Figure 1.
Postcranial remains of Casea broilii (FMNH UR 2512).
A–B: dorsal view. C–D: ventral view. E–F: right lateral view. Abbreviations: as, astragalus; ca1, first caudal vertebra; fe, femur; fi, fibula; lu, lumbar vertebra; lur, lumbar vertebral rib; il, ilium; isc, ischium; itr, internal trochanter; lur, lumbar rib; ns, neural spine; prz, prezygapophysis; psz, postzygapophysis; pu, pubis; s1–3, sacral vertebrae 1–3; sr1–3, scaral ribs 1–3; ti, tibia.
Figure 2.
Preserved left astragalus of Casea broilii (FMNH UR 2512) in lateral view.
Note the presence of a groove for the perforating artery that passes across the entire facet for the calcaneum. Anterior is to the right.
Figure 3.
Close-up of the sacral region of Casea broilii (FMNH UR 2512).
Note the overlap of the second sacral rib over the first sacral rib. Abbreviations: prz, prezygapophysis of first sacral vertebra; psz, postzygapophysis of first sacral vertebra; s1–3, sacral vertebrae 1–3; sr1–3, scaral ribs 1–3.
Figure 4.
Close-up and reconstruction (inset) of the sacral region of Eocasea martini (KUVP 9616b).
Abbreviations: ca1, first caudal vertebra; cr1, first caudal rib; lsr1–2, left sacral ribs 1–2; ns, neural spine; prz, prezygapophysis of first sacral vertebra; psz, postzygapophysis of first sacral vertebra; rsr1–2, right sacral ribs 1–2; s1–2, sacral vertebrae 1–2.
Figure 5.
Reconstructions of sacral vertebrae of basal synapsids examined in this paper and hypothesized homologous sacral positions.
A: reconstruction of sacral vertebrae of Eocasea martini based on the holotype (KUVP 9616b). B: reconstruction of sacral vertebrae of Casea broilii based on FMNH UR 2512. C: reconstruction of sacral vertebrae of Varanosaurus acutirostris modified from Sumida [19]. D: reconstruction of sacral vertebrae of Ophiacodon retroversus from Romer and Price [8]. E: reconstruction of the sacral vertebrae of the basal varanopid Archaeovenator hamiltonensis based on the holotype (KUVP 12483). F: reconstruction of the sacral vertebrae of Varanodon agilis based on FMNH UR 986. G: reconstruction of the sacral vertebrae of Edaphosaurus boanerges based on the description of MCZ 1372 from Romer and Price [8] and ROM 7985. H: reconstruction of the sacral vertebrae of Dimetrodon limbatus, modified from Romer and Price [8]. Dashed lines indicate portions of the skeleton that were either extrapolated from the literature or were damaged on the specimens.
Figure 6.
Sacral regions of the basal varanopid Archaeovenator hamiltonensis (KUVP 12483) and the varanodontine Varanodon agilis (FMNH UR 986).
A: Sacral region of KUVP 12483 and reconstruction of the sacral vertebrae (inset). B: Sacral region of FMNH UR 986 and reconstruction of the sacral vertebrae (inset). Abbreviations: ca1, first caudal vertebra; cr1, first caudal rib; ns, neural spine; s1–2, sacral vertebrae 1–2; sr1–2, sacral ribs 1–2; prz, prezygapophysis of first sacral vertebra; psz, postzygapophysis of first sacral vertebra.
Figure 7.
The distribution of sacral vertebral counts mapped on a time-calibrated phylogeny of basal synapsids (modified from Sues and Reisz [42], Laurin and Reisz [41], Fröbisch et al. [43], and Campione and Reisz [13]).
1, Diadectes sp.; 2, Orobates pabsti; 3, Casea broilii; 4, Eocasea martini; 5, Varanosaurus acutirostris; 6, Ophiacodon retroversus; 7, Archaeovenator hamiltonensis; 8, Heleosaurus scholtzi; 9, Varanodon agilis; 10, Aerosaurus wellesi; 11, Edaphosaurus boanerges; 12, Ianthosaurus hardestiorum; 13, Haptodus baylei; 14, Dimetrodon limbatus; 15, Biarmosuchus antecessor; 16, Suminia getmanovi. Solid lines indicate known stratigraphic distributions of taxa, dashed lines indicate ghost lineages. White circles and diamonds correspond to hypothetical sacral counts for clades based on observed sacral counts of representative taxa. Stratigraphic occurrences and descriptions of the sacral regions for these taxa were taken from [5], [6], [8], [13], [19], [21], [23], [25], [34], [35], [44]–[47].