Wednesday, September 1, 2021

Geology and Paleontology of Marrella splendens of the Middle Cambrian Burgess Shale Formation of British Columbia: Part I - Its Fortuitous Discovery and Eminent Discoverer

“The animals of the Burgess Shale are holy objects... 
We do not place them on pedestals and worship from afar. 
We climb mountains and dynamite hillsides to find them. 
We quarry them, split them, carve them, draw them, 
and dissect them, struggling to wrest their secrets. 
We vilify and curse them for their damnable intransigence. 
They are grubby little creatures of a sea floor 530 million years old, 
but we greet them with awe because they are the Old Ones,
and they are trying to tell us something.“

From Wonderful Life by Stephen Jay Gould, 1989

Every geologist and paleontologist has a favorite locale, stratum, rock or fossil that for them possesses a captivating and informative story about provenance, evolution and significance. In that regard, I offer one from my personal collection - the fossil of an ancient, extinct marine arthropod and iconic member of the Burgess Shale biota. From the ancestral shores of tropical western North America, it is entombed in blackish Burgess Shale from Walcott Quarry, located high in the southern Canadian Rockies of British Columbia. 

It's one of the most important sites for the exceptional preservation of uniquely soft-body tissues and organisms of a diverse Middle Cambrian paleo-community that thrived in the immediate aftermath of the biological Cambrian Explosion. The some 20-25 million year interval, considered short on the geologic timescale, is when motile and biomineralized multicellular animals made their first uncontested appearance and when nearly all major phyla 'suddenly' appeared in the fossil record.

Half a Billion Year Old Fossil of Marrella Splendens
With its distinctive cephalic array of spines and antennae and paired appendages that emanate from its thoracic flanks, the primitive marine arthropod is exceptionally well preserved in glossy, blackish Burgess Shale with both soft and hard body parts. The dark stain at the animal's posterior is thought to be either extruded biotic fluids, waste matter or blood-like hemolymph and related to the Burgess Shale biota's particular taphonomy.


During the period of COVID pandemic-restricted I travel, I decided to turn my blogging attention to various notable specimens in my personal collection, a convenient alternative. As a result, this post is the first of four that deal with the geology, paleontology, taxonomy, mode of preservation, discovery and discoverer of Marrella splendens of the Burgess Shale biota.

Italicized items are defined, and select names and locations are in boldface where first mentioned. Compass directions are in terms of the Cambrian unless stated otherwise. "Middle Cambrian of the formerly tripartite-divided Cambrian System that was abandoned in part due to the addition of a thick Cambrian succession of strata below the first appearance of trilobites. It roughly corresponds to the newly designated Miaolingian Series of Cambrian Stage 3. Lastly, GPS coordinates are provided for you to "Go there."

Astounding Detail of a Half-Billion Year Old Fossil of Marrella Spendens
Rapid entombment preserved specimens with extraordinary anatomical detail and with layered three-dimensionality in various captured postures such as oblique rather than strictly parallel to bedding planes. As a result, fossils could literally be dissected in layers under a stereo microscope. 


The discovery of Marrella and the Burgess Shale marine assemblage has revolutionized our understanding of evolution and how early animals experimented with different strategies to adapt to their environment. It has given us greater insight into the tectonic events and geo-chemical processes responsible for its formation and preservation.

And yet, after over 100 years since the discovery, many questions remain unanswered fully or with consensus. Here are a few of the more cogent ones that will be addressed over the next few posts.

• What is the taxonomic assignment of Marrella?
• What is the phylogenetic relationship of Marrella to its descendants and the biota to earlier metazoans of the Cambrian Explosion and the biotas of the latest Proterozoic?
• Why are many members of the Burgess Shale biota so difficult to classify in spite of the wealth of fossils at Walcott Quarry?
• What was Marrella's lifestyle and ecological relationship to the paleo-community?
• What is the evolutionary significance of the biota?
• Considering the diversity of the biota and sheer numbers of taxonomic orders, how did each species in competition manage to survive and propagate?
• How did the biota arrive so far inland and so dramatically elevated from their original marine habitat, and how and when did temporally-related Burgess Shale type-deposits arrive at so many seemingly disparate localities worldwide?
• How did the fossils became concentrated within such a narrow time interval at Walcott Quarry and other Burgess-type localities?
• What was the process that conferred such a high level of tissue preservation?
• Were the deposits preserved in situ or were they transported en masse?
• Why are there more exceptionally preserved deposits in the Cambrian than in the entirety of the remaining Phanerozoic?
• Why did multicellular life appear so late with no simpler precursors in the fossil record? 
• What was Walcott's explanation for the agonizingly fossil-depauperate Precambrian, and what did Darwin have to say about the "dilemma"? 
• Was the Cambrian Explosion of biological life one of animals or merely of fossils?
• Molecular clock predictions indicate that metazoans were plentiful in the Neoproterozoic. Is it possible to reconcile this assumption with what is found in the fossil record?
• What were the triggers and environmental changes that facilitated the biological revolution?
• What big picture views are there regarding the early course of evolution?
• And on a grand tectonic scale, did the fragmentation of supercontinent Rodinia at the end of the Proterozoic somehow trigger the Cambrian Explosion?
• Does the sudden explosion of the fossil record during the Cambrian represent a preservational bias or is it an indication of a massive environmental trigger for the biota's emergence? 
• Is the Cambrian Explosion a genuinely real evolutionary event or is it merely a short-lived manifestation in the fossil record of an evolutionary process that initiated well before?     

Delicate, Elegant and Feathery Marrella Splendens
From the Dorling Kindersley website


The 505 million year old marine arthropod is exceptionally well preserved within fine-grained sedimentary rock of the Burgess Shale Formation of British Columbia, Canada. It is thought to have been a bottom or near-bottom scavenger that lived on and just above the shallow continental slope (~100 m deep) of now-western Laurentia, the stable cratonic core of North America. 

Youngest known specimens are middle Middle Cambrian in age and oldest are from above the Lower-Middle Cambrian of southwest China. Thus, the range is relatively short, about 5 million years, slightly longer than that of T-Rex. Known immediate descendants of class Marrellomorpha persisted through the Devonian but possibly longer as new paleo-sites are discovered. 

Camera Lucida Drawing of Marrella Splendens in Dorsal View
Photographs taken under angulated UV light and diagrams created with a camera lucida were used during the restudy of the Burgess Shale biota in the mid-19??. It revealed intricate morphological detail and the gills of soft-body appendages, intestines (i) and 'heart' (h) in the cephalic region with blood vessels that course through the abdomen and branch into the appendages. From Garcia-Bellido, 2006.


Less than one inch in length, based on fossil evidence it was the most abundant member (~37.4%) and extinct icon of the world famous, shallow-marine paleo-community called the Burgess Shale biotaWith the exception of a heavily debated chordate (a pre-vertebrate animal) and a number of difficult to "problematic" members (challenging to categorize), it was a diverse, arthropod-dominated, "Middle" Cambrian faunal assemblage of more than 150 species. 

"Marrella splendens overwhelms anything else in the Burgess by sheer abundance."
Paleontologist and author Stephen Jay Gould in Wonderful Life

They thrived at the base of the Cathedral Escarpment, a ~400 m-high, interpreted "submarine cliff" of limestone that ran along the edge of the paleo-continent. Had it not been for the unique taphonomy (circumstance of fossilization) within the Burgess Shale deposit, entire soft-bodied organisms (that represent 98% of the Burgess organisms and 85% of genera) and certain body parts (such as Marrella's appendages, gills and circulatory and digestive viscera) would stand virtually no chance of preservation. 

As a result, the site and its entrained biota provide important clues to the early evolution of animal lineages, diversity, and early paleo-communities and lifestyles. 

Artist Depiction of the Burgess Shale Biota of the Middle Cambrian
From Marrella Science News for Students by Ken Doud


The youngest of four children and born in 1850 near the small town of Utica in Central New York State, Charles Doolittle Walcott (1850-1927) never finished high school. At a young age, he was captivated by and collected marine fossils of trilobites and brachiopods from the region's ubiquitous successions of sandstone, shale and limestone that, being from nearby Syracuse, was also my youthful source of geological and paleontological inspiration. 

Charles Doolittle Walcott in c.1908
From Wikimedia Commons 

By the age of 20, farmer-landowner William Rust and Charles established the small Walcott-Rust Quarry deep in the woods along a brook in 1870. It turned out to be the single richest and most varied source of trilobites in the limy muds of New York's Trenton Group of Ordovician age. 

Many of the specimens were enrolled (curved into a defensive ball like a pillbug), completely articulated with all arthropodal body segments and appendages intact and with an astounding level of soft tissue preservation. 

Seafloor Invertebrate Marine 'Hash' from Walcott-Rust Quarry
The Ordovician assemblage includes 18 species of trilobites with many exquisitely preserved, first-time appendages, and a diverse array of disarticulated brachiopods, gastropods, crinoids, corals and bryozoans. The presence of intact but fragmented columnar stalks and single crinoidal ossicles in the absence of holdfasts (attachment seafloor structures) and dissociated trilobite molts implies a turbulent, rapid burial event such as occurs during a severe storm, debris flow or tectonic depression of a foreland basin.

Separated by some 45 million years of origin and some 3,200 km (~1,990 mi), both Walcott-Rust Quarry, that he worked at the beginning of his career and Walcott Quarry in British Columbia that he worked at the end, are similar in geologic provenance, fossil-preserving processes and diversity of the invertebrate marine constituents. 

Walcott's insightful descriptions and publications of the strata's diverse invertebrate marine assemblage and previously unknown, well preserved trilobite appendages brought him scientific notoriety that is lauded to this day. 

Walcott-Rust Quarry in Central New York State
Nearly lost in the woods along a stream, the "Pit", as it's affectionately called by quarry excavator and trilobite aficionado Dan Cooper, was reopened adjacent to the original quarry. I had the honor and privilege of working the quarry with Dan and his fossil-hunting family, the subject of a future post.

Recently rediscovered and reworked by amateur paleontologist William Whiteley in the 1990s, the quarry's stratum came from the shores of Laurentia, the ancient, stable Proterozoic cratonic core of North America. As we shall see with greater geo-tectonic detail in a later post, Walcott Quarry originated on the western shores of the paleo-continent in the Cambrian.

In 1873, young Walcott sold the entire fossil collection to Louis Agassiz of Harvard's Museum of Comparative Zoology, who encouraged him to pursue a career in paleontology. It turned out to be good advice for Walcott and the world. The experience would propel Walcott on a lifelong trajectory of fossil exploration that took him from discoveries in New York State and nearby Vermont to the American Southwest and Canadian Rockies.

Positive and Negative Casts of Isotelus Gigas in Walcott-Rust Quarry
The four most common trilobites are Ceraurus pleurexanthemus, Flexicalymene senaria, Isotelus gigas and Meadowtownella trentonensis. It's the richest and most varied source of trilobites in the Trenton Group and perhaps the entire suite of New York's Paleozoic rocks.


Without formal education, he became an astute field geologist and the eminent paleontological and stratigraphic authority on early Paleozoic invertebrates and Cambrian trilobites in particular. He not only found the time to author hundreds of papers but travel and deliver countless lectures and convene conferences on the subject. 

At various times throughout his life, he served as director of the US Geological Survey, secretary of the Carnegie and Smithsonian Institutions and president of the American Association for the Advancement of Science, and the National Academy of Sciences. He was on the board of the National Parks Association, was science advisor to President Theodore Roosevelt and the recipient of countless awards, certificates, medals and honorary degrees in the US and Europe.

Charles Walcott strikes a pose in Walcott Quarry
To this day, the tools of the trade include shovels, chisels, wedges, pick axes and long iron bars (pictured). On occasion, dynamite was also used. Excavated slabs await lowering by rope down to camp where they were split, reinvestigated and sent by horse and then rail to the Smithsonian. Image from Royal Ontario Museum website 

Tireless and ever curious, his explorations took him to the Colorado Plateau, Grand Canyon and eventually to a remote ridge high in the Canadian Rockies of the province of British Columbia. Geo-aficionados will know Walcott penned many of the canyon's iconic features such as the Butte fault that marks the eastern edge of the Kaibab Upwarp

He was essentially the first geologist to challenge John Wesley Powell's antecedence theory regarding the genetic river-evolution of the Grand Canyon. You can read about it in renown geologist and author Wayne Ranney's Carving Grand Canyon book here


By accident (so often the case) while on horseback, as the legend goes, but essentially what he had been searching for his entire life, Walcott discovered a Middle Cambrian fossil of Marrella and its shallow marine cohorts on the steep, west-facing slopes of Fossil Ridge

North-Facing View of Mount Wapta from the Crest of Fossil Ridge
Rich in Burgess Shale genera of worms, sponges, arthropods and microbial stromatolites, the Eldon Formation forms the crest of the ridge. It's a thick sequence of cliff-forming carbonate rocks deposited in the Middle Cambrian (513 to 499 Ma) in an intertidal and supratidal zone and directly overlies the Stephen Formation. Walcott Quarry is downslope to the west (left). Metamorphosed and afossiliferous rocks of the Eldon Formation dominate Wapta's uppermost vertical cliffs. Image from Wikipedia Commons.

Located some 2,400 meters (almost 8,000 feet) above sea level between the summits of Mounts Wapta and Field in Yoho National Park of British Columbia, Walcott and at various times his fossil-hunting wife, sons and daughter established the world famous, UNESCO-acclaimed and protected Walcott Quarry.

Walcott Quarry
About the size of two school buses end to end, shovels, chisels and pick axes rest along the quarry's gradually excavated back wall. Note the slabs of shale awaiting analysis. To this day, even talus refuse are continually being reexamined and protected from theft. From the Royal Ontario Museum

Over the course of some six summers from 1909 to 1925, they hammered, chiseled, blasted, excavated, examined and cataloged thousands of fossils of the biota entombed in slabs of shale. It included over 12,000 specimens of Marrella alone and shipped them off by rail from camp to the Smithsonian some 3,000 miles to the East. 

Charles and Sons at Camp below Fossil Ridge and Walcott Quarry in 1913
Walcott gazes into the fire as his sons sharpen an ax and prepare dinner. From Royal Ontario Museum website

Subsequent expeditions to Walcott Quarry and two temporally and stratigraphically-related quarries higher on the slope of Fossil Ridge in the ensuing century by Raymond, Richter, Stormer, Whittington, Collins, Norris, Caron, Garcia-Bellido and a host of others have yielded well over 75,000 Burgess specimens. 

Their ongoing restudies and subsequent reclassifications are testimony to the enduring enigma and allure of the Burgess Shale biota and its many mysteries that continue to be conjectured upon and discovered.

East Facing View of Mounts Wapta and Burgess with intervening Fossil Ridge
 The blue color of lakes in western Canada is due to marl held in suspension. It's a fine glacially-derived, muddy mixture of calcium carbonate (<60%) and clay (rock flour is 10 to 50% carbonate). Shorter red and yellow wavelengths are absorbed, while longer greens and blues are reflected. As the glaciers retreat and disappear (~80% in the next 50 years), the lakes are changing to sapphire blue. Image from Wikimedia Commons.

Burgess Shale-type localities from the Early and Middle Cambrian are located globally. Their seemingly disparate distribution - in places afar such as Greenland, China, England, Scandinavia and Siberia - is related to their original location on the passive margins of paleo-continents that have rifted from supercontinent Rodinia in the latest Proterozoic and subsequently drifted worldwide in the ensuing Paleozoic.

Their modern location, oft-inland and at considerable elevation, such as at Fossil Ridge, is related to the collisional tectonic reassembly of supercontinent Pangaea in the Mesozoic and subsequent break-up in the Cenozoic. More detailed geology in a follow-up post!

Global Geographic Distribution of Burgess Shale-Type Localities
Nearly three dozen localities are currently known from England, Spain, Sardinia, eastern Siberia, north-central Russia, China, Quebec, Canada and California, Nevada, Utah and Pennsylvania. From J. Han, 2007


A few fossils from Walcott Quarry are in general circulation, but the majority (over 65,000) are housed in the National Museum of Natural History at the Smithsonian Institution in Washington, DC (15,000), Toronto's Royal Ontario Museum (9,000), Ottawa's Geological Survey of Canada (800) and Harvard's Museum of Comparative Zoology (202). 

After his death in 1927, his wife (the third) donated his personal and official papers and photographs to the Smithsonian. Established in 1934, the Charles Doolittle Walcott Medal is awarded by the National Academy of Sciences every five years for outstanding work in the field of Precambrian and Cambrian life and history. Walcott is buried in the Rock Creek Cemetery in Washington, DC.


Please join me for my second post on Marrella, the Burgess Shale biota and the Cambrian Explosion. We'll pay a visit to the UNESCO-protected Walcott Quarry on Fossil Ridge in the Canadian Rockies and the fortuitous events that up to the discovery by Charles Walcott. The posts to follow will include the taxonomy, phylogeny and geo-tectonics of Marrella.


  1. Hi Jack! Great to find this post. I read Wonderful Life in May, during a two-week geotrip in Nevada.
    best wishes, Hollis

  2. Fabulous post and very well written. I felt like I was there. Bravo! I think Walcott also served a term or two as President of the Geological Society of America.

    1. He did and many other associations, organizations and institutions. I'll cover that in the next post or two.