Evolution of Articulated Coralline Algae
A small articulate growing atop a broken rock.
The evolutionary history of the articulates is not well understood. Although coralline algae matter is often preserved over time due to its calcium carbonate cell walls, no useful fossils have been discovered that might help sort out the origin of the geniculates. In addition, no genetic analyses have been conducted to discern relatedness amongst the corallines. However, based on developmental, vegetative anatomy, and conceptacular structure, hypotheses can be made about the paths that led to current coralline forms.
The common ancestor to all articulates was likely a crustose coralline or an early articulate that diverged from the crustose line. Whether or not articulates diverged from the crustoses once or multiple times is a difficult question to answer? Parsimony would suggest that the articulates are likely not good examples of convergent evolution. Indeed, flexible genicula, a key trait common to all articulates, and other shared traits suggest that origination of the articulated form probably only occurred or was successful once so far in phylogenesis.
Coralline algae have evolved to tolerate the turbulent coastal areas they inhabit.
Within the corallines, evolutionary trajectory is rather difficult to trace due to immense variation in general morphological characteristics. However, some have suggested the best way to evaluate the evolutionary scheme of the corallines is to look at conceptacle placement and structure. Johansen proposes the axial conceptacle to be the most primitive within one coralline subfamily. Lateral and marginal conceptacles are likely to be more recently evolved. Even with conceptacle analysis, it still is very difficult to discern an overall scheme for articulate evolution. Further research into the origin of algae and corallines, in particular, could come up with some interesting and perhaps surprising results.
© 2001 Ian Ehrenreich. All rights reserved.