Ischyodus bifurcatus ( Case, 1978 )
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publication ID |
https://doi.org/ 10.4202/app.01117.2023 |
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persistent identifier |
https://treatment.plazi.org/id/03E18741-1307-EF05-DF68-FD0FFEAAF966 |
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treatment provided by |
Felipe |
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scientific name |
Ischyodus bifurcatus ( Case, 1978 ) |
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Ischyodus bifurcatus ( Case, 1978)
Fig. 2B View Fig .
Material.— Seventeen tooth plates: four left mandibulars, six right mandibulars, five left palatines, and two right palatines from the MFL of the Maastrichtian– Danian Hornerstown Formation , Jean and Ric Edelman Fossil Park, Mantua Township, New Jersey, USA. ( SOM 1 : table 1, Supplementary Online Material available at http://app.pan.pl/SOM/app69- Boles_etal_SOM.pdf) .
Description.—Mandibular tooth plates are robust, labiolingually narrow, and possess a short mesial ramus and “stepped” labial margin (cf. Cicimurri and Ebersole 2014). They possess four tritors in occlusal view: anterior inner tritor, “median” tritor (fused middle and posterior inner tritors; Case 1978), anterior external tritor, and posterior external tritor. The “median” tritor is expansive with an embayment into its mesial margin that creates bifurcations that are unequal in length: the outer bifurcation extends further mesially than the inner bifurcation. The mandibular symphysis is thin and curves longitudinally. The anterior external tritor is positioned slightly ventrally compared to the surface of the “median” tritor and is commonly angled slightly anterolaterally relative to the sagittal axis. Axes of the other tritors are nearly parallel and parasagittal. Where preserved, the labial face is smooth and slightly concave.
Palatine tooth plates are roughly triangular in occlusal view and narrower than the mandibular tooth plates. Four tritors are present: anterior inner tritor, posterior inner tritor, median tritor, and outer tritor. Each of these tritors are long and narrow labiolingually. The anterior inner tritor is positioned on an elevated, roughly flat platform compared to the rest of the lingual face and is commonly angled slightly anterolaterally relative to the sagittal axis. The posterior inner and median tritors are approximately equal in length, though the median tritor occasionally extends slightly further mesially in some specimens. The medial face is flat and straight. The aboral surface is roughly flat in most specimens, except those in which surficial bone and internal tissue of the anterior inner tritor have been lost; in these specimens, erosion has created a deep, rounded longitudinal groove representing the lingual border of the anterior inner tritor tissue. The labial face is flat with a sigmoidal lateral margin.
Remarks.— Gallagher (1993, 2003) listed five chimaeroid fish among the fauna recovered from the EFPQ, including Ischyodus bifurcatus , Ischyodus thurmanni Pictet & Campiche, 1858 , and three species of Edaphodon Buckland, 1838 . The specimens described here most notably differ from Edaphodon in their possession of four tritors (rather than three) in both mandibular and palatine tooth plates Cicimurri and Ebersole 2014; Ward and Grande 1991; Duffin and Reynders 1995; Stahl 1999). Additionally, the mandibular symphysis is thin and curved in mandibular tooth plates described herein, whereas in Edaphodon it is thick and nearly straight/flat ( Cicimurri and Ebersole 2014; Ward and Grande 1991). Finally, the aboral surface of the palatine tooth plates are generally flat, as in Ischyodus , whereas those of Edaphodon usually exhibit a deep longitudinal sulcus ( Cicimurri 2010, and references therein). Among the two species of Ischyodus listed by Gallagher (1993, 2003), the specimens described here can be assigned to I. bifurcatus based on attributes of the tritors: mandibulars exhibit an embayment into the anterior margin of the “median” tritor characteristic of this species, and the median and posterior inner tritors of the palatines extend approximately the same length anteriorly (whereas in I. thurmanni the posterior inner tritor commonly extends further anteriorly; Case 1978). Also as in I. bifurcatus ( Case 1978) , the outer tritor of the palatine is longer and narrower than in I. thurmanni . Mandibular tooth plates of I. rayhaasi Hoganson & Erickson, 2005 , also exhibit an embayment into the “median” tritor, but it is the inner bifurcation that extends further anteriorly as opposed to the outer as in our specimens and I. bifurcatus ( Hoganson and Erickson 2005; Hoganson et al. 2015).
Gallagher (1993, 2003) listed I. thurmanni as the only species of the genus present in the MFL, with I. bifurcatus being recovered from the underlying Navesink Formation. Elsewhere, I. bifurcatus has been described from Santonian to Maastrichtian deposits in Belgium and across the United States ( Case and Schwimmer 1992). Our identification of numerous specimens of I. bifurcatus in the MFL extends its range across the K/Pg boundary into (presumably) the initial few years of the Danian (cf. Esmeray-Senlet et al. 2017).
Stratigraphic and geographic range.— Upper Cretaceous Santonian ) to lower Paleocene (Danian) of California, Delaware, Montana, New Jersey, and North Carolina in the USA, Belgium , and Russia.
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