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. 2020 Jun 15:8:e9349.
doi: 10.7717/peerj.9349. eCollection 2020.

Quantitatively assessing mekosuchine crocodile locomotion by geometric morphometric and finite element analysis of the forelimb

Michael D Stein  1 , Suzanne J Hand  1 , Michael Archer  1 , Stephen Wroe  2 , Laura A B Wilson  1
Affiliations

Quantitatively assessing mekosuchine crocodile locomotion by geometric morphometric and finite element analysis of the forelimb

Michael D Stein et al. PeerJ. .

Abstract

Morphological shifts observed in the fossil record of a lineage potentially indicate concomitant shifts in ecology of that lineage. Mekosuchine crocodiles of Cenozoic Australia display departures from the typical eusuchian body-plan both in the cranium and postcranium. Previous qualitative studies have suggested that these crocodiles had a more terrestrial habitus than extant crocodylians, yet the capacity of mekosuchine locomotion remains to be tested. Limb bone shape, such as diaphyseal cross-section and curvature, has been related to habitual use and locomotory function across a wide variety of taxa. Available specimens of mekosuchine limbs, primarily humeri, are distinctly columnar compared with those of extant crocodylians. Here we apply a quantitative approach to biomechanics in mekosuchine taxa using both geomorphic morphometric and finite element methods to measure bone shape and estimate locomotory stresses in a comparative context. Our results show mekosuchines appear to diverge from extant semi-aquatic saltwater and freshwater crocodiles in cross-sectional geometry of the diaphysis and generate different structural stresses between models that simulate sprawling and high-walk gaits. The extant crocodylians display generally rounded cross-sectional diaphyseal outlines, which may provide preliminary indication of resistance to torsional loads that predominate during sprawling gait, whereas mekosuchine humeri appear to vary between a series of elliptical outlines. Mekosuchine structural stresses are comparatively lower than those of the extant crocodylians and reduce under high-walk gait in some instances. This appears to be a function of bending moments induced by differing configurations of diaphyseal curvature. Additionally, the neutral axis of structural stresses is differently oriented in mekosuchines. This suggests a shift in the focus of biomechanical optimisation, from torsional to axial loadings. Our results lend quantitative support to the terrestrial habitus hypothesis in so far as they suggest that mekosuchine humeri occupied a different morphospace than that associated with the semi-aquatic habit. The exact adaptational trajectory of mekosuchines, however, remains to be fully quantified. Novel forms appear to emerge among mekosuchines during the late Cenozoic. Their adaptational function is considered here; possible applications include navigation of uneven terrain and burrowing.

Keywords: Australian mekosuchine crocodiles; Finite element analysis; Forelimb biomechanics; Geometricmorphometrics; Locomotory evolution; Palaeoecology.

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Conflict of interest statement

Laura AB Wilson is an Academic Editor for PeerJ.

Figures

Figure 1
Figure 1. Fossil mekosuchine crocodile specimens from the Australian Eocene Murgon locality used in study.
Right humerus QM F56058 in (A), dorsal (B), ventral (C), medial and (D), distal views. QM F56060 in (E), dorsal (F), ventral (G), medial and (H), distal views. Scale bars = 2 cm. Specimens were originally published in Stein et al. (2012) and are reproduced here with permission.
Figure 2
Figure 2. Fossil mekosuchine crocodile specimens from the Australian Oligo—Miocene Riversleigh, Bullock Creek and Plio—Pleistocene Floraville localities used in study.
Right humerus NTM P907-70 in (A), dorsal (B), ventral (C), medial and (D), distal views. QM F57954 in (E), dorsal (F), ventral (G), medial and (H), distal views. QM F57955 in (I), dorsal (J), ventral (K), medial and (L), distal views. QM F57956 in (M), dorsal (N), ventral and (O), medial views. QM F57953 in (P), dorsal (Q), ventral (R), medial (S), distal and (T), proximal views. Scale bars = 2 cm.
Figure 3
Figure 3. Plots of biomechanical property measurements extracted from diaphyseal cross sections of 3D models of the humerus at 50% (midshaft).
Measures, after Meers (2002), are (A) torsional rigidity versus humeral length, (B) mediolateral rigidity versus anteroposterior rigidity, (C) torsional rigidity versus cortical area, (D) cortical area versus humeral length. These measures provide information on the overall rigidity of the humerus and cross-sectional distribution patterns for cortical bone (mediolateral versus anteroposterior plane) in relation to bone length.
Figure 4
Figure 4. Results of the principal component analysis.
Principal components analysis of 16 equiangular semi-landmarks collected on the periosteal outline of each 50% cross section. (A) Principal component 1 versus principal component 2, (B) principal component 1 versus principal component 3. The cross sections were extracted from virtual 3D models of the humerus, and the landmark data were corrected for body mass. Freshwater and saltwater crocodiles are shown in red, mekosuchines in blue.
Figure 5
Figure 5. Plots of cross-sectional properties and shape variance.
(A) Ratios of Imax to Imin for each specimen, indicating the magnitude of variance in cross-sectional axial symmetry (increased values, greater than 1.0, indicate greater departure from symmetry); (B) principal component 1 (major axis of cross-sectional shape variance in the sample uncorrected for body mass in this case) versus ratios of Ix and Iy;(C) rose diagram of theta values (in degrees) extracted from cross sections. Theta is the orientation of greatest bending rigidity. Red line indicates average orientation for theta in the sample (−11.8 degrees counter clockwise from 0). Numbers in shaded bin regions correspond to number of samples in bins. Freshwater and saltwater crocodiles are shown in red, mekosuchines in blue.
Figure 6
Figure 6. Results of finite element analysis used to assess how different loading conditions (sprawl, high-walk) affect the patterning of stress engendered by the humerus.
Contour map results of von Mises stresses with output scale chosen to maximise contrast, considering both force of bodyweight due to gravity and musculature. Models are pictured in dorsal and ventral views. Modern Crocodylus porosus XCb Cp4 under (A), (B) sprawl and (C), (D) high-walk conditions. Modern Crocodylus johnstoni AR 22161 under (E), (F) sprawl and (G), (H), high-walk conditions. Murgon QM F56060 under (I), (J) sprawl and (K), (L) high-walk conditions. Riversleigh QM F57954 under (M), (N) sprawl and (O), (P) high-walk conditions. Bullock Creek NTM P907-70 under (Q), (R) sprawl and (S), (T) high-walk conditions. Floraville QM F57953 under (U), (V) sprawl and (W), (X) high-walk conditions.
Figure 7
Figure 7. Results of finite element analysis used to assess how different loading conditions (sprawl, high-walk) affect the patterning of stress engendered by the humerus under gravity only.
Contour map results of von Mises stresses with output scale chosen to maximise contrast, considering force of bodyweight due to gravity solely. Models are pictured in dorsal and ventral views. Modern Crocodylus porosus XCb Cp4 under (A), (B) sprawl and (C), (D) high-walk conditions. Modern Crocodylus johnstoni AR 22161 under (E), (F) sprawl and (G), (H) high-walk conditions. Murgon QM F56060 under (I), (J) sprawl and (K), (L) high-walk conditions. Riversleigh QM F57954 under (M), (N) sprawl and (O), (P) high-walk conditions. Bullock Creek NTM P907-70 under (Q), (R) sprawl and (S), (T) high-walk conditions. Floraville QM F57953 under (U), (V) Sprawl and (W), (X) high-walk conditions.
Figure 8
Figure 8. Compression and tension results of finite element analysis.
Contour map results of the finite element analysis by mean of the principle stresses with output scale chosen to maximise contrast, considering both force of bodyweight due to gravity and musculature. Yellow to violet indicates the model is in tension, green to blue indicates in compression. Models are pictured in dorsal and ventral views. Modern Crocodylus porosus XCb Cp4 under (A), (B) sprawl and (C), (D) high-walk conditions. Modern Crocodylus johnstoni AR 22161 under (E), (F) sprawl and (G), (H) high-walk conditions. Murgon QM F56060 under (I), (J) sprawl and (K), (L) high-walk conditions. Riversleigh QM F57954 under (M), (N) sprawl and (O), (P) high-walk conditions. Bullock Creek NTM P907-70 under (Q), (R) sprawl and (S), (T) high-walk conditions. Floraville QM F57953 under (U), (V) sprawl and (W), (X) high-walk conditions.
Figure 9
Figure 9. Average extracted von Mises stress values.
Average extracted von Misses stresses at measured anatomical locations for each model. (A) C. porosus XCb Cp4, (B) C. johnstoni AR 22161, (C) Murgon QM F56060, (D) Riversleigh QM 57954, (E) Floraville QM F57953, (F) Bullock Creek NTM P907-70.
Figure 10
Figure 10. Average extracted compressive and tensile stress values.
Mean component stresses at measured anatomical locations for each model, where positive values indicate tension and negative values indicate compression. (A) C. porosus XCb Cp4, (B) C. johnstoni AR 22161, (C) Murgon QM F56060, (D) Riversleigh QM 57954, (E) Floraville QM F57953, (F) Bullock Creek NTM P907-70.
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