Strigilodus tollesonae, Hodnett & Toomey & Olson & Tweet & Santucci, 2023

Hodnett, John-Paul M., Toomey, Rickard, Olson, Rickard, Tweet, Justin S. & Santucci, Vincent L., 2023, Janassid petalodonts (Chondrichthyes, Petalodontiformes, Janassidae) from the middle Mississippian (Viséan) Ste. Genevieve Formation, Mammoth Cave National Park, Kentucky, USA, Historical Biology CLXVI (CLXVI), pp. 1-10 : 4-7

publication ID	https://doi.org/ 10.1080/08912963.2023.2231955
DOI	https://doi.org/10.5281/zenodo.10014268
persistent identifier	https://treatment.plazi.org/id/03B13829-FF9D-4A44-FCB5-F97FE159FAFF
treatment provided by	Diego
scientific name	Strigilodus tollesonae
status

Treatment

Strigilodus tollesonae gen et sp. nov

Holotype

MACA 62166 View Materials , a complete symphyseal tooth

Referred Specimens

MACA 62012, a lower mediolateral tooth , MACA 62030, an upper symphyseal tooth , MACA 62055, a upper symphyseal tooth , MACA 62058, an upper symphyseal tooth , MACA 62085, a juvenile upper symphyseal tooth , MACA 62165, a lower mediolateral tooth , MACA 62167, an upper symphyseal tooth , MACA 62168, a lower mediolateral tooth , MACA 62169, an upper mediolateral tooth , MACA 62170, an upper distolateral tooth , MACA 62171, an upper symphyseal tooth , MACA 62172, a juvenile distolateral tooth , MACA 62174, tooth fragment , MACA 62175, a fragmentary upper symphyseal tooth , MACA 62176, an upper symphyseal tooth , MACA 62177, a lower anterolateral tooth , MACA 62179, an upper anterolateral tooth fragment , MACA 62179, an upper anterolateral tooth , MACA 62180, a lower symphyseal tooth , MACA 62181, an upper mediolateral tooth , MACA 62182, an upper mediolateral tooth , MACA 62183, a lower distolateral tooth.

Type Locality and horizon

Mammoth Cave National Park, Edmondson County, Kentucky, Joppa Member, Ste. Genevieve Formation.

Etymology

In honour of Kelli Tolleson, Mammoth Cave National Park guide, educator and cave explorer. Her dedication and exploration helped locate new Mississippian shark localities throughout Mammoth Cave National Park.

Diagnosis

Small teeth, sigmoidal in mesiodistal view, up to 14 mm in crown height. Apical cusp is orally directed and spoon-shaped. Mediolateral and distolateral teeth have minute cusplets on edge of carina. Small labial ridge present at base of crown. Lingual surface of the tooth, from the basal heel to approximately two thirds up the tooth surface bearing 5–14 V- to U-shaped cristae. Tooth base is short and positioned on the labial surface of tooth. Symphyseal teeth diamond shaped. Distolateral positioned teeth broad mesiodistally.

Description

Teeth range between 3 and 14 mm in crown height, with teeth measuring less than 5 mm postulated as representing juveniles ( Figure 3 View Figure 3 ). All teeth show a sigmoidal shape when viewed mesiodistally. Crownsof all teethare directed orally, wide mesiodistally at the labial margin and narrow to a blunt spoon-like point. Alower symphyseal tooth, though overall bearing a spoon-like crown, does have two separate wear facets and could have been slightly bifid in shape. The carina of the symphyseal and anterolateral tooth families lack cusplets. Minute rounded cusplets are present on the carina on the mediolateral and distolateral tooth families. The labial base of the crown bears a thin ridge that can either be straight, slightly curved at the distal margins, or slightly W-shaped. Lingually, a shallow trough is present between the carina and the lingual cristae and which bears multiple fine striations from the lingual trough to the carina. Two thirds of the lingual surface, from the basal heel of the tooth to just below the lingual trough, is ornamented by fine V- to U-shaped cristae that originate from mesiodistal margins and direct towards the basal heel. The number and shape of these lingual cristae are variable based on either tooth position or age. Symphyseal teeth can have up to 14 of the more V-shaped cristae, while the more distal teeth range between 6 and 9 U-shaped cristae. The tooth base is positioned labially at the base of the heel, directed at an approximately 90° angle. The base is tab-like, wide mesiodistally and thin labiolingually. Anumber of small foramina are present on the lingual and labial surfaces of the tooth base.

The shape of teeth is also variable depending on tooth position ( Figure 4 View Figure 4 ). Upper teeth are proportionately longer in height compared to the lower teeth. The tooth base in lower teeth is proportionately longer than those of the upper teeth. The symphyseal teeth are diamond-shaped, with the upper symphyseal teeth having a much narrower shaped cusp compared to the lower symphyseal tooth. The lateral teeth have a straight mesial margin and a rounded distal margin, which the distal margin becomes wider in the successive tooth families. This results with the most distal tooth families being the proportionately wider than they are tall. Longitudinal lingual wear patterns have been observed on a few of teeth presented here.

Comparisons and remarks

The dentition of Strigilodus tollesonae gen et sp. nov shares the typical features of members of the Janassidae in having a sigmoidal mesiodistal profile, a single primary crown, and a relatively short labially placed tooth base that extends at approximately a 90° angle from the lingual heel ( Hansen 1985; Ginter et al. 2010; Lund et al. 2014; Duffin and Ward 2017). Strigilodus differs from Janassa in having teeth with narrow spoon-shaped crowns, rather than wide spatulate-shaped crowns ( Hansen 1985; Lund 1989; Ginter et al. 2010; Lund et al. 2014). Nearly all Janassa species have transverse cristae that are relatively completely horizontal across the lingual surface of the tooth. However, the number and general morphology of lingual cristae in Janassa is variable with the Permian J. bituminosa having multiple thin/fine lingual cristae and the Pennsylvanian J. linguaeformis has a few but pronounced cristae. One species of Janassa that is an exception to having horizontal transverse cristae is Janassa kochi from the middle Permian (Kazanian/Upper Guadalupian) Posidonia Shale member of east Greenland ( Nielson 1932). J. kochi was described from 2 lateral teeth and has approximately 16–17 lingual cristae which are more horizontal at the lingual base and are more of a wide V-shape just below the crown but retains the broad spatulate crown typical of the genus ( Nielson 1932).

Compared with Cypripediodens cristatus ( Duffin and Ward 2017) , the crowns of Strigilisodus are more rounded and less pronounced, with a larger number of thinner of V- to U-shaped cristae. Strigilodus also lacks a well-developed lingual cusp seen in Cypripediodens Duffin and Ward (2017) that may have derived from the lingual cristae. Cavusodus whitei ( Itano 2023) is similar to Strigilodus in having rounded spoon-shaped cusps but the cusps of Cavusodus are proportionately more robust with a blunt carina. The U-shaped lingual cristae in Cavusodus are also much more pronounce and less in number in comparison with Strigilisodus . Cavusodus is also unique in having a pronounce median ridge on the lingual surface and a corresponding labial trough ( Itano 2023). Cholodus inaequalis (St. John and Worthen 1875; Itano 2023) is similar to Cavusodus and Cypripediodens in having a few V- to U-shaped lingual cristae but differ from either taxon with the cristae being flatter and less pronounced in shape, as well as having a narrower chisel-like cusp and a greatly more sigmoidal profile in mesiodistal view.

The dental arrangements of janassids and other petalodonts have been proposed based on articulated or nearly articulated specimens that show the tooth family positions ( Hancock and Atthey 1869; Hancock and Howse 1870; Jaekel 1898, 1899; Woodward 1919; Schaumberg 1979; Lund 1984, 1989; Brandt 1996, 2009; Grogan et al. 2014; Lund et al. 2014). The first reconstructions of janassid dentitions for Janassa linguaeformis ( Atthey 1868; Hancock and Atthey 1869) and Janassa bituminosa ( Hancock and Howse 1870) in which the authors reconstructed the dental series with symphyseal tooth family flanked by three laterals families on either side are based on associated but distorted dentitions of both taxa. Jaekel (1899) illustrated at least two undistorted articulated dentitions of J. bituminosa which prominently shows the upper and lower symphyseal teeth being flanked by two to three tooth families. Though some authors cite that J. bituminosa may have ranged from having a total of five to seven tooth families ( Ginter et al. 2010; Lund et al. 2014), we feel this is a reflection of distolateral teeth being obscured by sediments and are present within the jaw. This can be seen in the illustrations by Jaekel (1899, plate XIV, Figure 1 View Figure 1 ) which shows distolateral teeth barely exposed along the distal margins of the mouth in J. bituminos a. Further suggestion that seven-tooth family arrangement model is most appropriatefor janassids comes from the sister group Petalorhynchidae ( Lund et al. 2014) . As proposed by Lund et al. (2014), the petalorhynchids include Petalorhynchus and ‘ Janassa ’ (‘ Ctenoptychius ’) korni . Lund et al. (2014) described a complete dentition of the Late Mississippian Petalorhynchus beargulchensis which showed a broad spade-like upper symphyseal tooth and a broad bifid lower symphyseal tooth that both were flanked by three chisel-like teeth ( Figure 5E View Figure 5 ). Brandt (1996; 2009) reconstructed the dentition of ‘ Janassa ’ (‘ Ctenoptychius ’) korni with a bifid upper and lower symphyseal tooth with the lower dentition and possibly the upper dentition withthree lateral cuspid teeth ( Figure 5F View Figure 5 ). This seven-tooth family trend may have been the norm for janassids and petalorhynchids. Outside of this group, Belantsea montana also has a large blade-like symphyseal teeth in the upper and lower jaws that are flanked by three lateral teeth ( Lund 1989). Other petalodonts such as members of the Obruchevodidae can have variable numbers in lateral tooth families after the symphyseal tooth ( Grogan et al. 2014; Lund et al. 2014). Fissodopsis robustus and Netsepoye hawesi appear to have approximately seven smaller lateral tooth families in the upper jaw and approximately four to five lateral tooth families in the lower jaws, respectively ( Lund et al. 2014). Conversely, Obruchevodus griffithi appears to have two lateral teeth in the upper jaw and four lateral teeth in the lower jaw from the symphyseal teeth ( Grogan et al. 2014).

From the relatively large sample of isolated teeth collected from Mammoth Cave National Park, we were able to hypothesise a model for the dentition of Strigilodus tollesonae gen et sp. nov using the seven-tooth family position model proposed above for janassid and petalorhynchid petalodonts ( Figure 5A View Figure 5 ). Whether Strigilodus retained its teeth like in Janassa to form a dental battery is unknown at this time, but the labial ridge may indicate the position of the preceding tooth being placed there. For simplicity of discussion, we omitted the retained teeth in our reconstructions.

Spanning from the larger diamond-shaped symphyseal teeth, the three lateral teeth of Strigilodus may have formed a dental fan-like arrangement, progressively becoming shorter in height but broader mesiodistally. This pattern is also reflected in Janassa linguaeformis and Janassa bituminosa ( Hancock and Atthey 1869; Hancock and Howse 1870) ( Figure 5 View Figure 5 B-C). This appears to allow the crowns to act as clipping structures while the lingual cristae act as gripping and crushing platforms for food processing in Janassa and Strigilodus . From the knowledge we gained from the tooth arrangements of Strigilodus and Janassa , we attempted a hypothetical reconstruction of the dental arrangement of Cypripediodens cristatus ( Duffin and Ward, 2017) . The small sample size of Cypripediodens teeth includes what appears to be the upper (NMMUK PV P 73964) and lower symphyseal (NHMUK PV P 73968) teeth, an upper anterolateral tooth (NHMUK PV P 73966) upper and lower mediolateral teeth (NHMUK PV P 73965 and NHMUK PV P 73969), and a possible distolateral tooth (NHMUK PV P 73967), which made a more arclike dental arrangement. Our dental reconstruction model proposes that the upper symphyseal tooth fit between the bifid gap of the lower symphyseal tooth and that upper anterolateral teeth occluded with the cusps of the lower symphyseal. This proposed dental reconstructionof Cypripediodens cristatus ( Figure 5D View Figure 5 ) is unique compared to Janassa and Strigilodus , perhaps suggesting Cypripediodens relied more on the cusps than the lingual cristae for food processing and may have a unique feed niche among the Janassidae .