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).

WHEN SPONGES LEAVE THEIR MARK: CLIONA PERFORATIONS IN THE FOSSIL RECORD

 

                   Pliocene bivalve Glycymeris sp Da Costa 1778 from Italy with small holes in the shell (Photo by Jordi Sanchez).

The small perforations you observe in some fossils, attributable to sponges of the genus Cliona, are the result of their bioerosion mechanism. These sponges are known as "boring sponges" due to their ability to excavate calcareous substrates, such as mollusk shells, coral skeletons, and limestone rocks, to create galleries and chambers where they live.

The boring process of Cliona is not carried out by a mechanical scraping action, as might be the case with some predatory mollusks. Instead, these sponges use a chemical and cellular method to dissolve the calcium carbonate of the substrate.

 Detail of the perforations (Photo by Jordi Sanchez).

The presence of Cliona sponge perforations in fossils is a well-documented phenomenon that provides valuable information about the paleoecology and evolution of these organisms. Here are some key points:

1. Bioerosive Activity:

• Sponges of the genus Cliona are known for their ability to bore into calcareous substrates, such as mollusk shells, corals, and rocks. This activity, called bioerosion, leaves characteristic traces in fossils.
• These fossil traces, known as ichnofossils, are classified under the ichnogenus Entobia. Entobia cretacea is a common ichnospecies that is attributed to Cliona sponge activity.

2. Fossil Record:

• Cliona perforations are found in the fossil record from the Mesozoic and cenozoic, indicating that this bioerosive activity has been common for millions of years.
• The presence of these perforations in fossils of different marine organisms provides evidence of the ecological interactions that occurred in the past.

 Miocene gastropod Melongena lainei Basterot 1825 with small holes (Photo by Jordi Sanchez).

3. Paleoecological Importance:

• The study of Cliona perforations in fossils allows paleontologists to reconstruct past marine ecosystems and understand the relationships between different organisms.
• The presence and distribution of these perforations can provide information about the abundance and diversity of Cliona sponges and their host organisms in different geological periods.

Additional Information:

• Cliona perforations can affect the preservation of fossils, as they weaken the structure of calcareous substrates.
• The study of Cliona perforations in fossils is an important tool for research in paleontology, geology, and marine biology.

THE MYTHOLOGY OF FOSSIL ECHINOIDS

 

 Insitu eocene echinoid sea urchin Schizaster leymeriei (Cotteau, 1851) from Barcelona (Catalonia) (Photo by Jordi Sànchez).

Fossil echinoids, commonly known as sea urchins, have a rich history intertwined with human culture and mythology. Here are some fascinating insights into their significance:

Fossil echinoids have been collected by humans for thousands of years, with evidence dating back to the Palaeolithic era. They have been found in various archaeological contexts, including burials and cremations, indicating their importance in ancient rituals and practices.

These fossils are steeped in folklore, often referred to by names such as "shepherd's crowns," "fairy loaves," and "thunderstones." Each name carries its own mythological significance, reflecting the beliefs of different communities. For instance, in some regions, fossil echinoids were believed to ward off evil spirits when placed on mantels.

In Iron Age and Romano-British contexts, fossil echinoids were sometimes included in cremation graves, suggesting they held spiritual or protective qualities for the deceased. The practice of blackening these fossils, akin to rituals observed in Denmark, further emphasizes their cultural importance.

Skeleton of female and child found in Bronze Age grave on Dunstable Downs with hundreds of fossil echinoids. From Smith (1894) (*).

Interestingly, some folklore suggests that fossil echinoids could predict weather changes. People believed that these fossils would "sweat" before rain, making them a natural barometer. This practice highlights the connection between nature and human observation in ancient times.

 

                   Cretaceous sea urchin Micraster coranguinum Leske, 1778 from Lleida (Catalonia) (Photo by Jordi Sànchez).

Fossil echinoids were also considered "lucky stones." In some traditions, finders would spit on the stone and throw it over their left shoulder for good luck, a practice that underscores the deep-rooted superstitions surrounding these fòssils.

The presence of fossil echinoids in medieval churches indicates a retention of pagan beliefs within Christian contexts, showcasing how ancient practices can persist and evolve over time.

 Nineteenth century window on south side of St Peter' s church, Linkenholt, framed by fossil echinoids (*)

In summary, fossil echinoids are not just geological curiosities; they are deeply embedded in the cultural and spiritual fabric, reflecting a long history of human interaction with the natural world. Their mythology continues to inspire curiosity and reverence today.

More info: 

(*) McNamara, K. J. (2007). Sheperds' crowns, fairy loaves and thunderstones: The mythology of fossil echinoids in England. In L. Piccardi & W. B. Masse (Eds.), Myth and Geology 1268 (Vol. 273, pp. 279-294). London, England: Geological Society of London.