DIRECTIONAL EVOLUTION IN THE HISTIODELLA LINEAGE

ALBANESI, G. L.; FELTES, N. A.

Valencia

Simposio; FOURTH INTERNATIONAL CONODONT SYMPOSIUM. ICOS IV; 2017

UNIVERSITY OF VALENCIA (SPAIN) IN COLLABORATION WITH: INSTITUTE OF GEOLOGY. ACADEMY OF SCIENCES (CZECH REPUBLIC), UNIVERSITY OF CAGLIARI (ITALY), UNIVERSITY OF GRAZ (AUSTRIA)

Several species of the Ordovician genus Histiodella have been used as index fossils (e.g., Sweet, 1984; Maletz, 2009; Stouge, 2012). This taxon is distributed through low to mid-high latitude latitudinal regions and characterizes deep platform or platform margin to slope depositional environments (Stouge, 2012).[But, many occurrences of Histiodella species are from shallow platform sites, e.g., Oklahoma (Bauer; McHargue); U.S. Appalachians; Durness Gp., Scotland (R.Raine)] Histiodella is particularly abundant in the Las Aguaditas creek stratigraphic section of the Central Precordillera, Argentina, where it presents a continuous record through the upper part of the San Juan Formation and the lower member of the Las Aguaditas Formation, Darriwilian in age (fi g. 1). The upper part of the San Juan Formation presents a high diversity and abundance of conodont elements (Feltes et al., 2016)[have not read this yet], including those of Histiodella. In the lower and middle parts of the lower member of the Las Aguaditas Formation the conodont diversity decreases sharply because of the fl ooding event that submerged the carbonate platform. The sea level change caused unfavourable conditions for the development of the conodont fauna in this geological setting. In the upper part of the lower member, the diversity recovers and Histiodella species reappears. In the study section, fi ve species of Histiodella were identifi ed following Ethington and Clark (1981), McHargue (1982), Stouge (1984) and Bauer (2010); i.e., H. sinuosa, H. serrata, H. holodentata, H. cf. holodentata, and H. kristinae, which share a certain ancestor-descendant relation according to the phylogenetic hypotheses of McHargue (1982) (from H. sinuosa to H. serrata), and Stouge (1984) (from H. holodentata to H. kristinae). Elements that occupies the Pa position in the apparatus of Histiodella are predominant in the collection; these are the most useful elements in the taxonomy to distinguish between different species of the genus. The aim of this work is to analyze the evolutionary transformation experienced by this taxon across the Yangtzeplacognathus crassus to the Eoplacognathus pseudoplanus zones. An elliptic Fourier analysis is carried out (Kuhl y Giardina 1982; Ferson et al., 1985), using the free access software SHAPE v.1.3 (Iwata and Ukai, 2002) to evaluate the morphometry of Histiodella specimens. The data is treated in PAST v.2.17 (Hammer et al., 2001) to perform a principal component analysis (Fig. 2) in order to represent the element distribution in the morphospace; also, a multivariate analysis of the variance is performed in order to consider if there exists significant difference between species. The outline of a specimen represents the inside shape of the element, linking the nadirs between denticles and the cusp (see Jones, 2006); thus, broken tips of denticles and cusp, do not modify the shape. Conodonts are characterized by con-tinuous growth, for that reason, the outline includes only the five denticles closer to the cusp in the anterior process, in order to reduce the variation of the element shape due to differences in the ontogenetic state of development. In the case of the Pa element of Histiodella it corresponds to the length of the anterior process by successive addition of denticles. We decided to take into account five denticles because each specimen of the study collection presents at least five denticles. Moreover, with this number of denticles the shape of the element is representative of the maximum height reached by the denticles of the anterior process, which is taxonomically relevant. A total of 206 Pa elements of Histiodella are analysed, which were recorded (or recovered) through 55 m thick of the stratigraphic column, i.e. the uppermost 12 m of the San Juan Formation and the whole lower member of the Las Aguaditas Formation (43m).The two first principal components are represented and explain 65.92% of the variance. The PC1 explains 43.49% of the change of the element form, and represents mostly a contrast in the element height. The spe- cimens with positive values in this component have a high anterior process, where the oral margin becomes nearly parallel to the base due to a widening of the distal part of the anterior process (the length/height ratio of the element tends to 1). Conversely, the elements with negative values present a thinner anterior process, and the element acquires an elongated shape. The PC2 explains 22.43% of the variance, and the most evident change through this component is the height of the distal part of the anterior process relative to the height of the cusp. Elements with positive values in the PC2 have a distal part of the anterior process lower than the cusp region and elements with negative values have the opposite measures.It is remarkable that the MANOVA does not reveal significant differences between H. cf. holodentata with H. sinuosa and H. serrata, due to the temporal and phylogenetic distance that separates them. On the other hand, it is not surprising that this method does not differentiate between H. sinuosa and H. serrata, since the distinctive characteristic of the latter is the beginning of the development of denticles in the posterior process, but preserving a similar inner outline. However, the 95% ellipse of H. sinuosa is located left in relation to that of H. serrata in the PC1 (Fig 2.). This topology means that elements of H. sinuosa are more elongated, or poses lower denticles in the anterior process, which is the principal variation in the shape as explained. The outline technique applied in our study results useful to detect continuous and subtle changes in the morphology of the elements. However, it implies the drawback of not considering the denticles shapes in the process, which is relevant in the evolution of this taxon. This may be the reason that explains the absence of differences between some species. It supports the hypothesis that the Histiodella lineage evolves anagene- tically, through continuous changes in shape, and the appearance of novel characters, which can continue its development in the descendant species. An evolutionary tendency of the denticles to surpass the height of the cusp is verified by this analysis. The scatter plot shows that the species limits are diffuse, while the elements are not grouped discreetly; which is supported statistically. However, the species are positioned in an order that coincides with the stratigraphic record and the phylogenetic hypothesis followed in this study; thus sup-porting the stratophenetic interpretations that were traditionally held for the Histiodella lineage. Finally, we suggest that H. holodentata descends from H. serrata, as supported by the PCA analysis.