Morphological structure of head and comparison between Agraulos longicephalus Hicks, 1872 and A. ceticephalus Barrande, 1846, from the Drumian of Spain and Czech Republic. Daniel Eduardo Rojas Ariza Proyecto de Grado para optar por el título de Geocientífico Asesor: Jorge Esteve Serrano Universidad de los Andes Facultad de Ciencias Departamento de Geociencias Bogotá, Colombia – diciembre 2020 Contents 1. Abstract 2. Introduction 2.1. Background 2.2. Ptychopariid morphologic features 3. Objective 3.1. General 3.2. Specifics 4. Study Material and Methodology 4.1. Fossils 4.2. Methodology 5. Results: Visualization of morphological variance 5.1. Regression Analysis 5.1.1. Individual regressions 5.1.1.1. A. longicephalus Hicks, 1872 5.1.1.2. A. ceticephalus Barrande, 1846 5.2. Principal Component Analysis (PCA) 5.3. Discriminant Function Analysis (DFA) 5.4. Procrustes ANOVA 6. Discussion 7. Conclusions 8. Acknowledgments 9. References 1. Abstract Order Ptychopariida has always been a problematic group concerning the taxonomic classification of Cambrian trilobites, and even though the temporality of this group is well known, the relation between this order with others, and, the difficulty for classifying ptychopariids have let to diverse problems in the phylogenetic classification of this, and other primitive orders of trilobites. Agraulos is one of the genera belonging to this order and is a common group of the middle Cambrian, it is a well-known genus inside ptychopariids, therefore, morphological knowledge about this taxon could be helpful to analyze morphological patterns among other species from the genus and even other genera inside the family Agraulidae, and even more, about the superfamily Ellipsocephaloidea. This paper provides a morphological statistical comparison based on landmark geometric morphometric techniques performed on specimens of two species of this genus, Agraulos longicephalus Hicks, 1872, and Agraulos ceticephalus Barrande, 1846, from the Drumian stage of Spain and the Czech Republic, respectively. The findings of this study suggest that, even though these species are morphologically similar, there are significant differences as to facial suture and glabella shape and size. In general, allometry in the samples seems to be an irrelevant factor related to morphological variation, but, individually, it is a significant source of variation in morphological patterns of A. ceticephalus. The results obtained from this study may contribute to future similar studies interested in analyzing the morphology of other species, genders, or even families inside order Ptychopariida. 2. Introduction 2.1. Background Morphological variation represents a fundamental study phenomenon in phylogenetic classification, as well as in the task of following the routes of evolution through inter and intraspecific variations. Particular evolutionary trends can be understood to a greater extent by supporting hypotheses and conclusions with knowledge regarding degrees and patterns of variation within the studied taxa (Webster, 2011). In the case of the fossil record, morphological variation applied in paleontological analyses may be caused by different additional factors different from the biological ones, unlike the common case, when the study object is an actual non-extinct organism. These factors are related directly to taphonomy, as Webster & Hughes (1999) explain, which can generate shape distorts, as a consequence of compaction, increase in the shape variation, and possible biological misinterpretations. A similar situation happens with geographic variation (morphologic changes range of taxa spread in different locations), because of the lack of enough samples in some cases, a common problem in paleontology, which causes the frequent difficulty of distinguishing between geographical intraspecific variation from interspecific disparity (Webster, 2011). On the other hand, one of the most impactful biological phenomena in morphological variation is allometry, which refers to the changes of shape associated with the size of the organism (Klingenberg, 2009, 2016). Allometry may have great unpredictable effects on the behavior of morphological analysis results, therefore, it is always useful to address the magnitude of the impact that this characteristic has on the morphological variation. Different authors agree with the fact that the Order Ptychopariida has been significantly problematic concerning the taxonomic classification of the families within it, its paraphyletic character among the general trilobite phylogeny, and the general implications that this group has regarding the Cambrian and post-Cambrian trilobite classification (Cotton, 2001; Fortey, 2001; Webster, 2011; Rasetti, 1951 in Cotton, 2001). Nevertheless, Webster (2011) states that analyzing ptychopariid trilobites can provide a substantial knowledge base for understanding the first steps in the evolution of this diverse group of arthropods, and, that is the reason why this study is a small contribution to the palaeobiological knowledge about this group. Order Ptychopariida appeared in the late lower Cambrian, as a descendant group of Redlichiids (Fortey, 1990), and, currently is thought that this order comprises 4 suborders, from which only 3 are partially known: Ptychopariina, Olenina, Harpina and a last uncertain suborder (Whittington et al., 1997). The common morphological features that characterize this order are the following: i) A tapering glabella; ii) preglabellar field; iii) presence of natant hypostome; iv) opisthoparian sutures; v) rim-like cephalic borders extending into genal spines; vi) commonly more than 12 thoracic spines; vii) a micropygidium and viii) a subdued surface sculpture, (Fortey, 2001; Fortey, 2009). Within this group, the genus Agraulos Hawle and Corda, 1847, is mostly known from middle Cambrian, commonly it’s preservation is relatively poor, and most specimens have been collected from Spain, England, Poland, Czechoslovakia, France, North America among other countries (Samson et al., 1990). Agraulos is an effaced Cambrian ptychopariid very common in the Cambrian strata of West Gondwana as well as in Laurentia and Baltica (Fletcher, 2017; Geyer & Landing, 2001; Gozalo et al., 2011; Liñán et al., 2008). However, the taxonomy of this group is highly complicated given the relatively simple cranidial shape and body pattern. Taxonomic studies of early ptychoparioid trilobites would be improved thanks to a compressive understanding of their morphological variation. These Cambrian trilobites have been assigned to the family Ptychopariidae Matthew, 1887, according to Fortey in Whittington et al. (1997) and were considered the plesiomorphic ancestral stock from which more derived trilobite clades arose (Whittington et al., 1997; Fortey, 2001). However, its general body pattern and relatively minor morphological variation make this group a challenge in the trilobite systematic. Further, there is no doubt that analyzing morphological variation in organisms of this, and similar groups, is always a complex task, as taphonomic factors, as mentioned above, may play a fundamental role in the generation of biases regarding the interpretation of morphological traits, therefore, is necessary to understand very well how taphonomy can distort fossils dimensions and shape, as well as the information- related limitations that the investigator has when describing morphological comparisons (Fletcher 2017). Researchers have done detailed studies regarding ptychoparioid cladistics (Fortey, 1990, 2001; Lochman et al., 1947; Rasetti, 1963; Webster, 2011; Whittington et al., 1997), and, even though there is still an immense amount of progress to be made, to describe and understand, with great precision, the morphological variation of this group, advances had allowed works like the present to be developed. Thus, the present paper proposes to address the assessment and comparison of the cranidial structure of two very well-known species from the Drumian Stage of Spain and the Czech Republic, Agraulos longicephalus Hicks, 1872, and Agraulos ceticephalus Barrande, 1846, respectively. This, by using landmark-based geometric morphometric procedures and statistical analyses, to compare the degree and patterns of interspecific morphological variation, in samples preserved in siliciclastic rocks. 2.2. Ptychopariid morphological features Figure 1: General morphological division of trilobites. Taken and modified from Gon III (2009). From a general morphological approach of the class Trilobita, the exoskeleton of this organisms is divided into three elements: cephalon, thorax, and pygidium, additionally, the lateral portions of these parts are called gena (cephalon), and the pleural region, composed of thorax and pygidium (Whittington, 1997). About this study, the morphological analyses are focused on the cephalon, as it is the only part that is preserved for both taxa in the studied record, and, is the most important feature for trilobite species classification (Gon III, 2009). For this reason, Figures 2, 3, and 4 show the four cephalic elements included in the seven typical morphologic features of ptychopariids, mentioned above. Figure 2 shows the outlines of the right cephalic borders (green curves) and the common glabellar tapering shape, Figure 3, the natant hypostome, and Figure 4 show the opisthoparian suture. Figure 2: Hypostome position from the lateral view of Agraulos ceticephalus Barrande, 1846, cephalon (Whittington, 1997). Figure 3: Typical morphological traits of