August 29. The Last Trilobites

We haven’t talked about trilobites for a while. From their incredible abundance, and even dominance in Cambrian and Ordovician time, trilobites declined in both numbers and diversity. They were still really quite abundant and diverse in Silurian and Devonian time, but the extinction at the end of the Devonian had a pretty significant impact on them. The decline continued, until at the start of the Permian, there were only three families of trilobites in existence. Earlier in the Paleozoic, there were dozens of trilobite families. 

Permian trilobite from Kansas, about 1.5 cm long. See below for image credit.

The Permian trilobite families were divided into about 30 subdivisions at the genus level. They suffered two blows during the Permian – first, in the Middle Permian, about 266 million years ago, when more than half the living genera were wiped out. Combined with earlier losses, that meant that there were only five trilobite genera that survived until the end of the Permian, when they too were destroyed in the mass extinction that eliminated more than 90% of all the species on earth. We’ll talk about that extinction the day after tomorrow, when the Permian ends.

Trilobites survived in one form or another for almost 300 million years, making them one of the most successful animal groups in all of earth history. Their decline and ultimate extinction was probably the result of many factors, including competition from other kinds of organisms, more predatory organisms, climate change, and loss of habitat, the typical reasons any plant or animal goes extinct. There’s some speculation that poor and inconsistent molting, combined with increases in animals that could – and did – eat trilobites while they were in the vulnerable molting state contributed to their demise over a period of millions of years. But ultimately, I don’t think we have a really good handle on the reasons trilobites declined. Their final extinction, at the end of the Permian, is no surprise, since that event caused wholesale destruction. But their slow decline until that final blow probably represents a combination of various factors.

—Richard I. Gibson

The Last Trilobites

Nice drawing showing the expansion and decline of trilobites through the Paleozoic 

Poor molting style

Photo by Dwergenpaartje under Creative Commons license : proetid trilobite Ditomopyge decurtata from Permian of Kansas

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.

* * *

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.

April 5. More trilobites

Trilobites continued a remarkable diversification during the Silurian. Some, such as Staurocephalus murchisoni, had these big bulbous things like noses at the front of their bodies. It’s actually the glabella, which in turn is the central, axial portion of the cephalon, or head. The sketches here give you an idea how bizarre it looks. 

Staurocephalus murchisoni (left); Deiphon forbesi (right)

Many trilobites have a simple dome-like glabella, slightly raised above the side parts of the cephalon, or head, where the eyes are. The glabella apparently covered and contained some of the trilobite’s forward digestive organs – the stomach, which was above and forward of the mouth. Exactly how it worked isn’t clear, at least not to me after checking around – if any trilobite specialist hears this and can enlighten us, please contact me at rigibson@earthlink.net. But at a first pass, it seems that the glabella is more or less the stomach, or maybe part of the intestine. It’s been speculated that larger glabellas indicate more complex food sources, which some infer to mean that the trilobite in question might have been carnivorous – but that seems to be a bit of a stretch to me.

How and why did these bulbous glabellas evolve into knob-like protrusions extending away from the animal’s body? Beats me.

—Richard I. Gibson

See also:

Richard Fortey, Lifestyles of the trilobites (PDF)
The drawings above are from a 19th century textbook.

March 20. Ordovician trilobites

Ordovician trilobite

Are you familiar with pill bugs? Roll-up bugs? I haven’t seen one in decades – maybe they don’t live here in the arid west, or heaven forbid, maybe I’ve lost my curiosity at what might be under a rock. If that’s it I’ll have to remedy that. But I recall well as a child in Michigan finding these little gray multi-legged critters in the garden. If you touched them, they’d roll up into a tight little ball. Their soft underbellies were protected by their relatively hard carapaces.

Pill bugs are arthropods like insects, centipedes, and spiders, but they aren’t closely related to them. They are actually crustaceans, isopods, more closely related to shrimp and lobsters. And of course, trilobites were arthropods too, and they shared with pill bugs the ability to roll themselves up into a defensive posture.

Flexicalymene, enrolled

Trilobites may have actually peaked during the Cambrian Period, but they certainly participated in the Ordovician diversification, with lots of new species appearing. They seem to get a little fancier in their ornamentation and development of spines and they clearly became adept at enrolling. Trilobites could definitely roll themselves into tight defensive balls early in the Cambrian, but for some time it was thought that the earliest trilobites couldn’t do it. But in 2013, a team from the University of Cambridge described some olenellids – early trilobites – from about 510 or so million years ago that could and did enroll themselves. So this is not strictly an Ordovician trilobite thing. But some Ordovician trilobites, such as Felexicalymene from the Ordovician of Ohio, Kentucky, and Indiana, made themselves famous by doing it.

—Richard I. Gibson

Ordovician trilobite photo by Vassil, under GNU free documentation license. Flexicalymene photo by Steve Henderson, used by permission.

Further reading:
http://www.trilobites.info/enrollment.htm
http://www.sci-news.com/paleontology/science-enrollment-trilobites-01420.html
http://www.livescience.com/39920-trilobites-curled-like-pill-bugs.html