Question

I think about 15 years ago I saw a new classification system that placed Plathyhelminthes worms with Annelids and Mollusks into a superphylum (or other systematic clade) Lophotrochozoa, while Nematode worms were grouped with Arthropods into a clade Ecdysozoa. This would suggest that Arthropods and Annelids did not have a common coelomate or pseudocoelomate ancestor (in the past believed to be a nematode), or that if they did, then flatworms evolved FROM the pseudocoelomate ancestor. What are the current views of the characteristics of the Protostomate progenitor? It looks like Roundworms may have diverged from a flatworm like ancestor, and then given rise to current roundworms and Arthropods, while Annelids and Mollusks may have arisen independently from a flatworm ancestor. It would violate something I was taught in evolution (as a loose rule) that you don't usually call features like a coelem homlogous if they arose independently which would be the case for the Mollusk/Annelid Coelem and the Arthropod coelem. Of course, it is possible since the genetic changes that allowed both coelom may have existed for some other purpose prior to divergence, but been activated by other similar evolutionary phenomenon, or if a gene became deactivate (for example, if a gene were deactivated in mammals making scales for instead of hair those scales would still bear homology with reptile scales.

Answer 1

Protostome classifications have been revised due to molecular data: Lophotrochozoa, with annelids and mollusks, and Ecdysozoa, with nematodes and arthropods, delineate separate evolutionary paths for these taxa. The ancestry of Protostomia, their body plans, and the concept of a common coelomate ancestor are under reconsideration, with the potential for convergent evolution influencing these relationships.

Step-by-step explanation:

The classification system of protostomes has undergone significant revisions due to molecular research. Despite historical groupings based on morphology, ecdysozoan clade now links unlikely partners: nematodes and arthropods, characterized by the molt of their cuticles. Conversely, Lophotrochozoa, including annelids and mollusks, showed a different evolutionary path with a common ancestor signified by trochophore larvae. Interestingly, Platyhelminthes, once considered close relatives of acoelomorphs, now branch separately suggesting a more complex evolutionary history of these taxa. The concept of a coelomate ancestor is thus challenged, pointing towards convergent evolution or genetic homology across various protostome lineages.

Unraveling the ancestral traits and relationships within Protostomia reveals that annelids and mollusks represent bilaterally symmetrical, schizocoelous eucoelomates, while ecdysozoans like arthropods present a distinct body plan with hardened exoskeletons requiring ecdysis. Meanwhile, the placement of acoelomorphs remains a topic of intense study, potentially shaking the foundations of bilaterian classification.

Overall, it illustrates that evolutionary relationships are complex and can demonstrate varied developmental pathways such as the independent evolution of coeloms in different protostome clades, reflecting the intricate nature of evolutionary processes.