Agnostus pisiformis ~ Mystery of the Commonplace

Yet another instance in which I think I've been paying attention and listening, only to discover I've missed nearly all of the conversation.

I’ve added Agnostus pisiformis to my fossil collection, a long standing objective.  Though this arthropod species is very ancient, dating principally from the latter half of the Cambrian Period (about 500 million years ago), it’s not rare.  Indeed, it’s well-known, rather commonplace, and because it existed worldwide in such numbers for a relatively limited period of time it serves as an important Cambrian index fossil.  Further, this tiny creature and its other brethren that belong to the order Agnostida have been studied for years.

My latest acquisition comes from the Alum Shale Formation of the Kinnekulle plateau in Sweden.  In the midst of this formation’s shale are extremely fossiliferous (for once, this adjective actually applies) limestone nodules commonly referred to as “orsten.”  Shown below is my piece of that limestone, covered in myriad fossils and looking like solidified bubble wrap with some of the bubbles popped.

Peering more closely at this matrix reveals the broken parts of the exoskeletons of many A. pisiformis in a mixture of convex pieces (this is the side of their shells that the creatures exposed fully to the world) and concave pieces (the underside of the shells which, in life, harbored the animals’ soft parts).

The fragmented nature of these fossils doesn’t make it clear what A. pisiformis looked like in its entirety.  The drawing below is of the genus Agnostus (Agnostidae is the superfamily of which Agnostus is a member).  It illustrates the complete dorsal view of the creature’s exoskeleton (anterior end is at the top).  Drawn by Dr. Samuel M. Gon, III, and reproduced with his permission, it appears on his marvelous website A Guide to the Orders of Trilobites.

Until these fossils came into my possession, my understanding of A. pisiformis was based largely on two sources –  paleontologist Raymond C. Moore’s classic treatise (and one of my favorites) titled Invertebrate Fossils (1952), and Richard Fortey’s exemplar of popular science writing Trilobite:  Eyewitness to Evolution (2000).  Moore sums up the order Agnostida as follows:  “The agnostids are specialized blind trilobites which stand far removed from others and are not known to have left any descendants.”  (p. 497)  Fortey describes A. pisiformis as “little blind trilobites,  . . . strange miniatures, specialized, sophisticated, and so successful that when there was plenty of plankton on which to feed they must have turned the late Cambrian . . . seas dark with their numbers.”  (p. 75)

So, that was what I knew – this species was a tiny, blind, Cambrian trilobite.

With the acquisition of this small portion of limestone, it was not surprising that I took a new look at the research on A. pisiformis and the Agnostida.  Sadly, it didn’t take long to realize that much of what little I thought I knew about this animal was in error and what I had to learn has been known and debated for years.  In fact, I had to grapple with that most fundamental question about these animals, a long-standing one for paleontology which still poses challenges:

What, in fact, is the A. pisiformis and its agnostid kin?  Trilobites?  Crustaceans?  None of the above?  Closely related to both?

That many scientists placed them among the trilobites is perhaps not too surprising.  Consider the prototypical trilobite which features a cephalon (its anterior shield), a thorax, and a pygidium (posterior shield).  Pictured immediately below is an Elrathia kingii (this is a Cambrian Period trilobite from the Wheeler Shale Formation, Utah); the next picture identifies the three parts of a trilobite just identified.  The length of this specimen is 27 millimeters or just over an inch.

As noted in a previous post on this blog, these three divisions from the anterior to the posterior of the organism are not the source of its common name.  Rather, the tripartite division described by the name trilobite applies to the three lobes that separates the organism from one side to the other as shown in the picture below of the same E. kingii specimen.

A. pisiformis and the whole Agnostida order share those basic trilobite attributes.  As the drawing presented earlier shows, these animals, like the prototypical trilobite, feature a cephalon, thorax, and a pygidium.  But one initial attribute that distinguishes them from other trilobites is their minute size.  Further, there’s the limited nature of their thorax which consists of just two segments; other trilobites have more than that, often significantly more.  In addition, as is clear from the drawing of Agnostus, the cephalon and pygidium are nearly identical in shape and size, apparently not a common trilobite trait.  (This also makes identification of disarticulated pieces quite difficult.)  Even some of those who classified the agnostids as trilobites, such as Moore, recognized that they stood apart.  How far they didn’t know.

Over the past several decades, paleontologists have been able to extract well-preserved specimens, primarily arthropods, from orsten nodules.  The soft parts of these small specimens are phosphatized (with calcium phosphate) and, thus, when specimens are teased out of the limestone, they offer an unparalleled view of these creatures.  (See, for example, Andreas Mass, et al., The ‘Orsten’ – More Than a Cambrian Konservate-Lagerstätte Yielding Exceptional Preservation, Paleoworld, Volume 15, 2006.)  Paleontologists Klaus J. Müller and Dieter Walossek pioneered the work to treat the orsten with acetic acid, leaving the phosphatized specimens untouched.  Their treatise on the Agnostus pisiformis presents a rich and compelling description of its life style and offers an array of photographs of its “soft parts," principally its appendages.  (Morphology, Ontogeny, and Life Habit of Agnostus pisiformis from the Upper Cambrian of Sweden, Fossils and Strata, 1987.)  By the way, the white material covering some of the fossils in my piece of limestone seen in the photos above is calcium phosphate.

Greater knowledge of the agnostids’ soft parts offered support to the hypothesis that the agnostids, and A. pisiformis among them, were quite possibly not trilobites.  In a recent article reviewing in broad strokes what is known about A. pisiformis, paleontologist Mats E. Eriksson and artist Esben Horn offer the following summation of where things currently stand with this taxonomic issue (Agnostus pisiformis – A Half a Billion-Year Old Pea-shaped Enigma, Earth-Science Reviews, volume 173, 2017):

For a long time, agnostids were regarded as trilobites.  However, the “soft tissue” (appendage) structures discovered from the “Orsten” Lagerstätte rather suggested a crustacean (sensu lato) affinity . . . .  Although this conclusion was drawn from empirical evidence from the fossil record and was based on uniquely well-preserved material, contrasting opinions with regard to the evolutionary relationships between polymerid trilobites and agnostoids do still persist . . . .  However, . . . , a sister group relationship of the Agnostina and Crustacea does not exclude a close relationship to the Trilobita.  (p. 69)

(In the quotation above, citations to references have been omitted.  Sensu lato means “in the broad sense.”  Polymerid trilobites are the lion’s share of all trilobites; they have more than two thoracic segments and pygidia that are typically smaller than their cephalons.  Agnostina is a suborder of the Agnostida order.)

Hmmm, this review by Eriksson and Horn leaves things a bit in limbo, I’m afraid.  Nevertheless, it's enough to prompt me to strike "trilobite" from what I thought I understood about A. pisiformis,  It was probably not a trilobite, though presumably closely related.  Eriksson and Horn describe it as "trilobite-like."

What about the rest of my so-called knowledge of this animal?  Well, that it was alive in the Cambrian (and into the Ordovician) isn’t being debated and clearly it was very, very small.

Blind?

At least as early as three decades ago, Müller and Walossek reported that, though there was no dorsal evidence of eyes on A. pisiformis, among the ventral soft parts were large lobes that might have acted to provide the animal some form of sight when its shell was open.  This idea of sight in these animals remains quite viable though perhaps no consensus has been reached.  Most recently Eriksson and Horn write that these prominent ventral structures were “probably light sensitive.”  “Thus, it was probably able to see while the shields were gaping, though not as well as many polymerid trilobites with their dorsal compound eyes.”  (p. 69)

Through the work of Eriksson and Horn, A. pisiformis has truly become a "living fossil."  They are responsible for the building of large scale, complex models of A. pisiformis based on the detailed knowledge they now have the animal’s soft parts.  Horn is a principal at 10 Tons, a studio that specializes in making detailed models of zoological and botanical specimens including those from deep time.  With Eriksson’s supervision, 10 Tons has created fabulous models of A. pisiformis.  Of note, the ventral eye structures occupy a prominent place in these models amid many appendages and other soft parts.  That they gave the animal some semblance of sight seems to me to be entirely reasonable.  So, I will strike blindness, as well, from what I thought I knew.

Eriksson and Horn are on an educational mission.  As they write in their research article, they are “breathing life” into this ancient creature in order launch an educational effort, including a traveling exhibit, to expose the public to life in the Cambrian.  Part of their agenda is to show “that paleontology is not just about dinosaurs and that the considerably older ecosystems of the Cambrian also hosted quite remarkable organisms, when animal communities first dominated the seas.”  (p. 74)

In the end, it’s entirely appropriate that some basic questions about A. pisiformis and the rest of the agnostid taxon persist and that, it turned out, I knew less than I thought I did about this commonplace fossil.  Indeed this species’ scientific name perfectly captures that idea of endless mystery.  Agnostus is based on the Greek agnoi which means “ignorance.”  Pisiformis is the juxtaposition of Greek and Latin roots:  pisi means “pea” and form means “shape.”  To my mind, Eriksson and Horn have come up with the best translation of the name:  pea-shaped enigma.