However, at least 95% of the multicellular species that ever lived are extinct (Sepkoski, 1992), and among those extinct taxa are many unexpected forms. The fossil menagerie is almost endless but includes echinoids with periscope-like test extensions, tightly coiled and helical oysters, uncoiled, helical, and spiny nautiloids, sharks with coiled tooth batteries, six-fingered tetrapods (basally and with a second derivation in ichthyosaurs), crocodilians with bulbous shell-crushing teeth, and giant ground sloths including aquatic ones (for sources see Amson, Argot, McDonald, & de Muizon, 2015;Caldwell, 2002;Coates & Clack, 1990;Gale & Smith, 1982;Oyston, Hughes, Wagner, Gerber, & Wills, 2015;Seilacher & Gishlick, 2014;Tapanila, Pruitt, Wilga, & Pradel, 2018;Vermeij, 2015;Westermann, 1998). The fossil menagerie is almost endless but includes echinoids with periscope-like test extensions, tightly coiled and helical oysters, uncoiled, helical, and spiny nautiloids, sharks with coiled tooth batteries, six-fingered tetrapods (basally and with a second derivation in ichthyosaurs), crocodilians with bulbous shell-crushing teeth, and giant ground sloths including aquatic ones (for sources see Amson, Argot, McDonald, & de Muizon, 2015;Caldwell, 2002;Coates & Clack, 1990;Gale & Smith, 1982;Oyston, Hughes, Wagner, Gerber, & Wills, 2015;Seilacher & Gishlick, 2014;Tapanila, Pruitt, Wilga, & Pradel, 2018;Vermeij, 2015;Westermann, 1998).…”

“…These extinct taxa inform us on ancestral states of developmental systems, and shift our understanding of what lineages can do (Marshall, 2017;Organ, Cooper, & Hieronymus, 2015;Raff, 2007;Sánchez, 2012;Urdy et al, 2013;Wilson, 2013). The fossil menagerie is almost endless but includes echinoids with periscope-like test extensions, tightly coiled and helical oysters, uncoiled, helical, and spiny nautiloids, sharks with coiled tooth batteries, six-fingered tetrapods (basally and with a second derivation in ichthyosaurs), crocodilians with bulbous shell-crushing teeth, and giant ground sloths including aquatic ones (for sources see Amson, Argot, McDonald, & de Muizon, 2015;Caldwell, 2002;Coates & Clack, 1990;Gale & Smith, 1982;Oyston, Hughes, Wagner, Gerber, & Wills, 2015;Seilacher & Gishlick, 2014;Tapanila, Pruitt, Wilga, & Pradel, 2018;Vermeij, 2015;Westermann, 1998).…”