A reassessment of the postcanine dentition and systematics of the tritylodontid Stereognathus (Cynodontia, Tritylodontidae, Mammaliamorpha), from the Middle Jurassic of the United Kingdom

Figures & data

TABLE 1. Measurements of U.K. Stereognathus material.

Specimen no.	Species	Locality	Description	Length (mm)	Width (mm)	Width/length
BRSUG 20572	hebridicus		Holotype upper	5.1	5.4	1.06
BRSUG 20573	hebridicus	Kilmaluag	Paratype upper	4.8	5.4	1.13
BRSUG 20574	hebridicus	Kilmaluag	Paratype lower right	5.1	3.2	0.63
BRSUG 20575	hebridicus	Kilmaluag	Paratype lower right	5.8	3.8	0.66
BRSUG 29000	hebridicus	Kilmaluag	Fragmentary: upper left molar	NA	4.7	—
			Fragmentary: lower molar	3.1e	2.3e	0.74
			Fragmentary: cusps	NA	3.5	—
BRSUG 29002	hebridicus	Kilmaluag	Upper right	3.6	NA	—
BRSUG 28996_A	hebridicus	Kilmaluag	Upper right	4.5	4.9	1.09
BRSUG 28996_B	hebridicus	Kilmaluag	Upper left	4.2	5.2	1.24
BRSUG 28996_C	hebridicus	Kilmaluag	Upper right	4.4	5.5	1.25
BRSUG 28996_D	hebridicus	Kilmaluag	Upper right	2.3	2.6	1.13
BRSUG 28996_E	hebridicus	Kilmaluag	Upper right	4.3e	5.1e	1.19
BRSUG 28997_A	hebridicus	Kilmaluag	Upper	3.2e	3.7	1.16
BRSUG 28997_B	hebridicus	Kilmaluag	Upper	4.1	5.0	1.22
BRSUG 28997_C	hebridicus	Kilmaluag	Upper	4.2e	5.0e	1.19
BRSUG 28997_D	hebridicus	Kilmaluag	Upper	4.9	5.5	1.12
BRSUG 28997_E	hebridicus	Kilmaluag	Upper	2.6	2.6	1.00
BRSUG 28997_F	hebridicus	Kilmaluag	Upper	3.6	4.2	1.17
BRSUG 28997_G	hebridicus	Kilmaluag	Upper	4.0	4.8	1.20
BRSUG 28998_A	hebridicus	Kilmaluag	Lower	5.5	3.6	0.65
BRSUG 28998_B	hebridicus	Kilmaluag	Lower	5.6	3.5	0.63
BRSUG 28998_C	hebridicus	Kilmaluag	Lower	4.9e	3.4	0.69
BRSUG 28998_D	hebridicus	Kilmaluag	Lower	5.2	3.2	0.62
BRSUG 28998_E	hebridicus	Kilmaluag	Lower	5.2	3.3	0.63
BRSUG 28998_F	hebridicus	Kilmaluag	Upper	4.6	5.0	1.09
BRSUG 28999_A	hebridicus	Kilmaluag	Lower	5.2	3.3	0.63
BRSUG 28999_B	hebridicus	Kilmaluag	Lower	5.1	3.2	0.63
BRSUG 28999_C	hebridicus	Kilmaluag	Lower	3.9	2.6	0.67
BRSUG 28999_D	hebridicus	Kilmaluag	Lower	2.6	1.8	0.69
BRSUG 28999_E	hebridicus	Kilmaluag	Upper fragment	2.7	NA	—
NMS G.1992.47.120	hebridicus	Kilmaluag	Lower	5.7	3.6	0.63
NMS G.2017.17.1	hebridicus	Kilmaluag	Lower	4.2	NA
NMS G.2017.17.2	hebridicus	Kilmaluag	Upper	3.8	4.3	1.13
NMS G.2017.17.3	hebridicus	Kilmaluag	Lower	5.5e	3.8	0.69
NMS G.2017.17.4	hebridicus	Kilmaluag	Upper fragmented	3.2e	4.5e	1.41
NMS G.2017.17.5	hebridicus	Kilmaluag	lower	NA	3.6
NMS G.2017.17.6	hebridicus	Kilmaluag	Lower	5.7	4.1	0.72
BGS GSM113834	ooliticus	Stonesfield	Jaw fragment with 3PCs: anterior	3.1e	NA	—
	ooliticus	Stonesfield	Middle	3.4	3.6	1.06
	ooliticus	Stonesfield	Posterior	3.5	3.5e	1.00
GLRCM G75710-ulm	ooliticus	Hornsleasow	Upper	4.5	4.5	1.02
GLRCM MLR 20–22	ooliticus	Hornsleasow	Upper	4.2	3.4e	0.79
GLRCM MLR 20–38	ooliticus	Hornsleasow	Upper	4.1e	4.0e	0.97
GLRCM H174	ooliticus	Hornsleasow	Lower	5.8	3.6	0.62
GLRCM TEMP2105_4	ooliticus	Hornsleasow	Lower	4.3e	3.7	0.86
GLRCM TEMP2105_6	ooliticus	Hornsleasow	Lower	3.9e	2.9	0.74
OUMNH J.79435	ooliticus	Kirtlington	Upper	3.1	NA	—
OUMNH J.79439	ooliticus	Kirtlington	Lower	3.2	NA	—
OUMNH J.79480	ooliticus	Kirtlington	Upper	3.2	3.8	1.19
DORCM G11048	ooliticus	Forest Marble	Lower	5.3	3.2	0.60
DORCM G10828-lrm	ooliticus	Forest Marble	Lower	5.4	3.3	0.61

Measurements where slight breakages or wear made measurement uncertain (estimated measurements referred to in text) are indicated with an ‘e’ and underlined where breakage was more significant.

FIGURE 1. Postcanine cusp terminology for Stereognathus used in this paper (modified from Watabe et al., 2007). The PIA (posterior interlocking area) and AIA (anterior interlocking area) are present on both teeth, but the PIA is not visible in occlusal view on upper postcanines and the AIA is not visible in occlusal view on lower postcanines, because they are located on the underside of the tooth.

FIGURE 2. Stereognathus ooliticus, BGS GSM113834, holotype. A₁, occlusal view; A₂, occlusal view digital reconstruction, with teeth segmented from jaw; B₁, buccal view; B₂, buccal view digital reconstruction, with teeth segmented from jaw; C₁, lingual view; C₂, lingual view digital reconstruction, with teeth segmented from jaw; D, dorsal view of maxilla. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 3. Stereognathus ooliticus, BGS GSM113834, holotype, postcanines only. Digitally reconstructed from microCT scans and segmented from the jaw. A, occlusal view of anterior-most postcanine; B, occlusal view of middle postcanine; C, occlusal view of posterior-most postcanine; D, original drawing by Owen (1857); E, occlusal view digital reconstruction; F, dorsal view digital reconstruction; G, anterolingual view digital reconstruction; H₁, anterior view of anterior-most postcanine digital reconstruction; H₂, posterior view of anterior-most postcanine digital reconstruction; I₁, anterior view of middle postcanine digital reconstruction; I₂, posterior view of middle postcanine digital reconstruction; J₁, anterior view of posterior-most postcanine digital reconstruction; J₂, posterior view of posterior-most postcanine digital reconstruction. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 4. Stereognathus hebridicus, BRSUG 20572, holotype. A₁, occlusal view; A₂, occlusal view digital reconstruction; B, dorsal view digital reconstruction; C₁, anterior view; C₂, anterior view digital reconstruction; D, buccal view digital reconstruction; E₁, posterior view; E₂, posterior view digital reconstruction; F, lingual view digital reconstruction. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 5. Stereognathus hebridicus, BRSUG 20573, paratype, upper postcanines, and new specimen NMS G.2017.17.2, both reconstructed digitally from micro CT scans. A–F, BRSUG 20573: A₁, occlusal view; A₂, occlusal view digital reconstruction; B₁, anterior view; B₂, anterior view digital reconstruction; C₁, posterior view; C₂, posterior view digital reconstruction; D₁, lingual view; D₂, lingual view digital reconstruction; E₁, buccal view; E₂, buccal view digital reconstruction; F, dorsal view. G–M, NMS G.2017.17.2: G₁, occlusal view; G₂, occlusal view digital reconstruction; H₁, anterior view; H₂, anterior view digital reconstruction; I, posterior view digital reconstruction; J, lingual view digital reconstruction; K, buccal view digital reconstruction; L, dorsal view digital reconstruction; M, ventrolingual view digital reconstruction. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 6. Stereognathus hebridicus, BRSUG 20574 and BRSUG 20575, paratypes, lower postcanines. A–E, BRSUG 20574: A₁, occlusal view; A₂, occlusal view digital reconstruction; B₁, posterior view; B₂, posterior view digital reconstruction; C₁, anterior view; C₂, anterior view digital reconstruction; D₁, lingual view; D₂, lingual view digital reconstruction; E₁, buccal view; E₂, buccal view digital reconstruction. F–J, BRSUG 20575: F₁, occlusal view; F₂, occlusal view digital reconstruction; G₁, posterior view; G₂, posterior view digital reconstruction; H₁, anterior view; H₂, anterior view digital reconstruction; I₁, lingual view; I₂, lingual view digital reconstruction; J₁, buccal view; J₂, buccal view digital reconstruction. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 7. New specimen NMS G.1992.47.120, a lower postcanine, reconstructed digitally from micro CT scans. A₁, occlusal view; A₂, occlusal view digital reconstruction; B, posterior view digital reconstruction; C, anterior view digital reconstruction; D₁, lingual view; D₂, lingual view digital reconstruction; E, anterolingual view digital reconstruction; F₁, buccal view; F₂, buccal view digital reconstruction; G, posterobuccal view digital reconstruction. Anterior direction indicated by longer black arrow, lingual by shorter arrow.

FIGURE 8. Scatterplots of postcanine measurements of Stereognathus. A, upper postcanines; B, lower postcanines. Key for B, as in A. Solid symbols denote complete specimens; open symbols denote incomplete specimens (orange square = S. ooliticus; blue diamond = S. hebridicus). Measurements in Table 1.

FIGURE 9. Distributions of dimensions of Stereognathus postcanine specimens. A, upper postcanine length; B, upper postcanine width; C, lower postcanine length; D, lower postcanine width. Orange striped bars = S. ooliticus; blue bars = S. hebridicus.

TABLE 2. The data set used for analysis, including estimated measurements. Measurements in mm.

	S. ooliticus				S. hebridicus
	Uppers		Lowers		Uppers		Lowers
Statistic	Length	Width	Length	Width	Length	Width	Length	Width
Mean	3.59	3.80	4.94	3.33	3.91	4.63	4.89	3.25
Sample size	9	5	5	5	19	15	16	15
Sample range	1.37	1.20	1.95	0.75	2.81	2.90	3.20	2.27
s^2	0.10	0.23	0.69	0.10	0.62	0.95	0.91	0.36
s	0.32	0.48	0.83	0.31	0.79	0.97	0.95	0.60

FIGURE 10. Trees generated by our phylogenetic analysis of tritylodontid taxa, using updated character codings for Stereognathus. A, strict consensus of the five parsimonious trees of 71 steps; B, the agreement subtree of 10 taxa.

APPENDIX 2. Character matrix used for phylogenetic analysis. Polymorphisms are as follows: A (0, 2); B (2, 6); C (1, 0); D (2, 3)

Taxon	10	20	30	35
Bienotherium	1100111011	-011200???	??02111010	21?11
Bienotherium	1110102100	1101A1????	??0BC11110	10000
Bocatherium	1100102100	110111???0	0210111220	?????
Dianzhongia	11????20??	?????0????	??11111001	3????
Dinnebitodon	11????21??	?1???1????	??01111002	?????
Kayentatherium	1100112001	-0?1200110	0002111010	??0??
Lufengia	11????20??	?????0????	???2111000	10?12
Montirictus	??????????	??????????	???0111220	20011
Oligokyphus	0000210011	-011300111	1107000000	21111
Polistodon	11?101????	?????000?1	01?0111220	??0??
Stereognathus	??????????	?1?1?1????	???0011110	D10?0
Tritylodon	00???10011	-011200??0	0212111000	11???
Xenocretosuchus	??????????	??????????	???0111220	??0??
Yunnanodon	11????????	??????????	???1111002	10?12
Yuanotherium	??????2100	11??101???	???4000011	?????
Shartegodon	1100?02100	010010000?	??05100110	00000
Nuurtherium	?????????0	?1???????0	0003100010	10000

Watabe, M., T. Tsubamoto, and K. Tsogtbaatar. 2007. A new tritylodontid synapsid from Mongolia. Acta Palaeontologica Polonica 52:263–274.

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Owen, R. 1857. On the affinity of Stereognathus ooliticus (Charlesworth) a mammal from the Oolitic slate of Stonesfield. Quarterly Journal of the Geological Society of London 13:1–11.

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