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Do morphometric data improve phylogenetic reconstruction? A systematic review and assessment

Abstract

Background

Isolating phylogenetic signal from morphological data is crucial for accurately merging fossils into the tree of life and for calibrating molecular dating. However, subjective character definition is a major limitation which can introduce biases that mislead phylogenetic inferences and divergence time estimation. The use of quantitative data, e.g., geometric morphometric (GMM; shape) data can allow for more objective integration of morphological data into phylogenetic inference. This systematic review describes the current state of the field in using continuous morphometric data (e.g., GMM data) for phylogenetic reconstruction and assesses the efficacy of these data compared to discrete characters using the PRISMA-EcoEvo v1.0. reporting guideline, and offers some pathways for approaching this task with GMM data. A comprehensive search string yielded 11,123 phylogenetic studies published in English up to Oct 2023 in the Web of Science database. Title and abstract screening removed 10,975 articles, and full-text screening was performed for 132 articles. Of these, a total of twelve articles met final inclusion criteria and were used for downstream analyses.

Results

Phylogenetic performance was compared between approaches that employed continuous morphometric and discrete morphological data. Overall, the reconstructed phylogenies did not show increased resolution or accuracy (i.e., benchmarked against molecular phylogenies) as continuous data alone or combined with discrete morphological datasets.

Conclusions

An exhaustive search of the literature for existing empirical continuous data resulted in a total of twelve articles for final inclusion following title/abstract, and full-text screening. Our study was performed under a rigorous framework for systematic reviews, which showed that the lack of available comparisons between discrete and continuous data hinders our understanding of the performance of continuous data. Our study demonstrates the problem surrounding the efficacy of continuous data as remaining relatively intractable despite an exhaustive search, due in part to the difficulty in obtaining relevant comparisons from the literature. Thus, we implore researchers to address this issue with studies that collect discrete and continuous data sets with directly comparable properties (i.e., describing shape, or size).

Peer Review reports

Introduction

Discrete morphological data have been the primary focus of traditional systematic methods for phylogenetic reconstruction. Even following the success of molecular phylogeny, discrete morphological data remain crucial for merging fossils into the tree of life and for calibrating molecular dating [1]. However, a major limitation of discrete morphological data is subjectivity in the definition of characters and character states, which in turn, may potentially introduce biases and mislead phylogenetic inference and divergence time estimation. The use of quantitative data e.g., geometric morphometric (GMM; shape) landmark data can allow for more objective integration of morphology for phylogenetic inference. Two- or three-dimensional landmarks can be used to capture the geometry of a biological structure, and shape differences can then be quantified using the relative positions of landmarks for each object in the sample [2,3,4,5] by scaling and superimposing the landmark configurations, typically using Procrustes analysis [6].

However, challenges associated with the application of GMM data to the inference of phylogenetic topology include the widespread non-independence (e.g., covariation) of landmarks due to functional or developmental correlation, which violates assumptions of standard trait evolution models. Therefore, a single landmark should not be treated as an independent character, but rather the entire landmark configuration [7]. To accommodate this concern, Álvarez-Carretero et al. [8] described a Bayesian method that extended the work of Parins-Fukuchi [9] by explicitly accounting for character correlation within a dataset of 3D cranium landmarks from carnivoran mammals, showing promise for the analysis of continuous characters for phylogenetic and divergence time estimation. Moreover, landmark configurations require Procrustes superimposition to remove non-shape differences (scaling, rotation, translation) between configurations in a dataset, allowing geometric comparisons to be made in a shape space. This procedure scales the landmark configurations to centroid size, and allows for further assessment of allometry, the covariation between shape and size. Allometry can be assessed within a geometric morphometric framework through regression of Procrustes superimposed landmarks against centroid size. Allometric variation can be difficult to tease apart from, but may also contribute to, true phylogenetic signal as the two sources of variation are often confounded [10].

Subjectivity may remain with GMM through the choice i.e., number and position, and manual placement of landmarks which can result in observer and measurement error [11,12,13]. However, the development of automated GMM (whole bone shape) methods [14,15,16,17,18] that aim to reduce observer and measurement error associated with manual landmark placement, and thus increase accuracy in approximations of shape, has received recent attention in evolutionary morphology research [12, 19,20,21]. Although, further methodological studies are required to determine the most appropriate approach(es) for individual taxa and for the goals of individual studies [11]. For example, some automated landmarking approaches [e.g., ALPACA; 19] may be better employed within species or among closely related species; while others [e.g., MALPACA; 21] are particularly suitable in broad phylogenetic contexts for landmarking the types of morphologically diverse (multi-taxa) samples commonly encountered in evolutionary studies.

If choosing to discretize GMM data for phylogenetic analysis [22, 23], another difficulty is in characterising and coding (i.e., discretizing) landmarks into character states. Moreover, arbitrarily delimiting discrete states from variation that is inherently continuous [24] (i.e., GMM; shape) can result in information loss [25]. However, improvements in discretization methods for continuous data have recently been proposed [22].

Methods for analysing undiscretized continuous morphological data i.e., as continuous quantitative characters in phylogenetic inference have been available using cladistic approaches such as parsimony [TNT; 25,26,27,28,29] and model-based methods such as maximum likelihood (ML) [CONTML in PHYLIP; 30] and Bayesian methods [31]. However, most attempts to integrate GMM data into phylogenetic inference have been criticised due to methodological concerns and unreliability [7, 32,33,34], although few studies have performed well in reconstructing ‘true’ phylogenetic relationships [35,36,37,38].

Analysis of continuous data (including GMM data) has been more commonly applied to taxonomy, especially in the delineation of species [e.g., 39, 40]. Among marsupial mammals cranial and/or dental linear measurement data have been applied in several studies on bandicoots (Peramelemorphia) [41, 42], whereas 3D landmark-based GMM of the cranium has been applied in Antechinus (Dasyuromporphia) [43].

Examples of reconstructing phylogeny from GMM data include the use of landmark configurations as continuous characters in what is referred to in the literature as ‘landmark analysis under parsimony’ (LAUP; spatial parsimony) or ‘Phylogenetic Morphometrics’ (PM) as proposed by Catalano et al. [27], Goloboff and Catalano [28], Catalano and Goloboff [26], [e.g., 23, 44,45,46]. Other studies have used landmark coordinates as continuous characters under parsimony [e.g., 47], squared-change parsimony [e.g., 23], minimum evolution [48, 49] [i.e., “Euclidean parsimony”; 50] [e.g., 32], and with Brownian motion modelling of evolutionary change under maximum likelihood (ML) [e.g., 23] and under Bayesian inference [e.g., 31]; while others have used principal component (PC) scores [e.g., 24, 47, 51] and eigenscores [eigenshape analysis descriptors; e.g., 37] as continuous characters under parsimony, and PC scores as continuous characters under maximum likelihood [e.g., 24, 35, 39, 51]. However, the use of PC scores and landmark coordinates in phylogenetic inference have been heavily criticized by some researchers, with Adams et al. [33] regarding PCs of shape data as both inappropriate and ineffective as cladistic characters [33], and Varon-Gonzalez et al. [32] suggesting the unreliability of phylogenetic estimation from shape data such as landmark coordinates [32], see also criticisms by Catalano et al. [27]; Goloboff [52].

An alternative approach to character-based methods for reconstructing phylogeny from GMM data is to create trees based on distances (i.e., Procrustes, Euclidean) between taxa using cluster analysis. For example, neighbour-joining [NJ; 53] [e.g., 23, 24, 35, 44, 54,55,56] and unweighted pair group method with arithmetic mean [UPGMA; 58] [e.g., 24, 35, 44, 56, 57, 59, 60] or other methods such as minimum evolution [ME; 61] [e.g., 59] and maximum likelihood using flexibly weighted least squares methods [fWLS; e.g., 62, 63].

Another procedure is to estimate the positions of fossil taxa along a scaffold (molecular) phylogeny of extant taxa using quantitative data. For example, Revell et al. [64] described a maximum likelihood method for placing individual taxa into a phylogeny of extant taxa using continuous character data. This approach performed well but was limited to the placement of only extant and recently extinct taxa, and with only a single taxon placed at a time. Extending this approach, Parins‐Fukuchi [36] presented a Bayesian method that places multiple fossil taxa on a phylogeny of extant taxa using quantitative characters modelled under Brownian Motion [65]. Importantly, their model treats branch lengths in terms of morphological divergence as opposed to time, and moreover allows the placement of long extinct fossil taxa to be estimated.

Probabilistic approaches (e.g., under Bayesian inference) allow estimation of branch lengths and evolutionary rates and can improve estimates of uncertainty, thus potentially improving the accuracy of morphological phylogenetics compared to cladistic methods (i.e., parsimony analysis) [9]. Recently, Zhang et al. [31] introduced a probabilistic total-evidence method for phylogenetic inference using multiple continuous (3D GMM landmark coordinates) characters in addition to discrete morphology and molecular data from both living and subfossil taxa, and fossil ages. Their method was implemented in a flexible Bayesian framework and was found to result in a general, extendable, and fast approach for phylogenetic inference from multiple continuous characters [31], thus demonstrating promise for leveraging quantitative (i.e., GMM) data in phylogeny estimation moving forward.

Given the conflicting conclusions on the efficacy of GMM data for phylogeny reconstruction, coupled with the recent advancement in whole bone GMM approaches that enable the option to rapidly collect phenotypic data for taxonomic purposes, we review the current state of the field in using morphometric data for inferring phylogeny using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses in ecology and evolutionary biology (PRISMA-EcoEvo v1.0.) reporting guideline. In addition, we assess the efficacy of morphometric data compared to discrete characters and offer some pathways for approaching this task with GMM data (which we here consider a subset of continuous data). Our objectives were to 1) test whether morphological phylogeny congruence with a reference molecular benchmark improves when morphometric data are included, and 2) identify the challenges and benefits that could arise from the use of GMM data in phylogenetic reconstruction.

Methods

A search for relevant published literature was performed on October 11, 2023, in the Web of Science database using the PRISMA-EcoEvo version 1.0 reporting guideline of O'Dea et al. [66].

Data sources and search strategy

The search strategy was developed through an initial scoping review and subsequent rearrangement of ‘phylogen*’ and ‘morpho*’ as root terms, and with ‘landmark*’ as an additional term to identify all relevant articles. Asterisks were used to include search results containing variations of the root terms ‘phylogeny’, ‘morphology’, ‘morphometric’ and of the additional term ‘landmark’. The final search string comprised the following combination of keywords and Boolean operators: phylogen* AND morpho* AND (continuous OR landmark* OR shape). The search fields selected were “All Fields”. Inclusion criteria were defined as: Publication Years “2003–2023”; languages “English”. Exclusion criteria were defined as: Document Types “Review Articles” and “Editorial Material” (see Table S1, Additional file 1 for inclusion and exclusion criteria). A date restriction of 2003 to 2023 was applied, allowing for articles published over the last 20 years, up to October 2023, capturing the advancement and maturity of the field of geometric morphometrics [67], which began in the late 1990s [68]. Deduplication, and both title/abstract and full-text screening of records were performed manually by one author in EndNote version 20 [69]. Articles were independently assessed for individual study quality by two authors to determine final inclusion (Table S2, Additional file 1; Fig. 1).

Fig. 1
figure 1

PRISMA-EcoEvo flowchart adapted from O’Dea et al. [66] for Web of Science literature search and study selection in the systematic review, showing stages of the workflow

Eligibility criteria

Studies were included or excluded in this review according to predefined criteria (Table S1, Additional file 1). These criteria would enable valid comparisons between phylogenies that are based on or included continuous data with those that did not, as well as comparison to a reference phylogeny based on independent (i.e., molecular) data.

Study selection

The search strategy identified an initial 11,123 records (Fig. 1). Five duplicates were found in records identified through the search. Of the 11,118 articles retrieved, 10,975 were excluded because they did not meet the inclusion criteria. Of the 132 potentially eligible studies, a further 120 were excluded, resulting in a total of 12 articles for final inclusion in the systematic review. We note that six of the twelve studies estimate phylogeny from GMM (i.e., 2D or 3D landmark) datasets, and seven studies are based on traditional linear morphometric data (one study includes both GMM and linear morphometric data; see Fig. 2, Table 1; Tables S3-S4, Additional file 1 for full details).

Fig. 2
figure 2

Overview of morphological data obtained from included studies and analysed in the systematic review. Percentage of studies that include a) each continuous morphometric data type, N = 12 studies, and b) the number of morphological structures (1, 2, 3 or ≥ 4) at the ‘element’ level e.g., cranium, femur in continuous vs discrete (N = 10 studies) datasets. c) The average number of morphological characters per dataset for continuous vs discrete data. *One study was counted twice as it includes both 2D GMM and linear morphometric data

Table 1 Overview of morphological phylogenetic studies included in the systematic review.Phylogenetic morphometric method’ refers to the treatment of the continuous morphometric data in the analysis with abbreviations adapted from Catalano and Torres [24]: Phylogenetic Morphometrics/ landmark analysis under parsimony (PM/LAUP); landmark coordinates as continuous characters AND (linear) parsimony (LC-P); Procrustes distances AND unweighted pair group method with arithmetic mean (PD-UPGMA); Procrustes distances AND neighbor-joining (PD-NJ); linear morphometrics AND linear parsimony (L-P); Procrustes distances AND minimum evolution (PD-ME); principal component (PC) scores as continuous characters AND parsimony (PC-P). Key: number of terminal taxa in comparison trees; SR systematic review, GMM  geometric morphometric (landmark-based) data, linear MM  linear morphometric data, discrete discrete morphological-only dataset; continuous continuous-only dataset; combined combined discrete morphological and continuous dataset; GW  gap-weighted, SM GW  step-matrix gap-weighted, 7 OG seven original continuous quantitative characters; 40 larger dataset of 40 measurement and meristic characters; 3 sig three continuous characters carrying significant phylogenetic signal; 5 sig five continuous characters carrying significant phylogenetic signal. Continuous and combined datasets (i.e., tree comparisons) analysed ‘as such’ unless otherwise stated

Data collection, extraction

The key data sought from each study were the phylogenies that represented valid comparisons, in computer-readable tree file format, Newick or nexus (see Table 1; S2-S3, Additional file 1) for phylogenetic reconstruction methods and tree comparisons). As the tree files were not available online for any of the included studies, the author(s) of each study were contacted and requested to provide these data. For six of the 12 studies the authors either could not be reached or were not able to provide the tree files. In these cases, phylogenies were written manually in Newick format based on article figures, using the function read.tree from the ape package [version 5.7-1; 79] in the R programming language [version 4.2.2; 80]. Data collection was performed by one author.

Analyses

Molecular constraint trees

Molecular phylogenies were constructed for three studies in which a molecular reference phylogeny was not available, to provide a constraint against which the morphology-based trees of these studies could be compared (see S4, Additional file 1 for molecular analyses and GenBank accession numbers).

Tree comparisons

Phylogenies were read into R using either ape::read.tree or ape::read.nexus [79]. To provide equivalence for comparability between morphological and molecular phylogenies, trees based on taxa (i.e., tree tips) that did not match exactly or that differed in the number of taxa within the same study required pruning, taxon matching and/or taxon re-naming. Pruning and matching was performed using functions from the ape package in R (see S5, Additional file 1 for taxon matching). To ensure consistency, branch lengths were not included in analyses if they were not available for all tree comparisons within a single study.

First, we compared the congruence of the morphology-based trees with the molecular reference phylogeny by computing generalized Robinson-Foulds (RF) distances [81] using functions from the Treedist v2.7.0 [82] package in R (R code available at https://doiorg.publicaciones.saludcastillayleon.es/10.5281/zenodo.13357792). In contrast to standard RF metrics which are strictly based on the number of identical splits shared between two trees, generalized RF metrics consider potential similarities between non-identical splits, thus quantifying overall similarity between two trees [81]. Second, the number of resolved nodes with branch support 50 percent or higher (≥ 50%) were calculated as a percentage of the total number of nodes in the tree based on article figures or tables, or tree files. Third, support values for clades shared (i.e., equivalent splits) between alternative morphology-based trees were obtained from article figures or tree files and then averaged for each tree (see Table S8, Additional file 1 for comparable clades). Note that support values were only available for four studies; and thus, the latter two analyses were based on a subset of the twelve included studies. The overall mean value for each data category across all studies was calculated for each of the three analyses.

Results and discussion

The inclusion of continuous morphological data did not improve phylogeny reconstruction in terms of the number of resolved nodes and their statistical support or congruence with molecular benchmark phylogenies (i.e., comparing tree distances), regardless of whether the continuous data were included alone or integrated into discrete morphological datasets. We note that branch support (i.e., bootstrap) is a measure of resolution (precision), while RF distance is more a measure of accuracy, although low resolution can also manifest as reducing apparent accuracy. Continuous or combined discrete and continuous data performed similarly well to discrete data in terms of tree distances to molecular benchmarks and support values for shared clades (Figs. 3a, 5a; see Tables 2 and 4 for full details), but are outperformed by discrete data in terms of the number of resolved nodes (≥ 50%) (Fig. 4a; see Table 3 for full details). Combined (continuous and discrete) data performed better overall than continuous data alone in two out of the three comparable analyses.

Fig. 3
figure 3

a) Normalized generalized Robinson-Foulds (RF) distance from the molecular tree to the comparison trees averaged across all studies, N = 12. Lower values correspond to a closer match (i.e., most similar topology) with the reference tree, with zero being an equal match. b) Overall performance of different morphological data types across all studies for each pairwise data comparison where the percentage indicates the number of studies in which a given data type performed better. n indicates the number of studies included for each data type in each pairwise comparison

Table 2 Normalized generalized Robinson-Foulds (RF) distance from the molecular tree to the comparison trees, N = 12 studies. Key: discrete discrete morphological-only dataset; continuous continuous-only dataset; combined combined discrete morphological and continuous dataset; GW gap-weighted; SM GW step-matrix gap-weighted; 7 OG seven original continuous quantitative characters; 40 larger dataset of 40 measurement and meristic characters; 3 sig three continuous characters carrying significant phylogenetic signal; 5 sig five continuous characters carrying significant phylogenetic signal. Abbreviations adapted from Catalano and Torres [24]: Phylogenetic Morphometrics/ landmark analysis under parsimony (PM/LAUP); landmark coordinates as continuous characters AND (linear) parsimony (LC-P); Procrustes distances AND unweighted pair group method with arithmetic mean (PD-UPGMA); Procrustes distances AND neighbor-joining (PD-NJ); linear morphometrics AND linear parsimony (L-P); Procrustes distances AND minimum evolution (PD-ME); principal component (PC) scores as continuous characters AND parsimony (PC-P). The tree with the smallest distance from (i.e., most congruent with) the molecular tree is shown in bold. Note that some studies include more than one comparison based on a continuous morphometric or combined discrete morphological and continuous morphometric dataset (i.e., representing different treatments of continuous data and analytical approaches to the phylogenetic reconstruction)
Fig. 4
figure 4

a) Number of resolved nodes with branch support 50 percent or higher (≥ 50%) as a percentage of the total number of nodes in the tree averaged across all studies, n = 5. b) Overall performance of different morphological data types across all studies for each pairwise data comparison where the percentage indicates the number of studies in which a given data type performed better. n indicates the number of studies included for each data type in each pairwise comparison. Note that discrete vs continuous comparisons were not available (hence the n/a values for this pairwise comparison). *One study was counted twice as it performed equally well for continuous and combined data

Table 3 The number of resolved nodes with branch support 50 percent or higher (≥ 50%) as a percentage of the total number of nodes in the tree, n = 5 studies. Molecular = molecular reference tree. The morphology-based tree with the highest number of resolved nodes (≥ 50%) is shown in bold

On average, the discrete- and combined data-based trees were (equally) most congruent with the molecular reference phylogeny (i.e., have the lowest generalized RF values), with the continuous data-based trees being least congruent (Fig. 3a; Table 2). When comparing discrete- vs continuous-based trees, discrete data-based trees performed better in four studies (67%), while continuous data-based trees performed better in two studies (33%) (Fig. 3b). When comparing discrete- vs combined-data based trees, discrete data-based trees performed better in two studies (25%), while combined data-based trees performed better in six studies (75%). When comparing continuous- vs combined-data based trees, continuous data-based trees performed better in one study (17%), while combined data-based trees performed better in five studies (83%). There appears to be a temporal trend in terms of the advancement of morphometric methods (i.e., from linear morphometrics to 2D and 3D GMM) implemented between 2003 and 2023 (Table 2). Approximately half of the twelve studies represent phylogenies based on datasets comprising GMM data, while the other half comprise linear morphometric data (Table 1; Table 2). We also note that two of the twelve studies do not include a discrete data comparison (see Tables 1 and 2).

On average, discrete-based trees outperformed the continuous and combined data-based trees in terms of the number of resolved nodes (≥ 50%), with discrete-based trees having the highest percentage and continuous-based trees having the lowest (Fig. 4a; Table 3). When comparing discrete- vs combined-data based trees, discrete data-based trees performed better in one study (33%), while combined data-based trees performed better in two studies (67%) (Fig. 4b). When comparing continuous- vs combined-data based trees, continuous data-based trees performed better in one study (33%), while combined data-based trees performed better in two studies (67%).

On average, discrete- and continuous-based trees performed similarly in terms of the average support value for shared clades, with continuous-based trees having the highest value and combined data-based trees having the lowest (Fig. 5a; Table 4). When comparing discrete- vs combined-data based trees, the discrete data performed better in one study (50%), while the combined data performed better in the other (50%) (Fig. 5b). When comparing continuous- vs combined-data based trees, the continuous data performed better in one study (50%), while the combined data performed better in the other (50%).

Fig. 5
figure 5

a) Average support value for shared clades (i.e., equivalent splits) across alternative morphology-based comparison trees averaged across all studies, n = 4 studies. b) Overall performance of different morphological data types across all studies for each pairwise data comparison where the percentage indicates the number of studies in which a given data type performed better. n indicates the number of studies included for each data type in each pairwise comparison. Note that discrete vs continuous comparisons were not available (hence the n/a values for this pairwise comparison)

Table 4 Average support value for shared clades (i.e., equivalent splits) across morphology-based comparison trees, n = 4 studies. The tree with the highest average support value is shown in bold

Similar results (i.e., average values) and relative performance of each data category were found when considering only those studies which include GMM data (see Additional file 1, Tables S9-11). When comparing the performance of linear morphometric vs GMM (both 2D and 3D, where possible) data (regardless of the underlying method), GMM-based studies with GMM data in combined datasets performed best, on average, in terms of congruence with the molecular reference phylogeny (i.e., had the lowest generalized RF values) (0.316) (Additional file 1, Table S12). Overall, combined datasets (0.530, 0.316) performed better than continuous-only datasets (0.617, 0.638) (Additional file 1, Table S12). Continuous-only datasets performed similarly for both GMM- (0.638) and linear morphometric-based (0.6174) studies, while combined datasets performed better for GMM-based studies (0.316) than for linear morphometric-based (0.530 studies.

When comparing the performance of linear morphometric vs GMM data, linear morphometric-based studies with linear morphometric data in combined datasets performed best, on average, in terms of the number of resolved nodes (≥ 50%) (41%) (Additional file 1, Table S13). Overall, combined datasets (41%, 36%) performed better than continuous-only datasets (26%, 11%) (Additional file 1, Table S13). Continuous-only datasets performed better for linear morphometric-based studies (26%) than for GMM-based (11%) studies.

When comparing the performance of linear morphometric vs GMM data, GMM-based studies with GMM data in continuous-only datasets performed best, on average, in terms of the average support value for shared clades (99%) (Additional file 1, Table S14). Overall, continuous datasets performed better than combined datasets. Continuous-only datasets performed better for GMM-based studies (99%) than for linear morphometric-based studies (87.3%), while combined datasets performed similarly for both GMM- (71.1%) and linear morphometric-based (70.4%) studies.

Limitations

Limitations of our study include the small sample size (12), in part due to the eligibility criteria excluding studies that did not contain valid comparisons. For example, when the study included a continuous-only tree, but no discrete-only or combined data tree to directly compare and thus determine the effect of including continuous data. In other cases, valid comparisons were available, but a molecular reference tree was not, and it was not possible to make one. For example, if the study focused on fossil taxa then it could not be included. We are limited to existing empirical data, which at present does not allow for large sample sizes. We found great difficulty in obtaining relevant (i.e., discrete vs continuous) comparisons from the literature despite a rigorous and exhaustive search, which hinders our understanding of the performance of continuous data. Thus, we implore researchers to address this issue with more comparable analyses (i.e., targeting discrete vs continuous comparisons from the outset). We also note that a more pure “like for like” comparison i.e., between discrete data that describe shape variation and the quantitative treatment of the same data, is also interesting from a theoretical point of view and construction of such datasets should be encouraged.

The lack of support values for phylogenies within some studies meant that analyses comparing resolved nodes and average support values could only be based on a subset of the total number of studies included in the review (five and four out of 12, respectively). Furthermore, phylogenetic accuracy between studies may not be directly comparable due to different dynamics/resolution in different taxonomic groups. Different support measures might also not be directly comparable, for example, for one study [74] Bayesian posterior probability for the molecular reference was compared to bootstrap values for all morphological comparison trees. We also note that molecular trees, used here as a benchmark comparison, are not errorless, but are commonly considered the ‘gold standard’ in phylogenetics.

We acknowledge that there are biases associated with the definition and scoring of discrete morphological data, however this does not preclude a comparison. Discrete characters represent an alternative and adopted route used for many phylogenetic studies, especially in the field of palaeontology. Therefore, our comparison does yield practical relevance for researchers using these approaches. The GMM datasets we analysed typically cover fewer elements or structures than do the discrete datasets. Thus, our comparisons more closely reflect the current state of the field for researchers deciding to use continuous vs discrete data, rather than the full potential of GMM data.

We note that only six of the twelve studies included in the systematic review estimate phylogeny from GMM (i.e., landmark) datasets. Seven studies are based on traditional linear morphometric data collected from morphological structures which are often utilised as taxonomically informative characters, which may be due to the time and labour required to collect GMM data. Furthermore, one study [76] did not include an ‘as such’ comparison of continuous morphometric data. Here, 2D GMM data were not analysed as quantitative variables but were discretized using a modification (addition) to Wiens’ [78] step-matrix gap-weighting method in both a continuous-only dataset and in a dataset combined with discrete morphological data, and in both cases were treated as continuous data in the analyses.

Future directions

Overall, our results suggest that continuous morphometric data do not perform well when applied to the inference of phylogenetic topology. Since none of the included studies used probabilistic methods, it would be valuable to explore how these methods might perform compared to parsimony or distance-based methods. Our findings suggest a temporal trend in terms of the advancement of morphometric methods (i.e., from linear morphometrics to 2D and 3D GMM) implemented between 2003 and 2023, however, no trend is shown in the same timeframe for the improvement in continuous data compared with discrete data. In the case of GMM this may be due to the high dimensionality of the data and variation within these types of datasets. Thus, ‘noise reduction’ techniques [83, 84] will be critical to the use of GMM data for phylogeny to enable the isolation of phylogenetic signal.

In addition, the full potential of GMM data e.g., whole body or whole osteology landmarking will be important to explore in future. Despite the challenges associated with estimating phylogenetic topology from GMM data, the advancement of morphological phylogenetics depends on improved methods for extracting and modelling quantitative morphological data. Hence, the potential benefits of leveraging quantitative data for this purpose warrant further investigations into how to best model the complexity and correlations inherent in GMM character data.

One promising approach is to objectively discretize the shape variation in landmark data, subdividing the landmarks into “characters” and discretizing each of these into states using clustering algorithms [22]. Additionally, some continuous GMM characters do not appear to contain phylogenetic signal at all which may be due to signal erosion over time, with rapidly evolving traits or phylogenetic signal being overwhelmed by functional covariation and phenotypic plasticity [74, 85]. Phylogenetic signal erosion/retention for molecular phylogenies have been quantified using stemminess analyses, saturation analyses and various other metrics [86,87,88,89,90,91]. Phylogenetic signal retention in morphological landmark data and across different taxonomic groups will be another important area for further research.

Conclusions

Our study provides a comprehensive and rigorous assessment of the performance of existing empirical continuous data using the PRISMA-EcoEvo version 1.0 reporting guideline, and identifies important challenges to overcome, as well as benefits that could arise from the use of GMM data in phylogenetic reconstruction. We find that the inclusion of continuous morphological data does not improve phylogeny reconstruction, and our analyses show that overall, continuous morphometric data do not perform well when applied to the inference of phylogenetic topology. However, despite the challenges associated with estimating phylogenetic topology from GMM data, the advancement of morphological phylogenetics depends on improved methods for extracting and modelling quantitative morphological data. Hence, the potential benefits of leveraging quantitative data for this purpose warrant further investigations into how to best model the complexity and correlations inherent in GMM character data.

Since morphological data is often used in conjunction with molecular data, it would also be valuable to explore how discrete and continuous morphological data respectively interact with molecular data in producing emergent phylogenetic signals. Our study demonstrates the problem surrounding the efficacy of continuous data as remaining relatively intractable despite an exhaustive search, due in part to the difficulty in obtaining relevant comparisons from the literature. Our study was performed under a rigorous framework for systematic reviews, which showed that the lack of available comparisons between discrete and continuous data hinders our understanding of the performance of continuous data.. Thus, we implore researchers to address this issue with studies that collect discrete and continuous data sets with directly comparable properties (i.e., describing shape, or size).

Data availability

The datasets analysed during the current study and the R code are available in the Zenodo repository, https://doiorg.publicaciones.saludcastillayleon.es/10.5281/zenodo.13357792.

Abbreviations

GMM:

Geometric morphometric

ML:

Maximum likelihood

PCMs:

Phylogenetic comparative methods

PRISMA-EcoEvo v1.0.:

Preferred Reporting Items for Systematic Reviews and Meta-Analyses in ecology and evolutionary biology version 1.0

LAUP:

Landmark analysis under parsimony

PM:

Phylogenetic Morphometrics

PC:

Principal component

NJ:

Neighbour-joining

UPGMA:

Unweighted pair group method with arithmetic mean

fWLS:

Flexibly weighted least squares

RF:

Robinson-Foulds

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Acknowledgements

We thank the authors of studies who responded and provided data that were used in this study.

Funding

This work was supported by an Australian Government Research Training Program Scholarship (to E.J.H.), and the Australian Research Council Discovery Program (FT200100822) (to L.A.B.W.).

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L.A.B.W. conceived the study. E.J.H. analysed the data and wrote the original draft. M.J.P. and M.A.C. were involved in planning and performing the analyses. L.A.B.W., M.J.P. and M.A.C. supervised the study. E.J.H. prepared all figures and tables. All authors reviewed and approved the final manuscript.

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Correspondence to Emma J. Holvast.

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12862_2024_2313_MOESM1_ESM.docx

Additional file 1: Supporting information for this publication: inclusion and exclusion, quality assessment criteria, phylogenetic reconstruction methods, tree comparisons, molecular analyses, taxon matching, comparable clades, results, GMM data-only and results, linear morphometric vs GMM data.

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Holvast, E.J., Celik, M.A., Phillips, M.J. et al. Do morphometric data improve phylogenetic reconstruction? A systematic review and assessment. BMC Ecol Evo 24, 127 (2024). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12862-024-02313-3

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