The missing fossils argument; all gaps are real (Introduction)

by David Turell , Tuesday, August 29, 2023, 14:46 (246 days ago) @ David Turell

New discoveries in a known Cambrian animal:

https://www.cell.com/current-biology/fulltext/S0960-9822(23)01065-5?dgcid=raven_jbs_aip...

"Highlights:

"CT imaging clarifies head structure in the Cambrian stem-group euarthropod Kylinxia

"Kylinxia has three eyes and four pairs of biramous head limbs

"The six-segmented head of living arthropods dates to shared ancestry with Kylinixia

"Phylogeny supports homology of frontal appendages across stem-group arthropods (my bold)

"The early Cambrian Kylinxia zhangi occupies a pivotal position in arthropod evolution, branching from the euarthropod stem lineage between radiodonts (Anomalocaris and relatives) and “great-appendage” arthropods. Its combination of appendage and exoskeletal features is viewed as uniquely bridging the morphologies of so-called “lower” and “upper” stem-group euarthropods. Microtomographic study of new specimens of Kylinxia refines and corrects previous interpretation of head structures in this species. Phylogenetic analyses incorporating new data reinforce the placement of Kylinxia in the euarthropod stem group but support new hypotheses of head evolution. The head of Kylinxia is composed of six segments, as in extant mandibulates, e.g., insects. In Kylinxia, these are an anterior sclerite associated with an unpaired median eye and paired lateral eyes (thus three rather than five eyes as was previously described ), deutocerebral frontal-most appendages, and four pairs of biramous appendages (rather than two pairs of uniramous appendages). Phylogenetic trees suggest that a six-segmented head in the euarthropod crown group was already acquired by a common ancestor with Kylinxia. The segmental alignment and homology of spinose frontal-most appendages between radiodonts and upper stem-group euarthropods is bolstered by morphological similarities and inferred phylogenetic continuity between Kylinxia and other stem-group euarthropods."[/b] (my bold)

Comment: An early stem group is further described with three eyes. Such complexity with no precursors is typical of the Cambrian Explosion.