| 41. | Neoproterozoic and Ordovician sequences are also found at La Hoyada.
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| 42. | What you see are the Cambrian, Ordovician and Carboniferous sandstones and limestones.
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| 43. | Most of the life killed in the Ordovician extinction were primitive sea creatures.
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| 44. | Bryozoans have contributed to carbonate sedimentation in marine life since the Ordovician period.
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| 45. | Several groups of endobiotic symbionts appeared in the Ordovician.
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| 46. | Trilobites in the Ordovician were very different from their predecessors in the Cambrian.
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| 47. | A belt of Devonian-era shale divides the Mississippian and Ordovician formations.
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| 48. | This happened at the end of the Ordovician and during the early Silurian.
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| 49. | This also marks the end of the Ordovician period.
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| 50. | North of the Illinois River the bedrock surface is much older Ordovician sandstone.
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