May 18. Devonian trilobites

Trilobites – again? Well, trilobites are cool and each period seems to have some that are pretty distinctive. For the Devonian, at least in the United States, I’d say it’s the genus Phacops. There are at least 26 species of Phacops, and like all trilobites, they’re all extinct. They had large heads and bulbous glabellas – that’s the nose-like section in the middle of the head. And they had really large compound eyes. 

Phacops

The eyes of some Phacops species are sort of like frogs’ eyes, and one common species, Phacops rana, takes its name from a large group of frogs. Rana means frog in Latin. The eyes were mounted on little turrets and stood above the basic level of the head, so Phacops probably had pretty close to a complete 360-degree range of vision. They likely lived in muddy sea floors, so it’s possible that their eyes evolved to serve in a sometimes murky environment.

Phacops rana grew to as much as 6 inches long, but a lot of specimens are rolled up like pill bugs. Enrolling was probably a defense mechanism. Phacops rana is pretty much Middle Devonian in age, dating to around 385 to 400 million years ago.

Fossils of Phacops rana are found in Devonian rocks of the US Midwest, in the northeastern states, and adjacent parts of Canada. It’s the state fossil of Pennsylvania.  If you recall that northwestern Africa was getting closer and closer to the northeastern United States during the Devonian, it may come as no surprise that Phacops rana is also abundant in Morocco. A word of warning however – if you are interested in collecting trilobite fossils, be aware that there are a lot of very well-done fakes coming out of Morocco. Not just Phacops, but lots of different trilobites including delicate spiny trilobites, mostly from the Devonian. They make casts in resin of one original and attach it to natural rock. If you don’t really care if it’s real, and want something cool and decorative, $10 or $20 is a reasonable price to pay. Just be wary before you lay out $200 or $500 or more for something that has been mass produced – and not by nature 400 million years ago, but by humans within the past decade or so. If you are planning to invest in collectable trilobites, be sure to check around for information about how to identify fakes. Some of them are really well done.

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Today, May 18, is the anniversary of the eruption of Mt. St. Helens in Washington State in 1980. It was the first eruption of a volcano in the 48 United States since Lassen Peak in 1915 to 1917. 57 people were killed in Mt. St. Helens’ eruption, the deadliest volcanic eruption in U.S. history. Mt. St. Helens and the Cascade Range are part of the volcanic arc related to the subduction of part of the Pacific Oceanic Plate – the part called the Farallon Plate. It’s still heading down under North America, and the small remnants of the Farallon Plate beneath the ocean west of northern California, Oregon, and Washington, are called the Juan de Fuca and Gorda Plates.

—Richard I. Gibson

Photo by Didier Descouens under Creative Commons Attribution License.