Lament for the Passing of Trilobites

Research in my lab uses information on fossil morphology to address a variety of evolutionary and geological questions. We’re interested in how to use fossil shape to tell us neat things about the evolutionary process and about interesting episodes in Earth’s history. We run a wide range, from lab-based and literature-based work through to trotting around at 16,500 ft in the Himalaya! In particular, I’m interested in the appearance and disappearance of trilobites from the rock record.

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The Cambrian Explosion

540 million years ago, at the base of the Cambrian time period, there was an explosive diversification in the variety and abundance of animals living in the Earth’s oceans. This “Cambrian explosion” is one of the most exciting areas in paleontology, and has been the subject of much public attention. Some of the interest is the result of the discovery and redescription of exceptionally preserved “Burgess Shale-type” faunas, where the soft parts of animals are preserved along with their hard skeletons. While we’ve learned a tremendous amount from these unique windows of preservation, the exceptional nature of these deposits prevents us from examining the diversification of the soft-bodied fossil groups in detail, because we only see such fleeting glimpses of their evolutionary histories. This is where the “traditional” macrofossil record comes in. Trilobites have a special advantage in this regard because their resistant hard skeletons present us with a quality fossil record throughout most of Paleozoic time. This abundance of trilobite fossils allows us to dissect their evolutionary history in detail.

Close-up of a ten to fifty million year old trilobite fossil

Why Trilobites?

Trilobites are by far the most abundant of all skeletonized Cambrian metazoans. This fact reflects the volumetric and taxonomic abundance of trilobites in a wide range of Cambrian sediments, their intricate and labile morphology, and their occurrence throughout the majority of Cambrian time. These attributes have given the group unrivaled utility as fossils in Cambrian strata. Paradoxically, while trilobites serve as the timekeepers by which we gauge the evolution of other Cambrian metazoans, many aspects of the evolutionary radiation of trilobites remains poorly resolved.

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Genes or Ecology?

One of the abiding problems of the Cambrian explosion is to explain how such diversity was generated so quickly. Some have argued that the diversity was generated by “sloppy genes” in these primitive animals, which allowed an unparalleled riot of forms to appear in a short period of geological time. Others suggest that the diversification is a natural consequence of the empty ecological space available in the Cambrian. Because niches were previously unoccupied organisms were able to adapt quickly to exploit these new opportunities without having to out-compete incumbents. Research in my lab is testing these alternatives using a high-resolution field-based approach. It’s long been noted that Cambrian trilobites show considerable variation in their patterns of trunk segmentation when compared to the stability exhibited by most post-Cambrian trilobites. Recent studies in developmental biology indicate steriotyped patterns of segmentation and tagmosis (the evolution of body segments) among extant arthropods, so that temporal variability in segmentation patterns is likely to signify an important control on trilobite evolution. How did trilobite body patterning change during the evolutionary history of the group? Recent advances in the developmental genetics of extant arthropods are giving us a context in which to review the basic controls of trilobite growth.

I’ve been fascinated by trilobites since the beginning of my academic career. Many years ago when I did my Ph.D. defense I wrote and played the following song, partly to avoid any difficult questions at the end, but also as a humorous song about these fossils and as a celebration of a life in science.

By Nigel Hughes, University of California, Riverside.

Lament for the Passing of the Trilobites

Oh, look, what can this be,
swimming all over the Cambrian sea?
Is it Spriggina?, no, it cannot be,
some new and strange innovation maybe.

Come, hear the tale of the Trilobite days,
niches exploited in so many ways!
Where did they come from and where did they go?                          I’ve just spent three years and I still do not know.

Paradoxides is not what it seems,
and its more or less segments, who knows what that means?Trilobite workers in such consternation,
instraspecific phenotypic variation.

Phacopids with schiz’croal eyes
and they’ve more of less lenses, was that really wise?
Species selection it made the decision,
advantages reap’d from peripheral vision.

So why did thee marvellous creatures decline?
Was it meteorite impact or just too much wine?
Was it drying of oceans on Pangea’s shores?
Was it nat’ral progression of on of Cope’s laws?
Alas! It seems there’s no end to the debate
on the ultimate cause that determined their fate,
for trilobite workers’d be out of a job,
so if you discover, Just keep shut your gob!

Come, hear the tale of the Trilobite days,
niches exploited in so many ways!
Where did they come from and where did they go?
I’ll spend all my life and I’ll still never know!

Words and music by Nigel Hughes
Music transcribed by Adrian Rushton at Kyrshabakty Camp, Kazakhstan, 1990
Trilobite Song2 Trilobite Song3

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