Molecular paleobiology of early-branching animals: integrating DNA and fossils elucidates the evolutionary history of hexactinellid sponges

Abstract

Abstract.-Reconciliation of paleontological and molecular phylogenetic evidence holds great promise for a better understanding of the temporal succession of cladogenesis and character evolution, especially for taxa with a fragmentary fossil record and uncertain classification. In zoology, studies of this kind have largely been restricted to Bilateria. Hexactinellids (glass sponges) readily lend themselves to test such an approach for early-branching (non-bilaterian) animals: they have a long and rich fossil record, but for certain taxa paleontological evidence is still scarce or ambiguous. Furthermore, there is a lack of consensus for taxonomic interpretations, and discrepancies exist between neontological and paleontological classification systems. Using conservative fossil calibration constraints and the largest molecular phylogenetic data set assembled for this group, we infer divergence times of crown-group Hexactinellida in a Bayesian relaxed molecular clock framework. With some notable exceptions, our results are largely congruent with interpretations of the hexactinellid fossil record, but also indicate long periods of undocumented evolution for several groups. This study illustrates the potential of an integrated molecular/paleobiological approach to reconstructing the evolution of challenging groups of organisms.

Ordovician sponge spicules from Spitsbergen, Nevada and Newfoundland: new evidence for hexactinellid and demosponge early diversification

“…Spicules and microscleres interpreted as sceptrules have been reported from Late Cambrian and Ordovician strata (in Dohrmann et al 2013). However, their poor preservation and the gap between the next appearances of sceptrules in the Triassic (Donofrio 1991;Krainer & Mostler 1992) raise doubts about the homology of the Palaeozoic and Mesozoic-Recent forms (Dohrmann et al 2013).…”

“…Modern Hexasterophora are restricted to hexaster bearing sponges; when microcleres are absent in fossil forms the hexasterophora is restricted to skeletons of Lyssacine or Dictyonine types (Reid 2004). The earliest unambiguous evidence for crowngroup Hexasterophora is the occurrence of dictyonal frameworks, rigid skeletons produced by fusion of hexactine megascleres (in Dohrmann et al 2013). For this reason, any record of isolated hexaster-type microcleres is important and even more relevant if it is associated with megascleres of a sponge body fossil.…”

“…However, their poor preservation and the gap between the next appearances of sceptrules in the Triassic (Donofrio 1991;Krainer & Mostler 1992) raise doubts about the homology of the Palaeozoic and Mesozoic-Recent forms (Dohrmann et al 2013).…”

“…The latest hexactinellid phylogenetic analysis using molecular and fossil evidence (Dohrmann et al 2013) points to an early separation of Hexasterophora and Amphidiscophora in the Cambrian. The age of crowngroup Sceptrulophora was constrained to be at least 237 Ma, according to the earliest unambiguous finds of sceptrules in the Middle Triassic (Krainer & Mostler 1992) or Late Devonian (dictyonal frameworks assigned to extant sceptrulophoran families by Rigby et al 2001).…”

Fossil and Transcriptomic Perspectives on the Origins and Success of Metazoan Multicellularity

“…The earliest fossils that most likely represent metazoans are indeed possible sponges, that are found in rocks deposited earlier than the Ediacaran Period (c. 630-542 Ma), perhaps significantly before 635 Ma (Maloof et al 2010), while biomarkers that characterize demosponges today also appear earlier than 635 Ma (Love et al 2009). Hexactinellids seem to be closely allied with and perhaps basal to demosponges, but most early records of their spicules are suspect, and their crown groups may have diversified as late as the early Cambrian (Dohrmann et al 2013). It has been postulated that the appearance of large sponge populations filtering the water column influenced the oxygenation of the oceans ) and could have played an important role in the eventual appearance of larger, active eumetazoans.…”