Editorial Type:
Article Category: Research Article
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Online Publication Date: 23 Nov 2015

Homology of Lateral Cusplets in the Teeth of Lamnid Sharks (Lamniformes: Lamnidae)

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Page Range: 961 – 972
DOI: 10.1643/CG-14-109
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Teeth are a potentially rich source of character data for phylogenetic studies of living and fossil sharks, but there are many uncertainties about the homologies of tooth loci as well as structural details of teeth in different taxa. Here, we provide new developmental data on the so-called lateral cusplets of the teeth in Lamniformes. We studied the five living species of Lamnidae (Lamna nasus, L. ditropis, Isurus oxyrinchus, I. paucus, and Carcharodon carcharias) and a more basal lamniform, the Sand Tiger Carcharias taurus, using light microscopy and micro-CT scanning. We also studied a single skeletal specimen of the smalltooth Sand Tiger, Odontaspis ferox. Structures in the teeth of juvenile White Sharks, C. carcharias, were interpreted as lateral cusplets in some previous studies, but we show that they develop differently from lateral cusplets in the teeth of Lamna and Carcharias. We conclude that the structures in Carcharodon are not homologous to the lateral cusplets of Lamna and Carcharias taurus and propose the new name, serrational cusplet, for them. The distinction is important because the presence of lateral cusplets has been used as a character in previous phylogenetic analyses of Lamniformes. According to our new interpretation, among the species we studied, C. taurus, O. ferox, L. nasus, and L. ditropis have lateral cusplets but I. oxyrinchus, I. paucus, and C. carcharias do not. We interpret that the loss of lateral cusplets is a synapomorphy of Carcharodon and Isurus. This new information and interpretation sets the stage for a more comprehensive reevaluation of dental characters and lamniform phylogeny.

Copyright: © 2015 by the American Society of Ichthyologists and Herpetologists
Fig. 1. 
Fig. 1. 

Three possible resolved trees and one unresolved tree for relationships among the three extant genera of Lamnidae.

Fig. 2. 
Fig. 2. 

Leriche’s interpretation of tooth types in the smalltooth Sand Tiger, Odontaspis ferox. In the palatoquadrate, Leriche recognized symphyseal (S), anterior (A), intermediate (I), and lateral (L) teeth. In the Meckelian cartilage, he recognized symphyseal (S), anterior (A), and lateral (L) teeth. There now are several competing ideas about the homologies of these types of teeth in Lamniformes, contributing to the impression that dental characters are “noisy.” Reproduced from Leriche (1905:fig. 16).

Fig. 3. 
Fig. 3. 

Dentition of the Sand Tiger, Carcharias taurus, to show numbering system for tooth loci. AMNH 53033SD. MS, mandibular symphysis; PQS, palatoquadrate symphysis.

Fig. 4. 
Fig. 4. 

Isolated tooth of Carcharias taurus to show terminology for parts of teeth. (A) View of the labial surface of tooth. The mesial root lobe is slightly longer than the distal root lobe. (B) View of the lingual surface of tooth. (C) View of the distal edge and lateral cusplet of tooth. CU 5944, tooth LM4 F1. CC, central cusp; CRN, crown; LC, lateral cusplet; LF, lingual foramen; LP, lingual protuberance of root; RNA, root notch angle; RT, root; RTL, root lobe, one of two extensions of a tooth’s root; TN, tooth neck.

Fig. 5. 
Fig. 5. 

Development of lateral cusplets in palatoquadrate of Carcharias taurus. AMNH 221727SD. CC, central cusp; LC, lateral cusplet; PQ, palatoquadrate cartilage.

Fig. 6. 
Fig. 6. 

Development and variation in lateral cusplet number in Meckel’s cartilage of Carcharias taurus. CU 5944. CC, central cusp; LC, lateral cusplet; OLC, osteodentine core of lateral cusplet.

Fig. 7. 
Fig. 7. 

Dentition of the Porbeagle, Lamna nasus, to show numbering system for tooth loci. Inset (A): Variation in lateral cusplets at loci LP4 and LP5. Inset (B): Lateral cusplet development for loci RM3, RM4, and RM5. CU 98002. LC, lateral cusplet.

Fig. 8. 
Fig. 8. 

Comparison of teeth in the anterior portion of the palatoquadrates of adult specimens of Isurus paucus and Carcharodon carcharias. CU 98003 and AMNH 53095. CC, central cusp; PQ, palatoquadrate cartilage; SR, serrations.

Fig. 9. 
Fig. 9. 

Lateral view of the teeth in a juvenile specimen of Carcharodon carcharias. The tooth at LM3F1 is broken at its root. MCZ 36470. SRC, serrational cusplet.

Fig. 10. 
Fig. 10. 

Comparison of teeth from juvenile and subadult specimens of Carcharodon carcharias. (A) Labial view of tooth of a juvenile specimen, CU 97912. (B) Lingual view of tooth of a juvenile specimen, CU 97912. (C) Labial view of tooth of a subadult specimen, NMFSN F1. (D) Lingual view of tooth of a subadult specimen, NMFSN F1. CC, central cusp; CRN, crown; LF, lingual foramen; RNA, root notch angle; RT, root; RTL, root lobe; SR, serrations; SRC, serrational cusplet.

Fig. 11. 
Fig. 11. 

3-D models of LM2 (or LM3 for Carcharias taurus) tooth file and stages in developing teeth rendered from CT scans of six species of Lamniformes. (A) Carcharias taurus, CU 5944. (B) Lamna nasus, CU 97646. (C) Lamna ditropis, CAS 55476. (D) Isurus oxyrinchus, CU 97455. (E) Isurus paucus, USNM 196024. (F) Carcharodon carcharias, NMFSN 5 (for tooth file reconstruction on the left) and NMFSN 4 (for the developing and fully formed teeth on the right). CC, central cusp; LC, lateral cusplet; SR, serrations; SRC, serrational cusplet.

Contributor Notes

Associate Editor: G. Arratia.

Received: 19 Jul 2014
Accepted: 23 Dec 2014
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