A Morphometric Assessment of the Functional Ecomorphology in … · 2015-12-03 · Hadrosaurs can...
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577|71 - Photos of the premaxillae of 121 extant species of ruminants, and 66 specimens of ornithopod representing 34 species were taken (or obtained) in ventral aspect. - Ruminants were categorised according to their ecological grouping extracted from numerous sources and based on a range of secondary criteria (e.g., hypsodonty index, % grass consumption). - Ornithopods were grouped into coarse chronostratigraphic age bins, and ‘grades’ that are associated with equivocally resolved taxonomic Family-level clades. - Both data sets were qualitatively demarcated based on whether their profiles were more ‘pointed’ or ‘blunt’. - Additional categories were created on the basis of new informal secondary shape descriptors that attempted to incorporate the shape of the lateral margin. RECONSTRUCTING THE PROFILE - The maxilla-premaxillary junctions on the lateral margins were used as the start and end point to digitially reconstruct a chain of 100 equally-spaced (i.e., interpolated to be topograhically homologous between specimens) semi-landmarks providing a geometrically faithful representation of the premaxillae. Procrustes Transformation - removes extraneous variations in scale (size), orientation and position for all specimens’ profile reconstructions (Fig. 1). Covariance-Based Principal Components Analysis - dimensionality reduction from 200 to four PC axes (based on associated eigenvalues), that for both data sets explained >95% of the total shape variance. Canonical Variates Analysis - to test and model the degree of sub-group structure within the datasets. Log Likelihood Ratio Test - used to test the statistical significance of alternative sub-group structures within the data sets (e.g., phylogeny, functional constraints). Modelling - used to obtain shape-based representations of the major between sub-group distinctions as a shape transformation series. The models are projections of sets of co-ordinate locations on the CVA space that have been deconvolved and back-projected into the original PCA spaces from which the CVA results were calculated. - Determine the extent to which premaxillary shape can delimit ruminants within a functional ecology context. - Demonstrate whether statistically defined morphotypes covary between putative feeding types. - To assess the validity of the terms ‘pointed’ and ‘blunt’ as formal shape descriptors. - If patterns do exist, to see how much can be inferred regarding the feeding styles of ornithopods. - Recently, traditional avenues of research regarding the ecology of ruminants have been applied to ornithopods based on parallelisms between trophic levels, herding, gross morphology and cranial kinematics. - Ruminant ecological diversity has typically been lumped into discrete categories, formally recognised as ‘Browsers’, ‘Grazers’, and ‘Intermediates’. Each is defined by numerous qualitative and quantitative morphophysi- ological characteristics, although this scheme has increasingly become the subject of criticism. - Of these ’traits’, snout morphology has been a prominent and integrative aspect of herbivore ecology, defining initial intake rate/amount and selection sensitivity, and aiding in the conventional categorisation of ruminants. - ‘Pointed’ snouts and ‘blunt’ snouts are typically ascribed to ‘browsing’ and ‘grazing’ ruminants, respectively. - This has been a recurrent theme in the ruminant literature and, to date, has not been tested in a robust quantitative geometric framework beyond specification of simple ratios, linear measurements, and/or inadequate reconstructions. 2D CVA score plot for ecologically-categorised ruminants (Figure 2). Overlap between all sub-groups implies that the intra-group variation is occluding the evidence for potential inter-group discontinuities. Back-projection of the CV-model axes into the respective PC shape space (Figure 3). ‘Strobe plot’ of the CV-models in PC shape space (Figure 4). The right hand column is an overlay plot showing the progressive deformation along the axes. Note that the shape range cannot be simply described as a transition from ‘blunt’ to‘pointed’ . The result is that snout shape is not sufficient to discriminate between functional feeding classes in ruminants, instead forming a continuum of shape variation that cannot be split into discrete groupings (Figure 5). No evidence of a confounding phy- logenetic signal is recovered either, when the ruminants are categorised according to various taxonomic sub-groupings (this needs to be tested further using comparative phylogenetic methods). Additional descriptive units regard- ing the mediolateral geometry of the snout only serves to increase the complexity of the ordinations. All results are confirmed as statisti- cally significant using a log likeli- hood ratio test, implying there is an additional constraint (possibly func- tional) on snout morphology. Figure 6. Projection of the ornithopod PCA scores into the space defined by the first two PC axes for ruminants. Ornitho- pods are categorised according to taxonomic family, and rumi- nants according to their ecology. Note how hadrosaurs project discretely from any ruminant sub-groups, challenging previous assertions that they are ecologically analoguous to ruminants. Figure 7. (A) CV-1 describes the heirarchical transition between taxonomic groupings. Group distinctions are broadly appar- ent, with overlap between more closely related clades. (B) When grouped according to chronostratigraphic age, ornitho- pods occupy an increasingly broader region of the snout pro- file morphospace through time. Hadrosaurs exhibit a wide range of shapes, implying adaptation to numerous feeding styles. When modelled, these CV axes describe a pattern of lat- eral broadening of the distal premaxilla. CONCLUDING REMARKS - Using a 2D profile of the ruminant snout, it was found that snout shape is not organized into consistent shape groups with respect to feeding strategy. These results are not a phylogenetic artefact, and pass the log-likelihood ratio test for statistical significance. - Qualitative shape descriptors such as ‘blunt’ and ‘pointed’ are inadequate to describe the complex range of morphology exhibited by ruminants. Increasing the complexity of these descriptors to include the lateral geometry is also insufficient. - The practise of attempting to place arbitrary thresholds on functional categories in ruminants should be discontinued, as it is evident that snout shape is a morphologically con- tinuous ‘trait’. Further, using snout shape as a proxy for reconstructing palaeodiets should be questioned, preferably discontinued. - Ecological classifications of ornithopods can potentially be applied by relating snout shape to feeding style in a manner used for sauropods (Whitlock, 2011), especially as they possess a strong phylogenetic and temporal heirarchy with respect to snout shape. Hypsilophodonts and heterodontosaurs were ‘browsers’, in terms of selectivity and intake rate. Iguanodontians underwent an experimental transitional phase. Hadrosaurs can confidently be classed as ‘grazers’, as corroborated by Williams et al. (2009) in an analysis of dental microwear. This transition may relate to the ascent of early angiosperms, framed in a co-evolutionary framework, as opposed to previous co-radiation hypotheses. REFERENCES Williams, V. S., Barrett, P. M. and Purnell, M. A. (2009) Quanti- tative analysis of dental microwear in hadrosaurid dino- saurs, and the implications for hypotheses of jaw mechan- ics and feeding, Proceedings of the National Academy of Sci- ences, 106(27), 11194-11199 Whitlock, J. A. (2011) Inferences of diplodocoid (Sauropoda: Dinosauria) feeding behaviour from snout shape and mi- crowear analyses, PLoS One, 6(4), 1-20 A Morphometric Assessment of the Functional Ecomorphology in Ruminant (Ungulata: Artiodactyla) and Ornithopod (Dinosauria: Ornithischia) Snouts Figure 1. Procrustes-transformed overlay plot for profiles of all (A) ornithopod and (B) ruminant specimens A B Browser Camptosauridae Dryosauridae Frugivore Grazer Hadrosauridae Heterodontosauridae Hypsilophodontidae Iguanodontidae Intermediate Rhabdodontidae 1 2 3 1 2 3 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Jonathan Tennant 1 and Norman MacLeod 2 1. Life Sciences Department, Imperial College London, South Kensington Campus, London SW7 2AZ 2. Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD BACKGROUND DATA COLLECTION AIMS METHODS OF ANALYSIS CONTACT: [email protected] Fig. 7 A Fig. 6 B 2 1 3 4 CV-2 CV-3 Fig. 7 B
Transcript of A Morphometric Assessment of the Functional Ecomorphology in … · 2015-12-03 · Hadrosaurs can...
577|71
- Photos of the premaxillae of 121 extant species of ruminants, and 66 specimens of ornithopod representing 34 species were taken (or obtained) in ventral aspect.
- Ruminants were categorised according to their ecological grouping extracted from numerous sources and based on a range of secondary criteria (e.g., hypsodonty index, % grass consumption).
- Ornithopods were grouped into coarse chronostratigraphic age bins, and ‘grades’ that are associated with equivocally resolved taxonomic Family-level clades.
- Both data sets were qualitatively demarcated based on whether their pro�les were more ‘pointed’ or ‘blunt’.
- Additional categories were created on the basis of new informal secondary shape descriptors that attempted to incorporate the shape of the lateral margin.
RECONSTRUCTING THE PROFILE
- The maxilla-premaxillary junctions on the lateral margins were used as the start and end point to digitially reconstruct a chain of 100 equally-spaced (i.e., interpolated to be topograhically homologous between specimens) semi-landmarks providing a geometrically faithful representation of the premaxillae.
Procrustes Transformation - removes extraneous variations in scale (size), orientation and position for all specimens’ pro�le reconstructions (Fig. 1).
Covariance-Based Principal Components Analysis - dimensionality reduction from 200 to four PC axes (based on associated eigenvalues), that for both data sets explained >95% of the total shape variance.
Canonical Variates Analysis - to test and model the degree of sub-group structure within the datasets.
Log Likelihood Ratio Test - used to test the statistical signi�cance of alternative sub-group structures within the data sets (e.g., phylogeny, functional constraints).
Modelling - used to obtain shape-based representations of the major between sub-group distinctions as a shape transformation series. The models are projections of sets of co-ordinate locations on the CVA space that have been deconvolved and back-projected into the original PCA spaces from which the CVA results were calculated.
- Determine the extent to which premaxillary shape can delimit ruminants within a functional ecology context.
- Demonstrate whether statistically de�ned morphotypes covary between putative feeding types.
- To assess the validity of the terms ‘pointed’ and ‘blunt’ as formal shape descriptors.
- If patterns do exist, to see how much can be inferred regarding the feeding styles of ornithopods.
- Recently, traditional avenues of research regarding the ecology of ruminants have been applied to ornithopods based on parallelisms between trophic levels, herding, gross morphology and cranial kinematics.
- Ruminant ecological diversity has typically been lumped into discrete categories, formally recognised as ‘Browsers’, ‘Grazers’, and ‘Intermediates’. Each is de�ned by numerous qualitative and quantitative morphophysi- ological characteristics, although this scheme has increasingly become the subject of criticism.
- Of these ’traits’, snout morphology has been a prominent and integrative aspect of herbivore ecology, de�ning initial intake rate/amount and selection sensitivity, and aiding in the conventional categorisation of ruminants.
- ‘Pointed’ snouts and ‘blunt’ snouts are typically ascribed to ‘browsing’ and ‘grazing’ ruminants, respectively.
- This has been a recurrent theme in the ruminant literature and, to date, has not been tested in a robust quantitative geometric framework beyond speci�cation of simple ratios, linear measurements, and/or inadequate reconstructions.
2D CVA score plot for ecologically-categorised ruminants (Figure 2). Overlap between all sub-groups implies that the intra-group variation is occluding the evidence for potential inter-group discontinuities. Back-projection of the CV-model axes into the respective PC shape space (Figure 3). ‘Strobe plot’ of the CV-models in PC shape space (Figure 4). The right hand column is an overlay plot showing the progressive deformation along the axes. Note that the shape range cannot be simply described as a transition from ‘blunt’ to‘pointed’ . The result is that snout shape is not su�cient to discriminate between functional feeding classes in ruminants, instead forming a continuum of shape variation that cannot be split into discrete groupings (Figure 5).
No evidence of a confounding phy-logenetic signal is recovered either, when the ruminants are categorised according to various taxonomic sub-groupings (this needs to be tested further using comparative phylogenetic methods).
Additional descriptive units regard-ing the mediolateral geometry of the snout only serves to increase the complexity of the ordinations.
All results are con�rmed as statisti-cally signi�cant using a log likeli-hood ratio test, implying there is an additional constraint (possibly func-tional) on snout morphology.
Figure 6. Projection of the ornithopod PCA scores into the space de�ned by the �rst two PC axes for ruminants. Ornitho-pods are categorised according to taxonomic family, and rumi-nants according to their ecology. Note how hadrosaurs project discretely from any ruminant sub-groups, challenging previous assertions that they are ecologically analoguous to ruminants.
Figure 7. (A) CV-1 describes the heirarchical transition between taxonomic groupings. Group distinctions are broadly appar-ent, with overlap between more closely related clades. (B) When grouped according to chronostratigraphic age, ornitho-pods occupy an increasingly broader region of the snout pro-�le morphospace through time. Hadrosaurs exhibit a wide range of shapes, implying adaptation to numerous feeding styles. When modelled, these CV axes describe a pattern of lat-eral broadening of the distal premaxilla.
CONCLUDING REMARKS - Using a 2D pro�le of the ruminant snout, it was found that snout shape is not organized into consistent shape groups with respect to feeding strategy. These results are not a phylogenetic artefact, and pass the log-likelihood ratio test for statistical signi�cance. - Qualitative shape descriptors such as ‘blunt’ and ‘pointed’ are inadequate to describe the complex range of morphology exhibited by ruminants. Increasing the complexity of these descriptors to include the lateral geometry is also insu�cient. - The practise of attempting to place arbitrary thresholds on functional categories in ruminants should be discontinued, as it is evident that snout shape is a morphologically con- tinuous ‘trait’. Further, using snout shape as a proxy for reconstructing palaeodiets should be questioned, preferably discontinued. - Ecological classi�cations of ornithopods can potentially be applied by relating snout shape to feeding style in a manner used for sauropods (Whitlock, 2011), especially as they possess a strong phylogenetic and temporal heirarchy with respect to snout shape. Hypsilophodonts and heterodontosaurs were ‘browsers’, in terms of selectivity and intake rate. Iguanodontians underwent an experimental transitional phase. Hadrosaurs can con�dently be classed as ‘grazers’, as corroborated by Williams et al. (2009) in an analysis of dental microwear. This transition may relate to the ascent of early angiosperms, framed in a co-evolutionary framework, as opposed to previous co-radiation hypotheses.
REFERENCESWilliams, V. S., Barrett, P. M. and Purnell, M. A. (2009) Quanti-tative analysis of dental microwear in hadrosaurid dino-saurs, and the implications for hypotheses of jaw mechan-ics and feeding, Proceedings of the National Academy of Sci-ences, 106(27), 11194-11199Whitlock, J. A. (2011) Inferences of diplodocoid (Sauropoda: Dinosauria) feeding behaviour from snout shape and mi-crowear analyses, PLoS One, 6(4), 1-20
A Morphometric Assessment of the Functional Ecomorphology in Ruminant (Ungulata: Artiodactyla) and Ornithopod (Dinosauria: Ornithischia) Snouts
Figure 1. Procrustes-transformed overlay plot for pro�les of all (A) ornithopod and (B) ruminant specimens
A B
BrowserCamptosauridaeDryosauridaeFrugivoreGrazerHadrosauridaeHeterodontosauridaeHypsilophodontidaeIguanodontidaeIntermediateRhabdodontidae
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Fig. 2 Fig. 3
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Jonathan Tennant1 and Norman MacLeod 2
1. Life Sciences Department, Imperial College London, South Kensington Campus, London SW7 2AZ 2. Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD
BACKGROUND DATA COLLECTION
AIMS
METHODS OF ANALYSIS
CONTACT: [email protected]
Fig. 7 A
Fig. 6
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CV-2 CV-3
Fig. 7 B
