Information

13.23: The Cambrian Explosion - Biology

13.23: The Cambrian Explosion - Biology



We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision (Figure 1).

The cause of the Cambrian explosion is still debated. There are many theories that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period (Figure 2).

Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist. There is also support for theories that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other theories claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Theories that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened when it did. There is evidence that both supports and refutes each of the theories described above, and the answer may very well be a combination of these and other theories.

However, unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Was there really an “explosion” of life at this particular time? Some scientists question the validity of the this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively rapid animal evolution and diversification that is unmatched elsewhere during history.

View an animation of what ocean life may have been like during the Cambrian explosion. Note that there isn’t any narration in the video.

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/biom2/?p=454


A juvenile-rich palaeocommunity of the lower Cambrian Chengjiang biota sheds light on palaeo-boom or palaeo-bust environments

The fossil record, including the record of Burgess Shale-type deposits, is biased towards late ontogenetic stages. Larval stages, juvenile and subadult specimens exist but are very rare and often preserved as phosphatic fossils, resulting in biased population structures. Here, we report a new Burgess Shale-type Lagerstätte from Haiyan, China. The Haiyan palaeocommunity is extraordinary in that it is rich in fossils of early and middle ontogenetic stages of various phyla, with eggs also commonly found in the studied interval. This Lagerstätte also hosts a considerable number of new taxa—many related to later biotas of Gondwana and Laurentia. We propose that the deposit may either preserve one of the earliest nurseries in the fossil record or, alternatively, records several attempted invasions. Our study highlights the complexity of biotas and their interactions in the lower Cambrian ocean and calls for a better understanding of the mechanisms responsible for the observed spatial variation of fossil community composition in the Cambrian.


Abstract

The biological pump is crucial for transporting nutrients fixed by surface-dwelling primary producers to demersal animal communities. Indeed, the establishment of an efficient biological pump was likely a key factor enabling the diversification of animals over 500 Myr ago during the Cambrian explosion. The modern biological pump operates through two main vectors: the passive sinking of aggregates of organic matter, and the active vertical migration of animals. The coevolution of eukaryotes and sinking aggregates is well understood for the Proterozoic and Cambrian however, little attention has been paid to the establishment of the vertical migration of animals. Here we investigate the morphological variation and hydrodynamic performance of the Cambrian euarthropod Isoxys. We combine elliptical Fourier analysis of carapace shape with computational fluid dynamics simulations to demonstrate that Isoxys species likely occupied a variety of niches in Cambrian oceans, including vertical migrants, providing the first quantitative evidence that some Cambrian animals were adapted for vertical movement in the water column. Vertical migration was one of several early Cambrian metazoan innovations that led to the biological pump taking on a modern-style architecture over 500 Myr ago.

References

. 2015 Zooplankton fecal pellets, marine snow, phytodetritus and the ocean's biological pump . Prog. Oceanogr. 130, 205-248. (doi:10.1016/j.pocean.2014.08.005) Crossref, ISI, Google Scholar

. 2012 The biological pump in a high CO2 world . Mar. Ecol. Prog. Ser. 470, 249-271. (doi:10.3354/meps09985) Crossref, ISI, Google Scholar

. 2013 Recalcitrant dissolved organic carbon fractions . Ann. Rev. Mar. Sci. 5, 421-445. (doi:10.1146/annurev-marine-120710-100757) Crossref, PubMed, ISI, Google Scholar

. 1997 Plankton ecology and the Proterozoic–Phanerozoic transition . Paleobiology 23, 247-262. (doi:10.1017/S009483730001681X) Crossref, ISI, Google Scholar

. 2009 Macroevolutionary turnover through the Ediacaran transition: ecological and biogeochemical implications . Geol. Soc. Spec. Publ. 326, 55-66. (doi:10.1144/SP326.3) Crossref, Google Scholar

Lenton TM, Boyle RA, Poulton SW, Shields-Zhou GA, Butterfield NJ

. 2014 Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era . Nat. Geosci. 7, 257-265. (doi:10.1038/ngeo2108) Crossref, ISI, Google Scholar

Brocks JJ, Jarrett AJM, Sirantoine E, Hallmann C, Hoshino Y, Liyanage T

. 2017 The rise of algae in Cryogenian oceans and the emergence of animals . Nature 548, 578-581. (doi:10.1038/nature23457) Crossref, PubMed, ISI, Google Scholar

. 1994 Burgess Shale-type fossils from a Lower Cambrian shallow-shelf sequence in northwestern Canada . Nature 369, 477-479. (doi:10.1038/369477a0) Crossref, ISI, Google Scholar

Harvey THP, Butterfield NJ

. 2008 Sophisticated particle-feeding in a large Early Cambrian crustacean . Nature 452, 868-871. (doi:10.1038/nature06724) Crossref, PubMed, ISI, Google Scholar

Antcliffe JB, Callow RHT, Brasier MD

. 2014 Giving the early fossil record of sponges a squeeze . Biol. Rev. 89, 972-1004. (doi:10.1111/brv.12090) Crossref, PubMed, ISI, Google Scholar

. 2018 Early sponge evolution: a review and phylogenetic framework . Palaeoworld 27, 1-29. (doi:10.1016/j.palwor.2017.07.001) Crossref, ISI, Google Scholar

Wallet E, Slater BJ, Willman S, Peel JS

. In press. Small carbonaceous fossils (SCFs) from North Greenland: new light on metazoan diversity in early Cambrian shelf environments . Pap. Palaeontol. (doi:10.1002/spp2.1347) ISI, Google Scholar

. 2008 Bacterial vs. zooplankton control of sinking particle flux in the ocean's twilight zone . Limnol. Oceanogr. 53, 1327-1338. (doi:10.4319/lo.2008.53.4.1327) Crossref, ISI, Google Scholar

. 2016 Making sense of ‘lower’ and ‘upper’ stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848 . Biol. Rev. 91, 255-273. (doi:10.1111/brv.12168) Crossref, PubMed, ISI, Google Scholar

. 2000 The Early Cambrian colonization of pelagic niches exemplified by Isoxys (Arthropoda) . Lethaia 33, 295-311. (doi:10.1080/002411600750053862) Crossref, ISI, Google Scholar

García-Bellido DC, Vannier J, Collins D

. 2009 Soft-part preservation in two species of the arthropod Isoxys from the middle Cambrian Burgess Shale of British Columbia, Canada . Acta Palaeontol. Pol. 54, 699-712. (doi:10.4202/app.2009.0024) Crossref, ISI, Google Scholar

. 2013 The affinities of the cosmopolitan arthropod Isoxys and its implications for the origin of arthropods . Lethaia 46, 540-550. (doi:10.1111/let.12032) Crossref, ISI, Google Scholar

Williams M, Siveter DJ, Peel JS

. 1996 Isoxys (Arthropoda) from the Early Cambrian Sirius Passet Lagerstätte, North Greenland . J. Paleontol. 70, 947-954. (doi:10.1017/S0022336000038646) Crossref, ISI, Google Scholar

Vannier J, García-Bellido DC, Hu SX, Chen AL

. 2009 Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas . Proc. R. Soc. B 276, 2567-2574. (doi:10.1098/rspb.2009.0361) Link, ISI, Google Scholar

Perrier V, Williams M, Siveter DJ

. 2015 The fossil record and palaeoenvironmental significance of marine arthropod zooplankton . Earth-Science Rev. 146, 146-162. (doi:10.1016/j.earscirev.2015.02.003) Crossref, ISI, Google Scholar

. 2011 Soft anatomy of the early Cambrian arthropod Isoxys curvirostratus from the Chengjiang biota of South China with a discussion on the origination of great appendages . Acta Palaeontol. Pol. 56, 843-852. (doi:10.4202/app.2010.0090) Crossref, ISI, Google Scholar

. 2015 Cephalic and limb anatomy of a new isoxyid from the burgess shale and the role of ‘stem bivalved arthropods' in the disparity of the frontalmost appendage . PLoS ONE 10, e0124979. (doi:10.1371/journal.pone.0124979) Crossref, PubMed, ISI, Google Scholar

Vannier J, Caron J-B, Yuan J, Briggs DEG, Collins D, Zhao Y, Zhu M

. 2007 Tuzoia: Morphology and lifestyle of a large bivalved Arthropod of the Cambrian seas . J. Paleontol. 81, 445-471. (doi:10.1666/pleo05070.1) Crossref, ISI, Google Scholar

. 2013 A taxonomical review of the Gnathophausia (Crustacea, Lophogastrida), with new records from the northern mid-Atlantic ridge . Zootaxa 3664, 199-225. (doi:10.11646/zootaxa.3664.2.5) Crossref, PubMed, ISI, Google Scholar

Bonhomme V, Picq S, Gaucherel C, Claude J

. 2014 Momocs: outline analysis using R . J. Stat. Softw. 56, 1-24. (doi:10.18637/jss.v056.i13) Crossref, ISI, Google Scholar

García-Bellido DC, Paterson JR, Edgecombe GD, Jago JB, Gehling JG, Lee MSY

. 2009 The bivalved arthropods Isoxys and Tuzoia with soft-part preservation from the lower Cambrian Emu Bay Shale Lagerstätte (Kangaroo Island, Australia) . Palaeontology 52, 1221-1241. (doi:10.1111/j.1475-4983.2009.00914.x) Crossref, ISI, Google Scholar

Wang Y, Huang D, Liu Q, Hu S

. 2012 Isoxys from the Cambrian Guanshan Fauna, Yunnan Province . Earth Sci. J. China Univ. Geosci. 37, 156-164. Google Scholar

. 2010 Analysis of flows past airfoils at very low Reynolds numbers . Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng. 224, 757-775. (doi:10.1243/09544100JAERO715) Crossref, ISI, Google Scholar

. 2000 Two dimensional mechanism for insect hovering . Phys. Rev. Lett. 85, 2216-2219. (doi:10.1103/PhysRevLett.85.2216) Crossref, PubMed, ISI, Google Scholar

. 1996 Life in moving fluids: the physical biology of flow , 2nd edn. Princeton, NJ : Princeton University Press . Google Scholar

Schoenemann B, Clarkson ENK

. 2011 Eyes and vision in the Chengjiang arthropod Isoxys indicating adaptation to habitat . Lethaia 44, 223-230. (doi:10.1111/j.1502-3931.2010.00239.x) Crossref, ISI, Google Scholar

Cowles DL, Childress JJ, Gluckj DL

. 1986 New method reveals unexpected relationship between velocity and drag in the bathypelagic mysid Gnathophausia ingens . Deep. Res. 33, 865-880. (doi:10.1016/0198-0149(86)90002-6) Crossref, Google Scholar

. 1988 Swimming speed and oxygen consumption in the bathypelagic mysid Gnathophausia ingens . Biol. Bull. 175, 111-121. (doi:10.2307/1541898) Crossref, ISI, Google Scholar

. 2007 Early Cambrian origin of complex marine ecosystems . In Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies (eds

Williams M, Haywood AM, Gregory FJ, Schmidt DN

), pp. 81-100. London, UK: The Geological Society . Google Scholar

. 1987 Early Cambrian large bivalved arthropods from Chengjiang . Acta Palaeontol. Sin. 26, 286-297. Google Scholar

Fu D, Zhang X, Budd GE, Liu W, Pan X

. 2014 Ontogeny and dimorphism of Isoxys auritus (Arthropoda) from the Early Cambrian Chengjiang biota, South China . Gondwana Res. 25, 975-982. (doi:10.1016/j.gr.2013.06.007) Crossref, ISI, Google Scholar

. 2010 Community structure and composition of the Cambrian Chengjiang biota . Sci. China Earth Sci. 53, 1784-1799. (doi:10.1007/s11430-010-4087-8) Crossref, ISI, Google Scholar

. 2008 Paleoecology of the Greater Phyllopod Bed community, Burgess Shale . Palaeogeogr. Palaeoclimatol. Palaeoecol. 258, 222-256. (doi:10.1016/j.palaeo.2007.05.023) Crossref, ISI, Google Scholar

. 1986 Soft-bodied animals in the fossil record: The role of decay in fragmentation during transport . Geology 14, 979-981. (doi:10.1130/0091-7613(1986)14<979:SAITFR>2.0.CO2) Crossref, ISI, Google Scholar

Stein M, Peel JS, Siveter DJ, Williams M

. 2010 Isoxys (Arthropoda) with preserved soft anatomy from the Sirius Passet Lagerstätte, lower Cambrian of North Greenland . Lethaia 43, 258-265. (doi:10.1111/j.1502-3931.2009.00189.x) Crossref, ISI, Google Scholar

. 2014 Burgess Shale-type preservation and its distribution in space and time . Paleontol. Soc. Pap. 20, 1-24. (doi:10.1017/S1089332600002837) Crossref, Google Scholar

Lerosey-Aubril R, Kimmig J, Pates S, Skabelund J, Weug A, Ortega-Hernández J

. 2020 New exceptionally preserved panarthropods from the Drumian Wheeler Konservat-Lagerstätte of the House Range of Utah . Pap. Palaeontol. 6, 501-531. (doi:10.1002/spp2.1307) Crossref, ISI, Google Scholar

Paterson JR, García-Bellido DC, Jago JB, Gehling JG, Lee MSY, Edgecombe GD

. 2016 The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana . J. Geol. Soc. London. 173, 1-11. (doi:10.1144/jgs2015-083) Crossref, ISI, Google Scholar

Guilbaud R, Slater BJ, Poulton SW, Harvey THP, Brocks JJ, Nettersheim BJ, Butterfield NJ

. 2017 Oxygen minimum zones in the early Cambrian ocean . Geochemical Perspect. Lett. 6, 33-38. (doi:10.7185/geochemlet.1806) ISI, Google Scholar

Harvey THP, Vélez MI, Butterfield NJ, Stanley SM

. 2012 Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation . Proc. Natl Acad. Sci. USA 109, 1589-1594. (doi:10.1073/pnas.1115244109) Crossref, PubMed, ISI, Google Scholar

Williams M, Siveter DJ, Popov LE, Vannier JMC

. 2007 Biogeography and affinities of the bradoriid arthropods: cosmopolitan microbenthos of the Cambrian seas . Palaeogeogr. Palaeoclimatol. Palaeoecol. 248, 202-232. (doi:10.1016/j.palaeo.2006.12.004) Crossref, ISI, Google Scholar

Williams M, Vandenbroucke TRA, Perrier V, Siveter DJ, Servais T

. 2015 A link in the chain of the Cambrian zooplankton: Bradoriid arthropods invade the water column . Geol. Mag. 152, 923-934. (doi:10.1017/S0016756815000059) Crossref, ISI, Google Scholar

. 2017 Metamorphosis is ancestral for crown euarthropods, and evolved in the Cambrian or Earlier . Integr. Comp. Biol. 57, 499-509. (doi:10.1093/icb/icx039) Crossref, PubMed, ISI, Google Scholar

Selden PA, Huys R, Stephenson MH, Heward AP, Taylor PN

. 2010 Crustaceans from bitumen clast in Carboniferous glacial diamictite extend fossil record of copepods . Nat. Commun. 1, 50. (doi:10.1038/ncomms1049) Crossref, PubMed, ISI, Google Scholar

Hu S, Steiner M, Zhu M, Erdtmann BD, Luo H, Chen L, Weber B

. 2007 Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian . Palaeogeogr. Palaeoclimatol. Palaeoecol. 254, 307-316. (doi:10.1016/j.palaeo.2007.03.044) Crossref, ISI, Google Scholar

2019 The Qingjiang biota-A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China . Science 363, 1338-1342. (doi:10.1126/science.aau8800) Crossref, PubMed, ISI, Google Scholar

Rahman IA, Darroch SAF, Racicot RA, Laflamme M

. 2015 Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems . Sci. Adv. 1, e1500800. (doi:10.1126/sciadv.1500800) Crossref, PubMed, ISI, Google Scholar

. 2018 Oxygen, animals and aquatic bioturbation: an updated account . Geobiology 16, 3-16. (doi:10.1111/gbi.12267) Crossref, PubMed, ISI, Google Scholar

Antcliffe JB, Jessop W, Daley AC

. 2019 Prey fractionation in the Archaeocyatha and its implication for the ecology of the first animal reef systems . Paleobiology 45, 652-675. (doi:10.1017/pab.2019.32) Crossref, ISI, Google Scholar

Steiner M, Li G, Qian Y, Zhu M, Erdtmann BD

. 2007 Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) . Palaeogeogr. Palaeoclimatol. Palaeoecol. 254, 67-99. (doi:10.1016/j.palaeo.2007.03.046) Crossref, ISI, Google Scholar

Kouchinsky A, Bengtson S, Runnegar B, Skovsted C, Steiner M, Vendrasco M

. 2012 Chronology of early Cambrian biomineralization . Geol. Mag. 149, 221-251. (doi:10.1017/S0016756811000720) Crossref, ISI, Google Scholar

Betts MJ, Paterson JR, Jago JB, Jacquet SM, Skovsted CB, Topper TP, Brock GA

. 2017 Global correlation of the early Cambrian of South Australia: Shelly fauna of the Dailyatia odyssei Zone . Gondwana Res. 46, 240-279. (doi:10.1016/j.gr.2017.02.007) Crossref, ISI, Google Scholar

Vinther J, Stein M, Longrich NR, Harper DAT

. 2014 A suspension-feeding anomalocarid from the Early Cambrian . Nature 507, 496-499. (doi:10.1038/nature13010) Crossref, PubMed, ISI, Google Scholar

Fakhraee M, Planavsky NJ, Reinhard CT

. 2020 The role of environmental factors in the long-term evolution of the marine biological pump . Nat. Geosci. 13, 812-816. (doi:10.1038/s41561-020-00660-6) Crossref, ISI, Google Scholar

. 2002 Active transport of particulate organic carbon and nitrogen by vertically migrating zooplankton in the Sargasso Sea . Mar. Ecol. Prog. Ser. 234, 71-84. (doi:10.3354/meps234071) Crossref, ISI, Google Scholar

Steinberg DK, Goldthwait SA, Hansell DA

. 2002 Zooplankton vertical migration and the active transport of dissolved organic and inorganic nitrogen in the Sargasso Sea . Deep. Res. Part I Oceanogr. Res. Pap. 49, 1445-1461. (doi:10.1016/S0967-0637(02)00037-7) Crossref, ISI, Google Scholar

Wilson SE, Steinberg DK, Buesseler KO

. 2008 Changes in fecal pellet characteristics with depth as indicators of zooplankton repackaging of particles in the mesopelagic zone of the subtropical and subarctic North Pacific Ocean . Deep. Res. Part II Top. Stud. Oceanogr. 55, 1636-1647. (doi:10.1016/j.dsr2.2008.04.019) Crossref, ISI, Google Scholar

Hannides CCS, Landry MR, Benitez-Nelson CR, Styles RM, Montoya JP, Karl DM

. 2009 Export stoichiometry and migrant-mediated flux of phosphorus in the North Pacific Subtropical Gyre . Deep. Res. Part I Oceanogr. Res. Pap. 56, 73-88. (doi:10.1016/j.dsr.2008.08.003) Crossref, ISI, Google Scholar

Bollens SM, Rollwagen-Bollens G, Quenette JA, Bochdansky AB

. 2011 Cascading migrations and implications for vertical fluxes in pelagic ecosystems . J. Plankton Res. 33, 349-355. (doi:10.1093/plankt/fbq152) Crossref, ISI, Google Scholar

Sperling EA, Frieder CA, Raman AV, Girguis PR, Levin LA, Knoll AH

. 2013 Oxygen, ecology, and the Cambrian radiation of animals . Proc. Natl Acad. Sci. USA 110, 13 446-13 451. (doi:10.1073/pnas.1312778110) Crossref, ISI, Google Scholar

. 2018 New suspension-feeding radiodont suggests evolution of microplanktivory in Cambrian macronekton . Nat. Commun. 9, 1-9. (doi:10.1038/s41467-018-06229-7) Crossref, PubMed, ISI, Google Scholar

Servais T, Owen AW, Harper DAT, Kröger B, Munnecke A

. 2010 The Great Ordovician Biodiversification Event (GOBE): the palaeoecological dimension . Palaeogeogr. Palaeoclimatol. Palaeoecol. 294, 99-119. (doi:10.1016/j.palaeo.2010.05.031) Crossref, ISI, Google Scholar

2016 The onset of the ‘Ordovician Plankton Revolution’ in the late Cambrian . Palaeogeogr. Palaeoclimatol. Palaeoecol. 458, 12-28. (doi:10.1016/j.palaeo.2015.11.003) Crossref, ISI, Google Scholar

Van Roy P, Daley AC, Briggs DEG

. 2015 Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps . Nature 522, 77-80. (doi:10.1038/nature14256) Crossref, PubMed, ISI, Google Scholar

Klug C, Kröger B, Kiessling W, Mullins G, Servais T, Frýda J, Korn D, Turner S.

2010 The Devonian nekton revolution . Lethaia 43, 465-477. (doi:10.1111/j.1502-3931.2009.00206.x) Crossref, ISI, Google Scholar


ARCHEA

Here, for more than a century, palaeontologists have been exploring over a dozen geologic outcrops that speak of a world when arthropods ruled the seas.

The rocks we walk across are made of shale, thin-bedded limestone, and siltstone deposited during the Middle Cambrian — 513 to 497 million years ago. And these are no ordinary rocks for what they contain — exceptionally preserved soft-bodied fossils of the Burgess Shale biota.

Charles Doolittle Walcott will be forever remembered for his extraordinary 1909 discovery of the Middle Cambrian Burgess Shale of Yoho National Park in southern British Columbia — delivering to the world one of the most important biota of soft-bodied organisms in the fossil record. Here we find a fairly complete look at an ancient ecosystem with algae, grazers and filter feeders, scavengers and active predators. Remarkably, soft-bodied organisms make up 98% of individuals and 85% of the genera. These animals lived and died in the deep waters at the base of what would later become the Cathedral Escarpment.

In 1908, Walcott wrote, "Nearly every fragment of shale found on the slopes from 2000 to 2600 feet above Field has fossils upon it not only fragments but usually entire specimens of trilobites.” It was for this reason he returned the following year to collect and the rest, as they say, is history.

The sheer volume and level of preservation were unknown at the time. Walcott's material came from a single section on the west side of the ridge between Mount Wapta and Mount Field and was collected from the main quarry in the Phyllopod bed and the smaller Raymond quarry some 23 m above.

The Burgess Shale section occurs in the lower two-thirds of the Stephen Formation where the basinal shales abut against the steep face of the adjacent dolomite reef of the Cathedral Formation. The conditions necessary for the preservation of the soft parts of the organisms appear to have been controlled by the proximity of this reef front. Away from the reef front, exceptional preservation is less common.

A view to Mount Stephen, Canadian Rockies
The Burgess Shale was long considered to be a unique occurrence. Then in 1977, Canadian geologist, Ian McIlreath, found that the Cathedral Escarpment or reef front, could be traced for about 20 km southeast of Walcott's quarry and that the contact between the reef and basinal shales cropped out again on Mount Field, Mount Stephen, Mount Odaray, Park Mountain and Curtis Peak.

Des Collins speculated that more localities of soft-bodied fossils might be found in the basinal shales near these contacts, and, indeed, a few indications were later reported by Aitken and McIlreath (1981) along the line of the Escarpment.

In 1981 and 1982, we expanded our knowledge of the region. Des Collins and others organized fieldwork that led to the discovery of about a dozen new localities, which Collins et al. published in 1983.

The most promising of the new localities occurred in a large in situ block of pale grey-blue siliceous shale about 1500 m southwest of the outcrop of the Cathedral Escarpment on the north shoulder of Mount Stephen.

This is about 5 km almost directly south of the Burgess Shale quarries. The site was excavated by a Royal Ontario Museum party in the summer of 1983. Further fieldwork in 1986 led to the discovery of the arthropod Sanctacaris was first described by Briggs and Collins in 1988.

Sanctacaris uncata, Mount Stephen Fossil Beds

The stratigraphic level where the block occurred is characterized by the trilobite, Glossopleura, which is the local zone fossil for the basal part of the basinal Stephen Formation (Fritz, 1971).

In the Stephen Formation section of about 1000 m to the north on Mount Stephen measured by Fritz, the top of the Glossopleura Zone is 40 m below the level equivalent to the main Burgess Shale quarry.

The block excavated was at least 40 m below the top of the Glossopleura Zone. This puts it 80 m or more stratigraphically below the level of the Burgess Shale Phyllopod bed.

The faunal assemblage from the block is dominated by the arthropods, Alalcomenaeua and Branchiocaris, which are very rare in the Burgess Shale. Many other Burgess Shale animals were found (Collins et al. 1983) but surprisingly not the most common — Marrella. They did find many new forms and published their finds in 1986 (Collins, 1986). By all accounts, this fauna is distinct from those in the Burgess Shale — and a shade older.

But as we learn and gain insight, we also realize how much we have yet to learn. These outcrops help us to gain an understanding of the biology, ecology, diversity and evolution of Cambrian animals in a way that other Cambrian sites cannot. Without this insight, we would have a very limited view of the Cambrian Explosion and see only the shelly fossil assemblages. The unique conditions in the Burgess Shale record species that under typical circumstances, would never have fossilized and would be lost to us forever.

There has been no end of mysteries and riddles to be solved in the designating and correlating units within the Stephen Formation, Burgess Shale Formation, and the Cathedral Formation. Much of the controversy stems from the extensive faulting in the area and especially from environmental (facies) differences between the stratigraphic units.

There are shelf platform sequences that include shallow water inner detrital belt, middle carbonate belt, and carbonate shelf edge facies, as well as deeper water (basinal) outer detrital belt facies. These have all have posed problems in correlation and descriptions of the formations in the area.

What used to be known as the Stephen Formation is now restricted to what was known as the "thin" Stephen Formation. The Stephen Formation now includes the Narao and Wapituk Members. What was formerly the "thick" Stephen Formation (basinal Stephen) is now called the Burgess Shale Formation.

Pirania sp., extinct sea sponges, Burgess Shale
This formation comprises units that include the classic Burgess Shale localities (Walcott Quarry (including the "phyllopod bed"), Raymond Quarry), the Mt. Stephen Trilobite Beds, as well as most of the soft-bodied faunas (Collins Quarry, S7, Ehmaniella Zone faunas, etc.).

The Burgess Shale is a UNESCO World Heritage site. The Burgess Shale and Stephen Formations outcrop mainly in Banff and Yoho National Parks in the Alberta-British Columbia border area. All known outcrops are in Canada's Rocky Mountain Parks, so collecting is strictly forbidden.

While you cannot collect in the parks, you can join in on a guided tour to hike, explore, capturing the beautiful scenery and fossils with your camera and through rubbings. If you fancy a hike to these exalted cliffs, follow the link below.

If an armchair visit is more your thing, pick up a copy of, A Geoscience Guide To The Burgess Shale. This illustrated guide immerses the reader in the history, geology, environment and, most importantly, fossils of the Burgess Shale in an easy-to-read, concise summary of life as it was over 500 million years ago. Excellent colour images of 3D interpretations of the organisms and photos of the fossils make this resource a must-have for anyone interested in the Burgess Shale.

Burgess Shale Hikes: https://www.burgess-shale.bc.ca/burgess-shale-hikes/ / Toll free: 1 (800) 343-3006 Tel: 1 (250) 343-6006 Email: [email protected]

A Burgess Shale Primer: History, Geology and Research Highlights Jean-Bernard Caron & Dave Rudkin: https://www.rom.on.ca/sites/default/files/imce/burgess_shale_primer.pdf

References: Palaeontology, Vol 31, Part 3. 1988, pp 779-798, pls 71-73) was discovered by Collins (1986),http://palaeontology.palass-pubs.org/pdf/Vol 31/Pages 779-798.pdf

Image: Reconstruction of Sanctacaris uncata, a Cambrian Habeliidan arthropod (stem-Chelicerata: Habeliida). by Junnn11 @ni075 Pirania sp. & photos: @Fossil Huntress


Lower Cambrian vertebrates from south China

The first fossil chordates are found in deposits from the Cambrian period (545–490 million years ago), but their earliest record is exceptionally sporadic and is often controversial. Accordingly, it has been difficult to construct a coherent phylogenetic synthesis for the basal chordates. Until now, the available soft-bodied remains have consisted almost entirely of cephalochordate-like animals from Burgess Shale-type faunas. Definite examples of agnathan fish do not occur until the Lower Ordovician ( ∼ 475 Myr BP), with a more questionable record extending into the Cambrian. The discovery of two distinct types of agnathan from the Lower Cambrian Chengjiang fossil-Lagerstätte is, therefore, a very significant extension of their range. One form is lamprey-like, whereas the other is closer to the more primitive hagfish. These finds imply that the first agnathans may have evolved in the earliest Cambrian, with the chordates arising from more primitive deuterostomes in Ediacaran times (latest Neoproterozoic, ∼ 555 Myr BP), if not earlier.


Watch the video: The Cambrian Explosion and the evolutionary origin of animals with Professor Paul Smith (August 2022).