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. 2020 Feb 19:8:e8418.
doi: 10.7717/peerj.8418. eCollection 2020.

Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper

S Christopher Bennett  1
Affiliations

Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper

S Christopher Bennett. PeerJ. .

Abstract

The six known specimens of Scleromochlus taylori and casts made from their negative impressions were examined to reassess the osteological evidence that has been used to interpret Scleromochlus's locomotion and phylogenetic relationships. It was found that the trunk was dorsoventrally compressed. The upper temporal fenestra was on the lateral surface of skull and two-thirds the size of the lower, the jaw joint posteriorly placed with short retroarticular process, and teeth short and subconical, but no evidence of external nares or antorbital fossae was found. The posterior trunk was covered with ~20 rows of closely spaced transversely elongate dorsal osteoderms. The coracoid was robust and elongate. The acetabulum was imperforate and the femoral head hemispherical and only weakly inturned such that the hip joint was unsuited to swinging in a parasagittal plane. The presence of four distal tarsals is confirmed. The marked disparity of tibial and fibular shaft diameters and of proximal tarsal dimensions indicates that the larger proximal tarsal is the astragalus and the significantly smaller tarsal is the calcaneum. The astragalus and calcaneum bear little resemblance to those of Lagosuchus, and the prominent calcaneal tuber confirms that the ankle was crurotarsal. There is no evidence that preserved body and limb postures are unnatural, and most specimens are preserved in what is interpreted as a typical sprawling resting pose. A principal component analysis of skeletal measurements of Scleromochlus and other vertebrates of known locomotor type found Scleromochlus to plot with frogs, and that finding combined with skeletal morphology suggests Scleromochlus was a sprawling quadrupedal hopper. Phylogenetic analyses found that Scleromochlus was not an ornithodiran, but was either within the Doswelliidae or outside the clade consisting of the most recent common ancestor of the Erythrosuchidae and Archosauria and all its descendants.

Keywords: Anatomy; Archosauriformes; Locomotion; Lossiemouth; Scotland.

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Conflict of interest statement

The author declares that he has no competing interests.

Figures

Figure 1
Figure 1. Skeletal reconstructions of Scleromochlus taylori.
(A) Dorsal view from Woodward (1907). (B) Left lateral view from Huene (1914: fig. 33). (C) Left lateral view redrawn after Benton (1999: fig. 14). Scale bar for C = 3 cm, whereas A and B are scaled to similar sizes.
Figure 2
Figure 2. Skull reconstructions of Scleromochlus taylori in dorsal (A, D and F), palatal (G) and left lateral (B, C, E and H) views.
(A) From Woodward (1907). (B) From Huene (1914: fig. 33). (C) Redrawn from Benton & Walker (1985: fig. 3g). (D and E) Redrawn from Sereno (1991: figs. 16a and 16b). (F, G and H) Redrawn from Benton (1999: fig. 8a and 8b). In (C)–(F) and (H) antorbital fossae are indicated with a darker gray. Scaled to uniform skull length.
Figure 3
Figure 3. Preserved body positions of Scleromochlus taylori specimens in dorsal view based on combined information from dorsal and ventral slabs where present.
Vertebral column in black; skull, limb girdles, propodials and epipodials with dark gray fill; dorsal osteoderms indicated by hatching; reconstructed parts indicated by black dots and light gray fill; and stippling denotes disturbed areas. For more detailed drawings, see Benton (1999; Figs. 2–7). (A) NHMUK R3146. (B) NHMUK R3557. (C) NHMUK R3914. (D) NHMUK R3556. (E) NHMUK R5589. (F) NHMUK R4823/4824. Scale bar = 3 cm.
Figure 4
Figure 4. Stereo pair and interpretive drawing of skull of Scleromochlus taylori, NHMUK R3146A, in dorsal view on dorsal slab.
Stereo pair of photographs and interpretive drawing of polyurethane rubber cast of the skull of Scleromochlus taylori, NHMUK R3146A, in dorsal view on dorsal slab. aof, antorbital fenestra; fr, frontal; gr, groove; ju, jugal; max, maxilla; me, median elements; na, nasal; or, orbit; pa, parietal; pm, posterior margin of parietal; pmx, premaxilla; po, postorbital; qj, quadratojugal; qu, quadrate; sq, squamosal; tf, temporal fenestra; and ?, indeterminate element. Scale bar = 5 mm.
Figure 5
Figure 5. Stereo pairs and interpretive drawings of skull of NHMUK R3146A, in ventral view on ventral slab.
Stereo pairs of photographs and interpretive drawings of polyurethane rubber cast of Scleromochlus taylori, NHMUK R3146A, in ventral view on ventral slab. (A) Skull. (B) Detail of mandibular tip and teeth. The irregular ridge (hatched) running obliquely across the cast is a crack in the slab. max, maxilla; hu, humerus; mr, mandibular ramus; pmx, premaxilla; ra, radius; tc, tooth crown; and vo, vomer. Scale bars = 5 and 3 mm.
Figure 6
Figure 6. Composite interpretive drawing of the skull of Scleromochlus taylori, NHMUK R3146A, in dorsal view combining information from polyurethane rubber casts of the dorsal and ventral slabs.
Elements on the dorsal slab indicated with light gray, elements on the ventral slab indicated with medium gray, and overlapping elements indicated with dark gray. Insets (A) and (B) show the drawings of the dorsal slab and ventral slab (flipped horizontally) to aid in interpretation. aof, antorbital fenestra; fr, frontal; gr, groove; ju, jugal; max, maxilla; me, median elements; mr, mandibular ramus; na, nasal; or, orbit; pa, parietal; pm, posterior margin of parietal; pmx, premaxilla; po, postorbital; qj, quadratojugal; qu, quadrate; sq, squamosal; tc, tooth crown; tf, temporal fenestra; vo, vomer; and ?, indeterminate element. Scale bar = 5 mm.
Figure 7
Figure 7. Stereo pairs and interpretive drawings of Scleromochlus taylori ribs.
Stereo pairs of photographs and interpretive drawings of Scleromochlus taylori. (A) Fortuitous alignment of dorsal ribs that resembles a chevron on right side of NHMUK R3146B on polyurethane rubber cast of dorsal slab. (B) Overlapping dorsal ribs and articulated dorsal vertebra and right rib of NHMUK R5589 in posterior view on PVC cast of dorsal slab. dv, dorsal vertebra; r, dorsal rib. Scale bars = 3 mm.
Figure 8
Figure 8. Stereo pair and interpretive drawing of polyurethane rubber cast of skull and mandible of Scleromochlus taylori, NHMUK R3556, in ventral view on ventral slab.
Stereo pair of photographs and interpretive drawing of polyurethane rubber cast of skull and mandible of Scleromochlus taylori, NHMUK R3556, in ventral view on ventral slab. The irregular sharp ridge (hatched) at far left is a crack in the slab. Lighting is from the left rather than the usual upper left so as to adequately illuminate the right side ramus and palate. bo, basioccipital; bs, basisphenoid; max, maxilla; pmx?, premaxilla?; pt, pterygoid; and v, vomer. Scale bar = 5 mm.
Figure 9
Figure 9. Stereo pair and interpretive drawing of polyurethane rubber cast of distal right femur of Scleromochlus taylori, NHMUK R3556, in oblique posterolateral view on ventral slab.
The hatched feature is interpreted as a fracture in the shaft. lc, lateral condyle; mc, medial condyle; and sh, shaft. Scale bar = 3 mm.
Figure 10
Figure 10. Stereo pairs of photographs and interpretive drawings of left tarsus and metatarsus of Scleromochlus taylori, NHMUK R3556, on dorsal slab.
(A) PVC cast showing distal tarsals 1, 2 and 4 and incomplete metatarsal V. (B) Polyurethane rubber cast (rotated ~90° counterclockwise relative to (A) showing astragalus flattened by pressure covering proximal ends of metatarsals. (C) Silicone rubber peel showing distal tarsals, surface where astragalus ripped off, and complete metatarsal V. Inset in (B) shows astragalus, redrawn and rotated from Padian (1984: fig. 3b). The hatched prominent vertical feature at left in (A) and horizontal feature in (B) and (C) is a crack in the slab. Note that (B) and (C) present metatarsals I–IV in oblique view. as, astragalus; dt, distal tarsal; fi, fibula; mt, metatarsal; ped, pedestal that supported astragalus; and ti, tibia. Scale bars = 3 and 5 mm.
Figure 11
Figure 11. Stereo pairs of photographs and interpretive drawings of PVC casts of Scleromochlus taylori, NHMUK R3557.
(A) Left pelvic girdle and acetabulum in oblique lateral view on side of dorsal slab. (B) Left proximal femur in oblique posterodorsal? view. ac, acetabulum; fe, femur; he, head; is, ischium; pu, pubis; tub, tubercle; and ?, indeterminate element. Scale bar = 3 mm.
Figure 12
Figure 12. Stereo pairs of photographs and interpretive drawings of polyurethane rubber cast of ventral slab of Scleromochlus taylori, NHMUK R3557.
(A) Left distal tibia and fibula, proximal tarsals and metatarsus. (B) Detail of proximal tarsals. Dashed lines indicate reconstructed outlines of elements based on information from the ventral slab, whereas dotted lines indicate information from the dorsal slab. Calcaneum lying atop the astragalus indicated with lighter gray. as, astragalus; ast, tubercle on astragalus; ca, calcaneum; cat, calcaneal tuber; fa, articular facet on astragalus; is, ischium; mt, metatarsal; pu, pubis; and ti, tibia. Scale bars = 5 and 3 mm.
Figure 13
Figure 13. Stereo pair of photographs and interpretive drawing of polyurethane rubber cast of left distal tibia, proximal tarsals, and metatarsus of Scleromochlus taylori, NHMUK R3557, on dorsal slab.
Dotted lines represent the position of the tibia and ischium on the ventral slab. as, parts of astragalus; dt, distal tarsals articulated with metatarsals I–IV; is, ischium; mt, metatarsal; and ti, tibia. Scale bar = 3 mm.
Figure 14
Figure 14. Stereo pairs of photographs and interpretive drawings of polyurethane rubber casts of right pectoral girdle and humerus of Scleromochlus taylori, NHMUK R3914.
(A) In dorsal view on dorsal slab with articulated vertebral column at left. (B) In ventral view on ventral slab with vertebral column at right. The irregular ridges (hatched) running obliquely across the lower right of (A) and right side of (B) are cracks in the slab. Note the small ridge (cross-hatched) in B that represents a crack in the matrix extending from the coracoid toward the vertebral column. co, coracoid; cr, crack in matrix; dvc, vertebral centra; gl, glenoid fossa; hu, humerus; ns, neural spines; r, rib; sc, scapula; and ?, indeterminate element. Scale bars = 5 mm.
Figure 15
Figure 15. Composite interpretive drawing of right pectoral girdle and humerus of Scleromochlus taylori, NHMUK R3914, in dorsal view combining information from polyurethane rubber casts of the dorsal and ventral slabs.
Note the small crack in the matrix extending from coracoid anterior to the humeral head toward the indeterminate element near the vertebral column, which may represent the medial end of an elongate coracoid. Elements on the dorsal slab indicated with light gray, elements on the ventral slab indicated with medium gray, and overlapping elements indicated with dark gray. Compare to Fig. 14. co, coracoid; cr, crack in matrix; dvc, dorsal vertebral centra; gl, glenoid fossa; hu, humerus; ns, neural spines; r, rib; sc, scapula; and ?, indeterminate element. Scale bar = 5 mm.
Figure 16
Figure 16. Stereo pair of photographs and interpretive drawing of polyurethane rubber cast of dorsal osteoderms of Scleromochlus taylori, NHMUK R4823, in ventral view on ventral slab.
The oblique feature (hatched) is a crack in the slab. The irregular stipple is the layer of matrix that broke away to expose the osteoderms. ns, neural spine; os, osteoderm; and vc, vertebral centrum. Scale bar = 3 mm.
Figure 17
Figure 17. Stereo pair of photographs and interpretive drawing of PVC cast of right humerus and ulna of Scleromochlus taylori, NHMUK R4823, in lateral view on dorsal slab.
Dashed lines indicate reconstructed midshaft of humerus. dpc, deltopectoral crest; ect, ectepicondyle; hh, humeral head; mc, medullary canal; and u, ulna. Scale bar = 5 mm.
Figure 18
Figure 18. Reconstruction of the skull of Scleromochlus taylori based primarily on NHMUK R3146A in dorsal (A), ventral (B) and left lateral (C) views.
Size and shape of the external naris is conjectural. aof, antorbital fenestra; bo, basioccipital; bs, basisphenoid; en, external naris; fr, frontal; ju, jugal; ltf, lower temporal fenestra; md, mandible; mx, maxilla; na, nasal; or, orbit; pa, parietal; pf, postfrontal; po, postorbital; pmx, premaxilla; pt, pterygoid; qj, quadratojugal; qu, quadrate; sq, squamosal; and utf, upper temporal fenestra. Scale bar = 5 mm.
Figure 19
Figure 19. Composite of the dorsal and ventral views of Benton’s (1999; Figs. 8A and 8B) reconstruction of the skull of Scleromochlus taylori as redrawn in Figs. 2F and 2G.
Note the basioccipital extending a full vertebral length behind the posterior margin of the occiput. bo, basioccipital; pm, posterior margin of parietal.
Figure 20
Figure 20. Skeletal reconstruction of Scleromochlus taylori in typical resting posture in dorsal (A) and left lateral (B) views.
The skull and the precaudal vertebral column proportions are based on NHMUK R3146A, the pectoral girdle is based on NHMUK R3914, the manus and the pedal digits are after Woodward’s (1907) reconstruction, and the limb proportions are after Benton’s (1999: fig. 14) reconstruction. Note that the ribs, humerus, and hindlimb are foreshortened in (A). The vertical extent of the dorsal rib ends is unknown. In (B), dorsal osteoderms are shown on the right and omitted on the left to show posterior ribs. Scale bar = 3 cm.
Figure 21
Figure 21. Results of the principal component analysis of limb segment and trunk lengths of Scleromochlus taylori reconstructions and representative vertebrates of various locomotor types.
(A) Components 1 and 2; and (B) Components 2 and 3. Ar, Archaeopteryx; Co, Compsognathus; Sa, Struthiomimus altus; Sd, Sinornithoides dongi; Sy, Sinornithomimus youngi.
Figure 22
Figure 22. Results of parsimony analyses of the 80 taxon 600 character data set (Ezcurra’s, 2016 79 taxon data set plus Scleromochlus taylori).
A total of 50% majority-rule consensus tree with Scleromochlus (3rd from bottom) in the Doswelliidae as a sister taxon to Tarjadia ruthae + Archeopelta arborensis and Jaxtasuchus salomoni + Doswellia kaltenbachi. The dashed line shows the position of Scleromochlus (as sister taxon of a clade equivalent to the clade Erythrosuchidae + Eucrocopoda) in the 50% majority-rule consensus tree from analysis #3 with Chars 558 and 559 set to irreversible and weighted 2, though in that analysis the relationships within the Eucrocopoda are different. Compare to Ezcurra’s (2016: figs. 48 and 50) 79 taxon strict consensus tree. Taxa at right in all capital letters are collapsed but otherwise identical in composition to those in Ezcurra’s tree.
Figure 23
Figure 23. Single most parsimonious tree resulting from the parsimony analysis of Bennett’s (2013) 19 taxon 134 character Updated Data Matrix, etc.
Single most parsimonious tree resulting from the parsimony analysis of Bennett’s (2013) 19 taxon 134 character Updated Data Matrix with the coding of Scleromochlus taylori corrected on the basis of the new information and interpretation in the present article, with Scleromochlus as sister taxon of the Pterosauria + (Erythrosuchidae + Eucrocopoda).
See this image and copyright information in PMC

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