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Commun Biol
2022 Jan 10;51:14. doi: 10.1038/s42003-021-02971-9.
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Miniaturization during a Silurian environmental crisis generated the modern brittle star body plan.
Thuy B
,
Eriksson ME
,
Kutscher M
,
Lindgren J
,
Numberger-Thuy LD
,
Wright DF
.
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Pivotal anatomical innovations often seem to appear by chance when viewed through the lens of the fossil record. As a consequence, specific driving forces behind the origination of major organismal clades generally remain speculative. Here, we present a rare exception to this axiom by constraining the appearance of a diverse animal group (the living Ophiuroidea) to a single speciation event rather than hypothetical ancestors. Fossils belonging to a new pair of temporally consecutive species of brittle stars (Ophiopetagno paicei gen. et sp. nov. and Muldaster haakei gen. et sp. nov.) from the Silurian (444-419 Mya) of Sweden reveal a process of miniaturization that temporally coincides with a global extinction and environmental perturbation known as the Mulde Event. The reduction in size from O. paicei to M. haakei forced a structural simplification of the ophiuroid skeleton through ontogenetic retention of juvenile traits, thereby generating the modern brittle star bauplan.
Fig. 1. Arm plates of Ophiopetagno paicei and Muldaster haakei from the Silurian of Gotland, Sweden, in comparison to a living relative (Ophiopholis aculeata), and in the context of the local stratigraphic framework12 with localities sampled (Mulde T.: Mulde Tegelbruk), palaeoenvironmental events, and trends in average surface area of the lateral arm plates as an approximation for body size.Ophiopholis aculeata: lateral arm plate in external (a) and internal (b) views. Vertebra in lateral (c), ventral (d), and distal (e) views. Muldaster haakei: Specimen MnhnL OPH088 (all numerical codes in the figure legend refer to specimen numbers in the MnhnL collection), holotype of M. haakei. Lateral arm plate in external (f) and internal (g) views. Specimen OPH089. Vertebra in lateral (h), ventral (i), and distal (j) views. k Specimen OPH090. Arm segment in ventral view. Ophiopetagno paicei: Specimen OPH087. Lateral arm plate in external (l) and internal (m) views. Specimen OPH092. Vertebra in lateral (n), ventral (o), and distal (p) views. q Specimen OPH93. Lateral arm plate, detail of tentacle notch in internal view.
Fig. 2. Evolutionary tree of the Ophiuroidea to show the position of Ophiopetagno paicei and Muldaster haakei from the Silurian of Gotland, Sweden.Arrows indicate synapomorphies at the respective nodes. DAP dorsal arm plate, LAP lateral arm plate, VWC water vessel canal, Ceno. Cenozoic, Meso. Mesozoic. Gray lines indicate documented fossil range of taxa.
Fig. 3. Time-calibrated phylogeny from the tip-dating analysis with mean per-branch rates of discrete character evolution mapped to indicate low (blue colors) vs. high rates (red colors) of morphological evolution inferred from a model allowing each branch to have its own rate of change.The star indicates the branch with the highest rate in the tree (rate multiplier = 24.76), which links Ophiopetagno paicei and Muldaster haakei and occurs concomitant with the Mulde Event.
Fig. 4. Summary diagram of the Mulde Event (the “Big Crisis”11) in the Silurian of Gotland, Sweden.Occurrences of the ophiuroid species O. paicei and M. haakei and biotic events are plotted.
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