EXPLORING THE EVOLUTION AND CLASSIFICATION OF SAND DOLLARS

 

A sample of the diversity of the sand dollars (Photo: Jordi Sanchez).

 

 Sand dollars, fascinating members of the Class Echinoidea, are currently at the center of significant scientific inquiry. Recent research (*) has led to a major revision in their classification, driven by emerging molecular phylogenetic evidence. Here are some key insights from the latest findings: 

 

• Revised Classifications: The relationships among families within the superorder Luminacea (Echinoidea: Irregularia) have been updated, reflecting a more accurate understanding of their evolutionary history. The study proposes three new superfamilies: Astriclypeoidea, Mellitoidea, and Taiwanasteroidea, which replace the previously recognized superfamily Scutelloidea.

 

Topology, classification, and phylogenetic signal for relationships between sand dollars, sea biscuits, and close relatives. (A) Traditional morphological hypothesis (Kroh & Smith (2010)) (B) Phylogenomic topology and classification (Mongiardino Koch et al. (2022)). (C) Phylogeny recently proposed by Lee et al., 2023. 

• Unresolved Family Placements: Despite advancements in understanding, many family placements remain unresolved. This is largely due to conflicting evidence from various sources, including morphological, paleontological, and genetic data.

 

 

 Some fossil sea urchins of the genus Encope, an echinoderm that belongs to the Mellitidae family (Photo by Jordi Sanchez).

 1️ Encope micropora Agassiz, 1841 from the pliocene of Baja California (Mexico).

 2️ Encope grandis Agassiz 1841 from the pliocene of Sonora (California-USA). 

3️ Encope micropora californica Verrill, 1870 from the pliocene of Baja California (Mexico).

 4️. Encope macrophora (Ravenel, 1843) from the miocene of Florida (USA).

 5️ Encope macrophora subsp. tamiamiensis Mansfield, 1932 from the pliocene of Charlotte (Florida - USA).

• Biogeographic Insights: The research highlights the tropical western Pacific and eastern Indian Oceans as crucial regions for the early diversification of sand dollars. These areas are considered the cradle of their evolutionary development, suggesting a rich history of adaptation and change.

 Echinodiscus bisperforatus Leske, 1778 from the miocene of Sukabumi (Java-Indonesia).

 

• Historical Context: The study also connects the diversification of sand dollars to hothouse conditions during the late Cretaceous and early Paleogene periods. This correlation indicates that environmental factors played a significant role in shaping the evolutionary trajectory of these organisms.
• Importance of Taiwan Fauna: The fauna of Taiwan is emphasized as a key player in understanding major Cenozoic migration and dispersal events in the evolutionary history of Luminacea. This highlights the importance of regional studies in piecing together the broader narrative of sand dollar evolution.

In conclusion, the ongoing research into sand dollars not only sheds light on their complex evolutionary history but also underscores the importance of molecular data in resolving long-standing classification issues. As scientists continue to explore these intriguing creatures, we can expect further revelations that will enhance our understanding of marine biodiversity.

More info:

Kroh, A. & Smith, A. B. The phylogeny and classification of post-Palaeozoic echinoids. J. Syst. Palaeontol. 8, 147–212 (2010).

(*) Lee, H. et al. Phylogeny, ancestral ranges and reclassification of sand dollars. Sci. Rep. 13, 10199 (2023).

Mongiardino Koch, N. et al. Phylogenomic analyses of echinoid diversification prompt a re-evaluation of their fossil record. eLife 11, e72460 (2022).

Mongiardino Koch, N. Embracing the taxonomic and topological stability of phylogenomics. Sci Rep 14, 4088 (2024).