アメリカ自然史博物館のインスタグラム(amnh) - 12月14日 03時15分

It's time for #TrilobiteTuesday! It's relatively easy to confuse a ventral trilobite specimen with a “negative” example. They may initially look the same, but under closer inspection the differences between the two quickly become apparent. Negative, or counterpart, trilobites are exactly that… the reverse impression in the rock caused by the actual trilobite. In contrast, ventral trilobites, such as this Asaphus kowalewski from the Ordovician rocks of Russia, are complete examples of the actual fossil that have been preserved and subsequently prepared from the reverse, or bottom side. It has been speculated by some scientists that trilobites, much like modern horseshoe crabs, actually swam upside down. Thus the discovery of various specimens “flipped” within the sedimentary bedding plane isn't particularly surprising. When carefully prepared, these ventral examples can often showcase internal muscle attachment “hooks” and scars along with occasional appendage remnants.

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