Figure 1.
Torosaurus and Triceratops compared.
A, Triceratops prorsus YPM 1822 and B, Torosaurus latus ANSP 15192. Triceratops is characterized by a short frill with a flat squamosal, an upturned caudal margin of the frill, the absence of fenestrae, and a midline epiparietal. Torosaurus is characterized by an elongate frill with a straighter edge, a concave squamosal, and lack of upturning of the frill.
Figure 2.
Distribution of Torosaurus and Triceratops.
1, Scollard Formation, Alberta; 2, Frenchman Formation, Saskatchewan, 3, Hell Creek Formation, Montana; 4, Hell Creek Formation, North Dakota; 5, Hell Creek Formation, South Dakota; 6, Lance Formation, Wyoming; 7, Denver Formation, Colorado; 8, North Horn Formation, Utah; 9, Javelina Formation, Texas.
Figure 3.
Fusion of the skull roof in Chasmosaurinae.
A, Triceratops prorsus YPM 1823; B, Torosaurus latus YPM 1830. Abbreviations: fr, frontal; lac, lacrimal; nas, nasal; pos, postorbital; pre, prefrontal.
Figure 4.
Fusion of the rostrum in Chasmosaurinae.
A, Triceratops horridus USNM 1201; B, Triceratops prorsus YPM 1822. Abbreviations: epn, epinasal; nas, nasal; pmx, premaxilla; ros, rostral.
Figure 5.
Clustering diagram with Triceratops and Torosaurus specimens arranged in developmental sequence.
Inferred sequence of ontogenetic character changes is mapped onto the diagram. An initial run found that MNHN 1912.20, YPM 1830, YPM 1828 and USNM 5740 are adults and that USNM 15583 was immature. Due to missing data these specimens caused a lack of resolution and a second run of the analysis was conducted excluding these specimens. Asterisks indicate character change mappings that are provisional owing to missing or conflicting data.
Figure 6.
Immature features in Torosaurus latus ANSP 15192, a young adult.
A, lateral view; B, dorsal view. Abbreviations: epn, epinasal; max, maxilla; nas, nasal; pmx, premaxilla; pos, postorbital horncore, rec, reconstruction; rgr, rostral groove.
Figure 7.
Immature features in Torosaurus latus YPM 1831.
A, unfused epijugal. A1, anterior view, A2, lateral view; A3, medial view. B, unfused rostral. B1, dorsal view; B2, ventral view. C, occipital condyle formed of unfused exoccipitals and basioccipital. D, caudal margin of parietal showing rounded margin where unfused epoccipitals attach. E, dorsal surface of parietal showing striated surface texture. Scales = 50 mm for A and B, 20 mm for E.
Figure 8.
Comparison of the size of Torosaurus latus specimens.
A, YPM 1831, a subadult, skull length >2.6 m, and B, ANSP 15192, an early adult, skull length 1.8 m. The rostrum of YPM 1831 is reconstructed and would probably have been slightly longer, as in ANSP 15192.
Figure 9.
Comparison of frills in Torosaurus and Triceratops, showing the different position of the parietal fenestrae of Torosaurus and parietal fossae of Triceratops.
A, Triceratops YPM 1823; B, Torosaurus ANSP 15192. Abbreviations: fen, fenestra; fos, parietal-squamosal fossa; par, parietal; sq, squamosal. P0, midline epiparietal; p1–p5 epiparietals 1–5; eps, epoccipital crossing the parietal-squamosal suture.