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. 2011;6(11):e27343.
doi: 10.1371/journal.pone.0027343. Epub 2011 Nov 16.

Three-dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae)

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Three-dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae)

Johan Lindgren et al. PLoS One. .
Free PMC article

Abstract

The physical properties of water and the environment it presents to its inhabitants provide stringent constraints and selection pressures affecting aquatic adaptation and evolution. Mosasaurs (a group of secondarily aquatic reptiles that occupied a broad array of predatory niches in the Cretaceous marine ecosystems about 98-65 million years ago) have traditionally been considered as anguilliform locomotors capable only of generating short bursts of speed during brief ambush pursuits. Here we report on an exceptionally preserved, long-snouted mosasaur (Ectenosaurus clidastoides) from the Santonian (Upper Cretaceous) part of the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas, USA, that contains phosphatized remains of the integument displaying both depth and structure. The small, ovoid neck and/or anterior trunk scales exhibit a longitudinal central keel, and are obliquely arrayed into an alternating pattern where neighboring scales overlap one another. Supportive sculpturing in the form of two parallel, longitudinal ridges on the inner scale surface and a complex system of multiple, superimposed layers of straight, cross-woven helical fiber bundles in the underlying dermis, may have served to minimize surface deformation and frictional drag during locomotion. Additional parallel fiber bundles oriented at acute angles to the long axis of the animal presumably provided stiffness in the lateral plane. These features suggest that the anterior torso of Ectenosaurus was held somewhat rigid during swimming, thereby limiting propulsive movements to the posterior body and tail.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Ectenosaurus clidastoides FHSM VP-401.
(A) Skull, partial axial and appendicular skeleton, and calcified sternal cartilage in oblique ventro-lateral view. (B) Slab FHSM VP-401-05 showing phosphatized integumentary structures in medial view. Black and white arrow indicates anterior. Scale bars, (A) 10 cm and (B) 10 mm.
Figure 2
Figure 2. Ectenosaurus clidastoides FHSM VP-401.
Squamation. (A) External view of imbricating and obliquely arrayed scales (FHSM VP-401-01). Note underlying phosphatized dermal fiber bundles oriented parallel to the long axis of the animal. Black and white arrow indicates anterior. (B) External view of a flattened, rhomboidal scale (FHSM VP-401-01). Note presence of a distinct central keel. (C) External view of a three-dimensional, ovoid scale (FHSM VP-401-01). The parallel furrows adjacent to the central keel probably represent artifacts of preservation. (D) Medial surface of a rhomboid scale (FHSM VP-401-08) showing the interior ridged support of the ß-layer of the epidermis (arrows). Note medially inclined scale hinge (arrowheads). (E) Medial surface of an ovoid scale (FHSM VP-401-08) showing the supportive sculpturing of the ß-layer of the epidermis (arrows). Note folded scale hinge (arrowheads). Inset, inside of a Varanus gouldi body scale (MZLU L867/3039) showing supportive sculpturing (arrows) similar to that seen in Ectenosaurus. Scale bars, (A) 2 mm and (B–E) 0.5 mm.
Figure 3
Figure 3. Ectenosaurus clidastoides FHSM VP-401.
Dermal fiber bundle architecture. (A) FHSM VP-401-01 showing partially degraded, longitudinal fiber bundles underlying epidermal scales. Black and white arrow indicates anterior. (B) Close-up of FHSM VP-401-04 showing superficial layers of fiber bundles with a predominantly longitudinal orientation. Note that the fiber bundles located somewhat deeper in the dermis (at the right hand side of the picture) appear to be somewhat thicker than are those immediately below the epidermal scales (at the center and left hand side of the image). (C) Longitudinally oriented and partly degraded fiber bundles located immediately below the epidermis (FHSM VP-401-04). Inset, longitudinal (arrow) dermal fiber bundles from the neck region of Eunectes sp. (LO 11215). (D) Transverse and tangential sections through helically arranged fiber bundles (FHSM VP-401-05). Arrowheads show fiber bundles that appear to be cross-weaved with those of adjacent layers, whereas arrows indicate the principal fiber bundle directions. (E) Layers with fiber bundles that extend in opposing directions (arrows) from deep within the dermis in tangential view (FHSM VP-401-04). Despite being heavily encrusted with phosphate, the fiber bundles are still readily visible. Inset (also in tangential view), a better preserved fiber bundle from the base of the section. (F) Backscatter image of an isolated fiber bundle comprised of multiple apatite aggregates (FHSM VP-401-01). Scale bars, (A, C–E) 2 mm, (B) 1 mm and (F) 30 µm.

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