Standardized terminology and potential taxonomic utility for hadrosaurid skin impressions: a case study for Saurolophus from Canada and Mongolia

Abstract

The characterization of palaeospecies typically relies on hard-tissue anatomy, such as bones or teeth that is more readily fossilized than soft parts. Among dinosaurs, skin impressions are commonly associated with partial and complete hadrosaurid skeletons, and consist of non-imbricating tubercles or scales. Skin impressions from various parts of the body of two species of the hadrosaurine Saurolophus (S. angustirostris from Mongolia and S. osborni from Canada) are described from multiple specimens. These species, recently validated on osteological grounds, can be differentiated based solely on soft-tissue anatomy, namely scale shape and pattern. This study demonstrates for the first time the applicability of soft-tissue (i.e., scale impressions) as a means to differentiate species within the Dinosauria. Differences are most spectacular in the tail, where S. angustirostris is differentiated by the presence of vertical bands of morphologically distinct scales, a grid-like arrangement of circular feature-scales, and tabular scales along the dorsal midline. Preliminary results indicate scale architecture remained consistent throughout ontogeny in S. angustirostris. These results support previous assertions that hadrosaurid scale architecture has a positive phylogenetic signal. As such, future taxonomic descriptions should include, where possible, the standardized description of skin impressions including the position and orientation of these impressions on the body.

Figure 1. Hadrosaur scale morphology.

(A) Polygonal basement-scales (Ms) with a shield feature-scale (fs). Interstitial tissue (It) occurs between scales (Saurolophus angustirostris); B. Pebbles (S. angustirostris); (C) Radially-ornamented, irregular basement-scales (Edmontosaurus annectens ROM 801); (D) Imbricated shell basement-scales (S. angustirostris). Scale bars = 1 cm.

Figure 2. Cranial integument of Saurolophus.

(A, B) Saurolophus osborni (AMNH 5220) right dentary. (C, D) Juvenile S. angustirostris (ZPAL-MgD-1/159). Itf, infratemporal fenestra; Ju, jugal; Q, quadrate. Grey areas denote regions of skin impressions.

Figure 3. Field photograph of the integument from the flank of a subadult Saurolophus angustirostris.

Note impressions of the ribs. Scale bar in cm.

Figure 4. Tail integument of Saurolophus osborni (AMNH 5271).

(A) Composite photograph and (B) interpretive drawing of caudal series. Note a section of distal vertebrae is missing. i, ii, and iii identify enlarged images in (C, D), (E, F), and (G, H), respectively. Light grey regions denote areas of skin impressions. Dark grey indicates plaster reconstruction. C, centrum; Cv, chevron; Na, neural arch; Ot, ossified tendons.

Figure 5. Proximal-most tail integument of adult S. angustirostris (PIN 3738).

(A) photograph and (B) interpretive illustration. Light grey indicates extent of preserved integument. Dashed lines denote wrinkles in the skin. (C) Close-up showing narrow vertical bands of zone B scales (arrowheads). (D) Detail of transition between polygonal basement-scales in zone A and shell basement-scales of zone B. Fs, feature-scale; Os, ossified tendon; Tp, transverse processes of caudal vertebrae.

Figure 6. Tail integument of Saurolophus angustirostris.

(A) field photograph and (B) interpretive illustration of a juvenile caudal series showing distribution of midline feature-scales (Mfs). (C, D) Adult proximal-mid caudal series showing enlarged midline feature-scales. (E, F) Three dimensional skin preservation on mid-distal caudal series of subadult PIN 4216/49. (G) Close-up of midline feature-scales indicated by boxed region in (F). Light grey indicates extent of skin impressions. Dark grey denotes bone. Dashed lines are inferred outlines of structures.

Figure 7. Field photographs of pebbly basement-scales in Saurolophus angustirostris.

(A) Forearm and (B) shoulder girdle, superficial to the scapula. Longitudinal ridges in A are folds in the integument; distal is to the right.

Figure 8. Pelvic integument of Saurolophus.

(A, B) S. osborni (AMNH 5220) from the lateral surface of the iliac body. (C, D) Field photograph of S. angustirostris integument preserved between the sacral ribs of a subadult individual. Grey areas denote areas obscured by plaster.

Figure 9. Skin impressions from the left pes of S. osborni (AMNH 5220).

(A) Skin from the anterior surface of the metatarsus. (B) ?Lateral view of an unidentified pedal digit with the ungual (un) still intact showing irregular scales (inset).

Figure 10. Hind limb skin impressions of Saurolophus angustirostris.

Right leg of juvenile S. angustirostris (ZPAL-MgD-1/159). (A) Photograph and (B) interpretive drawing showing distribution of shield feature-scales (fs). Thick dashed line indicates outline of femur (f) and ?tibia (?t) exposed on the opposite surface of the block. (C) Close-up of large multi-pointed shield feature-scale. (D) Detail of integument.

Figure 11. Miscellaneous skin impressions of Saurolophus osborni.

(A) AMNH 5220; (B) AMNH 5221; (C) AMNH 5221; (D) AMNH 5220. Fs, feature scale.

Figure 12. Regions of skin impressions (light grey) currently known for species of Saurolophus.

(A) S. osborni and (B) S. angustirostris.

Figure 13. Soft tissue reconstructions of Saurolophus based on preserved skin impressions.

Variations in scale arrangement and patterning are used here as a basis for possible colour patterns, which are particularly evident in the caudal region. (A) Saurolophus osborni showing mottled appearance of tail integument. (B) Saurolophus angustirostris showing midline feature-scales and banded pattern on the tail. Illustration by L. Xing and Y. Liu.