Morphology, phylogeny, and paleobiogeography of the late Paleozoic flagship radiolarian genus <i>Albaillella</i>

In the late Paleozoic to earliest Mesozoic, the radiolarian genus <i>Albaillella</i> was highly diverse, comprising over 100 species, with a great range of complex morphologies, and therefore has been hugely valuable in biostratigraphy. Despite this, there are no comprehensive and inclusive quantitative analyses of morphological macroevolution and phylogeny in the genus. With the collected dataset of all the well-documented late Paleozoic taxa of <i>Albaillella</i>, we use Hayashi's Quantification Theory II (HQT-II) multivariate statistical method to morphologically classify all <i>Albaillella</i>, and reveal four distinct clusters based on discrete morphological features. Parsimony phylogenetic analyses recover three distinct clades, which are roughly consistent with the morphological clustering classification. Reconstructing patterns of morphological evolution in <i>Albaillella</i> using ancestral state reconstruction highlights multiple noteworthy trends. There was a decreasing trend in the height ratio of the H-frame and shell in the genus. The symmetry of the H-frame decreased from the Carboniferous to the late Permian. In addition, the Early Triassic species <i>Al. aotearoa</i> shows evidence for proteromorphic retrogradation of apical cone curvature, which is simplified and straight, like in distantly related and older taxa. Using results from our morphological, phylogenetic, and a biogeographic network analyses, we also constrain and revise the biostratigraphic zonal taxa of <i>Albaillella</i>. This study develops an integrative quantitative framework to analyze both morphological macroevolution and phylogenetic relationships in the radiolarian genus <i>Albaillella</i>, and provides an important basis for supplementing the late Paleozoic radiolarian biostratigraphy.