Research Article 
Corresponding author: Catherine Girard ( catherine.girard@umontpellier.fr ) Academic editor: Florian Witzmann
© 2022 Catherine Girard, AnneLise Charruault, Thomas Gluck, Carlo Corradini, Sabrina Renaud.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Girard C, Charruault AL, Gluck T, Corradini C, Renaud S (2022) Deciphering the morphological variation and its ontogenetic dynamics in the Late Devonian conodont Icriodus alternatus. Fossil Record 25(1): 2541. https://doi.org/10.3897/fr.25.80211

Identification of relevant taxonomic and evolutionary units is a recurrent issue in the fossil record, and all the more for ancient fossils devoid of modern equivalents such as conodonts. Extensive morphological variation has often led to the description of numerous species, subspecies or morphotypes, which may correspond to endmember morphologies reached through ontogeny. The platform elements of the Late Devonian conodont species Icriodus alternatus were characterized by rows of denticles coming into occlusion between opposite elements; each element grew by the incremental addition of lamellae and by the addition of successive triads during ontogeny. During the late Frasnian and the early Famennian, the important morphological variation within this species led to the description of three subspecies. An extensive sample of early Famennian Icriodus alternatus was quantified using 2D biometric measurements and denticle counts on 2D pictures, showing that the subspecies mainly differed in their size range but not in their general morphology. A 3D morphometric analysis was further performed on a subsample to characterize the shape of the ontogenetically older part of the elements. During ontogeny, early valleys between denticles tended to be filled, and the asymmetry between the inner and outer side of the element increased. These ontogenetic trends are responsible for the morphologies formerly described as the subspecies Ic. alt. mawsonae and Ic. alt. helmsi. Slight discrepancies between temporal ranges of the subspecies may be achieved through variations in range of size reached by the elements as a response to environmental changes. Disparity along ontogeny seems to follow an “hourglass model” suggesting a shift from relatively loose developmental constraints to a pattern of growth modulated by functional constraints during occlusion.
Devonian, disparity, geometric morphometrics, hourglass model, Icriodus alternatus
Conodonts are long extinct animals known from the Cambrian to the endTriassic, without modern equivalent. These early vertebrates were an important part of the nektonic fauna increasing in richness through the Paleozoic era (
This morphological diversity repeatedly raised the question of how to deal with an extensive phenotypic variation, leading description of numerous species, subspecies, or morphotypes (
The genus Icriodus is documented from the Lower Devonian up to the Upper Devonian period. Icriodus, as many other genera, has a cosmopolitan distribution but several endemisms at specific level have been documented (
One natural cluster of Icriodus alternatus has been described (
The ontogenetic growth of Icriodontan elements, as described based on the species Icriodus expansus (
Morphological descriptors of I elements of Icriodus alternatus. A. “Small” specimen (UM BUS 031), upper (occlusal) view. 2D counts: Number of triads (Triads_nb): estimation of the number of triads, based on the presence of the pair of outer and inner denticles; Number of median denticles on the spindle area (Med_dent_nb); Number of lateral denticle on the blade area (Lat_blade_dent); Number of median denticle on the blade area (Med_blade_dent). B. Icriodus alternatus helmsi (UM BUS 035), upper (occlusal) view. Biometric measurements (D1, MaxLength, MaxWidth, area and spindle area). Triads in yellow; spindle area in blue dotted line, area of the 2D specimen in red, D1 distance between the inner and the outer denticles of the first triad in orange. C. Icriodus alternatus alternatus (UM BUS 033), upper and lower views (not to scale) with the position of the 3D landmarks (3D measurements). In green the valleys; in yellow the denticles.
Morphological variation within the species Icriodus alternatus. Note the “hybrid” with morphological characteristic of both helmsi and mawsonae. A. UM BUS 034; B. UM BUS 035; C. UM BUS 036; D. UM BUS 037; E. UM BUS 038; F. UM BUS 039; G. UM BUS 033; H. UM BUS 040; I. UM BUS 041; J. UM BUS 042; K. UM BUS 043; L. UM BUS 044; M. UM BUS 045; N. UM BUS 031.
Icriodus alternatus was initially characterized as bearing “denticles of the median row (…) small, rounded and isolated, alternating with the lateral denticles in position” (
These characteristics are highlighted in the revised diagnosis of the species (
A third subspecies, Ic. alt. mawsonae, was later introduced by
However, according to the recent revision of the Famennian conodont zonation (
The Buschteich outcrop (Thuringia, Germany), is a condensed pelagic section entirely composed of limestones (Fig.
Conodonts were extracted from the limestone samples following the classical procedure. Rock material was dissolved using diluted formic acid (10%). Residues were rinsed through 100 µm and 1 mm sieves; the fraction in between was dried and the conodonts were picked using a Nikon stereomicroscope. All P1 elements attributed to Icriodus alternatus sensu lato, hence including Ic. alt. helmsi and Ic. alt. mawsonae, were selected for the present study. Specimens with 3 triads or less were considered separate in a “small” elements group.
4.1.1. 2D counts and measurements
A total of 191 elements were deemed to be in a suitable preservation state for a 2D quantitative study realized on SEM (Scanning Electron Microscope) pictures. Variables included the following counts (Fig.
Among these counts, the number of lateral and median denticles on the blade area were designed to characterize the helmsi morphotype, whereas the number of median denticles on the spindle area relative to the number of triads should characterized the mawsonae morphotype.
Furthermore, several 2D quantitative measurements were taken on 2D pictures of the upper (occlusal) surface of these 191 specimens using an image analysis device (Nikon NIS Elements software), in to describe the general size and shape of the elements (Fig.
For 133 of these elements, measurements were taken on MEB pictures. A subset of 58 elements was µCTscanned and measurements were taken on 2D snapshots of the 3D surface oriented with the occlusal surface up as for MEB pictures. The total sampling included 95 Ic. alt. alternatus s.s., 36 Ic. alt. helmsi, 17 Ic. alt. mawsonae, and 42 “small” specimens. One specimen displayed characters typical of both, Ic. alt. helmsi (most posterior denticle shifted towards the inner line of denticles) and of Ic. alt. mawsonae (absence of expressed first median denticle, md1) (Fig.
4.1.2. Acquisition and extraction of 3D surfaces
A subset of 58 elements were glued on a toothpick and scanned at a cubic voxel resolution of ~1 µm using Phoenix Nanotom S microtomograph (μCT) on the AniRAImmOs platform of the SFR Biosciences, Ecole Normale Supérieure, Lyon (France). The scanning parameters were as follow: 100 kV, 70 µA, 3000 projections at 360° with no filter. The surface of the element was extracted semiautomatically using the thresholding tool in Avizo (v. 9.1—Visualization Science Group, FEI Company).
These elements were selected to document the morphological variation over a broad range of ontogenetic stages (from two to eight triads) and across subspecies. Most corresponded to alternatus specimens (N = 29), but the sampling included the helmsi (N = 11) and mawsonae (N = 5) subspecies. One specimen displayed characters typical of both, helmsilike with the most posterior denticle shifted towards the inner row of denticles and of mawsonae with absence of expressed first median denticle, md1; it was depicted as “hybrid” but attributed to the subspecies mawsonae in statistical analyses (Fig.
All the specimens are housed in the collections of the University of Montpellier. The reconstructed 3D surfaces of the specimens illustrated on Fig.
4.1.3. Length, width and 3D landmarks
3D surfaces of the left elements, spur to the right when the element is seen in occlusal view), were subjected to a mirror transformation and measured as right elements. Length, measured as the greatest anteroposterior distance, and width, the largest innerouter dimension, were manually measured on the 3D surfaces using Avizo. Fifteen 3D landmarks (Suppl. material
Correlations involving counts were assessed using Kendall’s rank order test; correlations between numerical variables were assessed using the Pearson correlation coefficient. Differences between the alternatus, helmsi, mawsonae, and small groups were tested using nonparametric KruskalWallis tests and pairwise Wilcoxon comparisons.
Linear models using Maximum Length as the independent variable were used to assess relationships of the different variables along ontogenetic growth, and to provide sizecorrected residuals. Models including Maximum Length and group as factor were also performed. They allowed to test the significance of the differences between groups while taking size into account; the interaction between both factors indicated whether slopes were similar in the different groups.
All these tests were performed using R (
4.3.1. Length and width
The relationship between element length and width was assessed using a linear regression. Since width was available for all elements, the relationship length ~ width was used to interpolate length values for the three anteriorly broken specimens.
4.3.2. Characterization of the element geometry using interlandmark distances
In order to document the growth of the different parts of the elements, interlandmark distances were calculated from the landmark coordinates. The following distances were considered:
How these distances varied along the ontogeny was investigated using linear regression of each distance vs element length. Interpolated values were used for visual representations but were not included in the calculation of the regressions.
4.3.3. Denticle height and pit depth: estimation by the triangle geometry
In order to estimate the height of the denticles above the valleys, geometric properties of the triangle were used. For any triangle, the semiperimeter (p) and the area (S) can be derived can be calculated from the sides of the triangle a, b, and c, following to the two equations p = (a + b + c) / 2 and S = sqrt[p (pa) (pb) (pc)]. The altitude h at the summit A (opposed to side a) can then be obtained as h = 2S/a.
According to these formulas, the height of md1 can be approximated as the altitude of a triangle defined by the tip of the denticle (md1) and the two valleys surrounding it along a same line (iv1 and ov2, or alternatively ov1 and iv2) (Suppl. material
The height of the outer and inner denticles were assessed using the valleys along the same line: hid1, based on the triangle id1iv1ov2; hod1 based on od1ov1iv2; hid2 based on id2iv2ov1, and hod2 based on od2ov2iv1.
The relationship of denticle height and pit depth with element length was assessed using linear regression. Linear models including, groups and their interaction were further investigated.
4.3.4 3D geometric morphometrics
The configurations of the fifteen 3D landmarks were superimposed using a generalized Procrustes analysis (GPA) standardizing size, position, and orientation while retaining the geometric relationships between specimens (
Sizerelated variations in shape and differences between groups (alternatus, helmsi, mawsonae and “small” groups were investigated using Procrustes ANOVA. With this approach, the Procrustes distances among specimens are used to quantify the components of shape variation, which are statistically evaluated via permutation, here, 9999 permutations (
The Procrustes superimposition, allometric analysis, and Procrustes ANOVA were performed using the R package geomorph (
The 2D sampling included elements with two to eight triads (Fig.
Differences between subspecies, for maximum length, ratio between the number of median denticles and triad number, the number of lateral denticles on the blade, and the residuals of √Area, √Spindle Area, maximum width and distance D1 vs. maximum length. KW: pvalue of a KruskalWallis test; if significant, pvalues of pairwise Wilcoxon tests are provided below. In bold, pvalue < 0.001, in italics pvalue < 0.05.
Maximum Length  KW  < 2.2e16  

small  alternatus  helmsi  
alternatus  < 2e16     
helmsi  < 2e16  9.2e05   
mawsonae  2.9e14  9.2e05  0.5 
Med_dent_nb/Triad_nb  KW  3.804e06  
small  alternatus  helmsi  
alternatus  0.0068     
helmsi  0.0038  0.2104   
mawsonae  5.2e05  0.0017  0.0437 
Lat_Blade_Dent_nb  KW  < 2.2e16  
small  alternatus  helmsi  
alternatus  0.1370     
helmsi  6.9e14  < 2e16   
mawsonae  0.0140  0.106  6.9e06 
residuals sqrtArea  KW  0.1367  
residuals sqrtSpinArea  KW  0.2194  
residuals Max Width  KW  0.1759  
Residuals D1  KW  0.6757 
All variables, being counts or numeric measurements, were significantly related together (Fig.
Correlogram of the relationship between each pair of counts and numeric variables in the Icriodus dataset. Size of the circle is proportional to the strength of the correlation, color varies with the value of the correlation. Kendall’s tau correlation coefficient has been used for the representation. Variables are ordered according to a hierarchical clustering of the correlations.
The variables specifically designed to capture the characteristics of the subspecies indeed showed significant differences. The number of median denticles on the spindle area of course increased with triad number, but to a lesser degree for mawsonae (Fig.
The number of median blade denticles decreases with triad numbers (Fig.
The homogeneity of the subspecies when considering their general aspect was confirmed using linear models including maximum length and groups as factors (Table
Effects of maximum length and subspecies (Ssp) on total area (√), spindle area (√), maximum width and the distance between the inner and outer denticle of the first triad (D1). Pvalues of linear models including maximum length and groups are given; left, including the four groups; right, excluding the “small” group.
Four groups  Large only  

sqrtArea  
MaxLength  < 2e16  ***  < 2e16  *** 
Ssp  0.05268  0.5478  
ML:Ssp  0.16214  0.1800  
sqrtSpinArea  
MaxLength  < 2e16  ***  < 2e16  *** 
Ssp  0.03443  *  0.7423  
ML:Ssp  0.08720  0.3426  
MaxWidth  
MaxLength  < 2e16  ***  < 2e16  *** 
Ssp  0.03522  *  0.9998  
ML:Ssp  0.66452  0.7563  
D1  
MaxLength  < 2e16  ***  8.632e09  *** 
Ssp  0.27271  0.4680  
ML:Ssp  0.06596  0.0228  * 
In order to assess how morphological disparity varied along ontogeny, the variance of the different variables was assessed for each triad number. Triads 1 and 2, and 78, were grouped together because of insufficient data. The relative number of median denticles (Med_Dent_nb/Triads_nb) was considered together with maximum length and width, √ of the total area and spindle area, the distance between the inner and outer denticles of the first triad (D1), and the number of lateral and median denticles on the blade. To level out the effect of different scales and of size increase, variables were centered to the mean and divided by the standard deviation).
The patterns of variance through ontogeny were compared using Kendall rank order tests (Fig.
Disparity through ontogeny. The variance of the different reducedcentered variables was estimated for each triad number (triads 12 and 78 grouped). A. Correlogram showing the congruence of the pattern between variables, estimated using Kendall’s tau coefficient; B. Change in the level of variance through ontogeny. The variance of each variable has been scaled by its mean in order to represent the different lines on a single graph. Variables considered were: MaxLength: maximum length, sqrtArea: √ of the total area of the element; sqrtSpindleArea: √ of the spindle area; MaxWidth: maximum width; D1: distance between the inner and outer denticles of the first triad; LatBladeDent: number of lateral denticles on the blade; MedBladeDent: number of median denticles on the blade; relMedDent: number of median denticles on the spindle, divided by triad number.
Length vs width in the 3D dataset
When focusing on the 3D subset, length and width measured on the 3D surfaces were strongly related (Pearson’s productmoment correlation R = 0.920, pvalue: < 2.2e16), with the length increasing 1.5 faster than the length (slope of the regression: 1.509) (Suppl. material
The analysis of the heights and distances focused on the most posterior part of the element, including the first triad, because these features can be measured in almost all specimens, including small ones, and document the growth of the elements. Most variables increase with growth as indicated by increasing length (Table
Relationship of element length with interlandmark distances, denticle height and pit depth. Above, linear model of variable vs length: R, Pearson’s productmoment correlation R, associated pvalue, and slope of the regression. Below, pvalues of linear models with two factors: length, group [alternatus, helmsi, mawsonae {including the “hybrid”}, small {three or less developed triads}], and their interaction. In bold pvalues < 0.001, in italics p < 0.01.
Pit depth  Height md1  dlat1  dlat2  Height id1  Height od1  d(mpd, id1)  d(mpd, od1)  d(mpd, md1)  d(cusp, mpd)  

pvalue lm(~ Length)  
R  0.661  0.639  0.896  0.797  0.434  0.599  0.018  0.468  0.104  0.690 
pvalue  4.02e08  1.49e07  <2.2e16  3.32e13  0.0009  1.38e06  0.8962  0.0003  0.4498  5.70e09 
Slope  0.06  0.04  0.11  0.11  0.03  0.04  0.00  0.03  0.01  0.07 
pvalue lm(~ Length * Group)  
Length  2.25e08  1.65e07  <2e16  1.20e12  0.0013  8.66e07  0.8968  0.0006  0.4298  6.12e09 
Group  0.0796  0.7163  0.1306  0.7346  0.4869  0.3032  0.2930  0.6098  0.1266  0.1992 
*  0.2868  0.1185  0.5474  0.2235  0.8240  0.0982  0.6529  0.9989  0.1875  0.3448 
Relationship between Length (xaxis on all plots) and various interlandmark distances, denticle height and pit depth. Dotted lines: significant linear relations. Interpolated values of length for the three elements with the anterior part missing (diamond symbol) are used for visualization but are not included in the regressions.
Three variables clearly depart from this general growth pattern. Two distances involving the most posterior denticle show no relationship with length: distance from mpd to od1 and md1. Most strikingly, the height of the first median denticle (md1) is not only stable through growth, but even decreases. Concomitantly, pit depth increases slightly faster than the height of md1. Pit depth roughly assesses the thickness of the element in the vicinity of md1, from the same valley points from which md1 height is calculated. The balance between increasing pit depth and decreasing md1 height may therefore be largely due to a filling of the valleys, ultimately leading to a md1 which is not visible anymore.
Variables can thus be associated according to the way they covary (Fig.
Correlations between univariate variables: element length (including the three interpolated values), denticle height, pit depth, and distances between denticles. In each case, the diameter of the circle and its color is proportional to the strength of the correlation, estimated using the Pearson coefficient. Variables are order to a hierarchical clustering approach.
These analyses were complemented by linear models including length, groups, and their interaction, in order to assess whether the growth dynamics differed between alternatus, helmsi, mawsonae, and the small specimens (Table
The Procrustes superimposition delivered the centroid size of the configurations as another proxy of element size, which was related to element length (Pearson’s productmoment correlation R = 0.673, pvalue = 1.811e08) (Suppl. material
The morphometric analysis of the 3D shape showed along the first axis of the PCA on the aligned coordinates (PC1 = 35.2%), an opposition between small elements, towards negative scores, and helmsi, and especially mawsonae, tending to display high positive scores (Fig.
3D Geometric morphometric analysis of Icriodus alternatus shape variation; A. First two axes of a PCA on the aligned coordinates; B, C, D. Visualization of the deformation in profile and oral view; in yellow tip of the denticles, in green valley landmarks, in black the pit; B. Configuration corresponding to the minimum score along PC1; C. Shape change from the minimum (dots) to the maximum (tip of the vectors) scores along PC1; D. Configuration corresponding to the maximum score along PC1.
Therefore, the variation along PC1 mostly depicts the growth of the element. Towards negative scores corresponding to small elements, outer and inner denticles are close to and approximately at the same distance from the median axis of the element (Fig.
The relationship of shape with size and groups were investigated using Procrustes ANOVA. As expected, the size / shape relationship was very strong (Procrustes ANOVA, shape ~ Length including interpolated values: P = 0.0001) (Suppl. material
The allometric size/shape relationship can be visualized using scores along the Common Allometric Component, which highly resembles scores on PC1 (Pearson’s productmoment correlation R = 0.998, pvalue < 2.2e16), underlining the importance of the allometric signal in the total shape variation. Accordingly, shape deformation associated with allometry (Fig.
Allometric shape variation in Icriodus alternatus; A. Relationship between the total length of the element and the Common Allometric Component, based on the aligned coordinates of the posterior part of the element; B, C, D. Visualization of the deformation in profile and oral view, corresponding to extreme size values; in yellow tip of the denticles, in green valley landmarks, in black the pit; B. Configuration corresponding to the minimum length; C. Shape change from the minimum (dots) to the maximum (tip of the vectors) length; D. Configuration corresponding to the maximum length.
The 3D analysis was focused on the posterior, ontogenetically oldest part of the Icriodontan element. Results showed that independently of the addition of new triads along ontogeny, the geometry of this posterior part changed deeply along growth, the main components being: (1) a growth of the inner and outer first denticles with both, a vertical and lateral component, the centrifugal growth being more pronounced for the outer denticles; (2) a growth of the cusp towards a more posterior direction, increasing its distance from the first triad; (3) a shift of the pit towards a more anterior position, together with an increase in the thickness of the element between the pit and the valleys surrounding the first median denticle; (4) a decrease in height of the first median denticle, up to its disappearance in most extreme cases. (5) In contrast, the zone comprising the most posterior denticle, the first median denticle and inner denticle, which seem to form a “core” area of the element in which the geometry, designed early during ontogeny, is little affected later on.
The subspecies Ic. alt. alternatus, Ic. alt. helmsi, and Ic. alt. mawsonae appeared to share the same ontogenetic trajectory, and once sizerelated variation is accounted for, they did not differ in any quantitative variable describing their general shape or the relationships between posterior denticles. The most prominent criterion distinguishing Ic. alt. helmsi and Ic. alt. mawsonae is their large size, suggesting that actually, these subspecies simply represent endmember geometries achieved at late growth.
The Ic. alt. helmsi subspecies has been characterized by the alignment of the most posterior denticle with the inner row of denticles. This morphology is achieved due to pronounced centrifugal growth of the outer denticles. Combined with the invariant relationships between the most posterior denticle and the first inner one, this trend tends to twist the shape of the large elements, up to orienting the most posterior denticle with the inner row in the most extreme cases.
As for the Ic. alt. mawsonae subspecies, it corresponds to an endmember of the trend of decreasing height of the first median denticles along ontogeny. Since the removal of material from a denticle seems unlikely, this decreasing height, together with the increasing thickness of the element at the vertical of the first median denticle expressed by the increase in pit depth, suggest a progressive filling of the initial deep valleys. Without a concomitant growth of the median denticle, this leads to a progressive reduction, up to its disappearance. In agreement with this interpretation of the subspecies as part of a morphological continuum including all Ic. alternatus subspecies, several specimens first identified as Ic. alt. mawsonae displayed “clear” traces of the first median denticle on the 3D scans. Furthermore, at least one specimen displayed the diagnostic features of both Ic. alt. helmsi and Ic. alt. mawsonae.
As a consequence, the described subspecies appear to belong to a single, homogeneous taxonomic and evolutionary unit, corresponding to the species Ic. alternatus. Although they can be seen as a way of describing an extensive morphological variation, the use of the “subspecies” concept in this context is misleading. For modern organisms, this notion corresponds to geographically isolated pools (
Variation in the oldest, posterior part of the element: a general feature of the genus Icriodus
The posterior part of the platform Icriodontan element is the first to be formed in ontogeny. As such, it is exposed to remodeling during all subsequent growth, consequently being the most variable zone in different Icriodus species. A morphotype differing in the expression of the denticles in the posterior area of the spindle middle row, hence similar to Ic. alt. mawsonae, has been described in Icriodus subterminus (MiddleLate Devonian) (
The blade, the other part of the element that composes the initial growth stage, displays similar trends of denticle progressive disappearance or fusion, as shown by the progressive diminution of blade denticle number along growth in Ic. alternatus (Dreesen and Houllenbergs 1980).
Slight discrepancies exist in the temporal extension of the “subspecies” Ic. alt. alternatus, Ic. alt. helmsi and Ic. alt. mawsonae, apparently arguing for them being distinct evolutionary units. Icriodus alternatus as a whole appeared during a period marked by a succession of environmental perturbations. The mass extinction marking the Frasnian – Famennian boundary was the culmination of the Upper Kellwasser event materialized by anoxic deposits in many marine environments: it was associated with a pronounced temperature decrease and sealevel fall biosphere (
The ontogenetic pattern of Icriodus alternatus I elements further displayed heterogeneous disparity, with minimum morphological variance at the stage of four triads. This pattern is reminiscent of the “hourglass” developmental model (
2D and 3D biometric and geometric morphometric analyses have shown here that the platform (Icriodontan) elements of Icriodus alternatus display two major morphological trends along ontogeny, besides the addition of successive triads elongating the element: (1) a filling of the initially deep valleys between denticles on the posterior part of the element, leading to the progressive disappearance of the first median denticle on the spindle; and (2) an increasing asymmetry between the inner and outer denticles of a same triad, due to a more pronounced centrifugal growth of the outer denticle. These results suggest that the subspecies of Icriodus alternatus described for the end Frasnian and early Famennian constitute endmember morphologies characterizing the different growth stages. Icriodus alternatus helmsi and especially Ic. alt. mawsonae represent phenotypes achieved when large element sizes are reached, due to remodeling in relation with the continuous growth. Icriodus alternatus alternatus included smaller forms along the same ontogenetic trajectory. The term “subspecies” should thus be avoided to prevent the risk of artificially inflating biodiversity estimates.
Morphological disparity seems not to be homogeneous along the ontogenetic trajectory, following an “hourglass” pattern, suggesting loose developmental constraints at the beginning of the development, and increasing variance in late stages, due to continuous remodeling possibly modulated by occlusal functioning. In between, morphological variance reaches a minimum for elements with four triads. This stage may represent a “phylotypic” stage characterized by the highest canalization and hence the most discriminant between species. Taxonomic efforts should concentrate on such stages to identify relevant evolutionary relevant taxonomic units.
Depending on the journal, the morphological data will be deposited in Dryad or as Supplementary Files. Illustrated specimens (collection numbers UM BUS 031 to UM BUS 045) are available in MorphoMuseuM.
CG initiated and coordinated the study. CG, ALC and CC were responsible for all aspects relevant to paleontological and geological expertise (sampling, picking, identification, dating). TG and CG acquired the 2D measurements, CG and SR analyzed the corresponding data. SR performed the 3D analyses. CG and SR wrote the first draft and all authors contributed and approved the final version of the manuscript.
The authors declare that they have no conflict of interest.
We acknowledge Jeff Over (Geneseo, NY) and Tomas Kumpan (Masaryck, Czech Republic) for their comments on the manuscript, and the contribution of SFR Biosciences (UMS3444/CNRS, US8/Inserm, ENS de Lyon, UCBL) AniRaImmOs facility, and we particularly thank Mathilde Bouchet and Louise Souquet for their kind assistance during the scanning sessions. This work was supported by the ANR Project ECODEV (ANR13BSV7005; 2014–2017) and the LabEx CeMEB project MARCON (2018–2020). This is publication ISEM 2022003.
Table S1
Data type: table
Explanation note: Description of the 15 landmarks.
Table S2
Data type: table
Explanation note: Correlations between variables, tested using Kendall rank order tests. Below the diagonal, Kendall’s tau correlation coefficient, above the diagonal, pvalue. In bold, p < 0.001, in italics p < 0.05.
Figure S1
Data type: figure
Explanation note: Illustration of the calculation of md1 height by the geometry of the triangle iv1md1ov2.
Figure S2
Data type: figure
Explanation note: Relationship between width and length of the elements. The regression (dotted line) allowed for the interpolation of the length for the three elements with their anterior part missing (pointed by arrows on the graph).
Figure S3
Data type: figure
Explanation note: Relationship between length and geometric morphometrics of the posterior part of the element. A, Length vs Centroid Size of the posterior part. B, Length vs PC1 based on the aligned coordinates. Symbols with the thick black outline correspond to the interpolated length values.