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Biology

A “Jaw-dropping” Cambrian Fish!

Article: 

Conway, S., and Caron, J. (2014). A primitive fish from the Cambrian of North America. Nature, 512, 419-422. doi:10.1038/nature13414

Background

Scientists recently have redescribed an ancient primitive fish from the Cambrian Period (~520 – 500 million years ago) called Metaspriggina walcotti, based on newly collected fossils from the Canadian Rockies of British Columbia (Fig 1). The fossils were found in fine-grained sedimentary rocks, including the Burgess Shale and other similar deposits.

Screen Shot 2014-09-03 at 6.17.19 PM Figure 1. Metaspriggina walcotti fossils from this study are approximately 505 million years old, placing them in the Series 3 Epoch of the Cambrian Period. Older fossils have been identified from rock formations in Pennsylvania and Vermont.

Figure 1. Metaspriggina walcotti fossils from this study are approximately 505 million years old, placing them in the Series 3 Epoch of the Cambrian Period. Older fossils have been identified from rock formations in Pennsylvania and Vermont.

 

 

 

The Burgess Shale preserves both hard and soft material through a unique type of fossilization. The exceptional quality and rareness of this preservation has been rightfully named Burgess Shale-type preservation. The combination of alkaline seawater during the Cambrian, low oxygen and highly impermeable sediment are required for this unique type of preservation (Gaines et al., 2012). The age of these sedimentary rocks and the abundance of fossils within them capture what some describe as “the big bang” of evolutionary biodiversity: the Cambrian Explosion. The Cambrian Era marks a period of rapid evolution when advanced multicellular life became prevalent on Earth. Scientists have inferred that the depositional environment was shallow marine, located at the base of a large ancient reef (Fig 2) (The Burgess Shale Geoscience Foundation).

 

 

Figure 2. Although the Burgess Shale currently outcrops in the Canadian Rockies, the depositional environment during the Cambrian Period was a continental slope or sedimentary basin at the foot of a submerged ocean reef (The Burgess Shale Geoscience Foundation).

Figure 2. Although the Burgess Shale currently outcrops in the Canadian Rockies, the depositional environment during the Cambrian Period was a continental slope or sedimentary basin at the foot of a submerged ocean reef (The Burgess Shale Geoscience Foundation).

 

Metaspriggina is a small primitive fish, approximately 6 cm long and 1 cm in height. There is an absence of fins, resulting in a tapered, streamlined body shape (Fig 3). The presence of myomeres, the same “W”-shaped muscle tissue that is evident in a properly prepared salmon steak, and streamlined body indicate that Metaspriggina was a capable swimmer, as is shown in this modeled animation. Also preserved in the fossils are well-developed sensory organs, such as paired eyes with lenses, as well as nasal sacs (see “Ey” and “Na” in Fig 3).

Screen Shot 2014-09-03 at 6.17.02 PM

Figure 3. The exceptional preservation of Metaspriggina fossils in the Burgess Shale allow scientists to identify eyes (Ey), nasal sacs (Na), “W”-shaped skeletal muscle called myomeres (My) and branchial region (shaded blue).

 

 

 

 

 

Results

The most remarkable finding pertains to the branchial arches, structures responsible for supporting the gills of a fish. Lampreys, jawless fish with extinct species dating back more than 300 million years, have a completely different branchial configuration. Their branchial basket greatly differs from the two sets of seven branchial arches found in Metaspriggina. Evolutionary intermediate species have been identified which bridge these two branchial configurations, with the branchial arches of Metaspriggina considered to be the primitive form for jawless fish. The front most pair of arches found in Metaspriggina is significantly thicker than the others, a characteristic that is reminiscent of the mandibular arch in jawed fish. The thickening of this front-most branchial arch may have served as an important step towards the evolution of jaws in vertebrates.

 

Significance

Prior to these recently found Metaspriggina fossils from British Columbia, only two incomplete Metaspriggina fossils had been identified and described. The abundance of well-preserved fossils has greatly improved scientists’ understanding of these very early vertebrates. The discovery that the configuration of branchial arches is strikingly similar to that of jawed fish is a critical step to understanding the evolutionary development of primitive vertebrates.

 

 

 

 

References

Gaines, R.R., Hammarlund, E.U., Hou, X., Changshi, Q., Gabbott, S.E., Zhao, Y., Peng, J., and Canfield, D.E. (2012). Mechanism for Burgess Shale-type preservation. Proceedings of the National Academy of Sciences of the United States of America, vol. 109, no. 14, 5180-5184. doi:10.1073/pnas.1111784109

The Burgess Shale Geoscience Foundation, http://www.burgess-shale.bc.ca, accessed September 2014.

Resources

http://www.youtube.com/watch?v=Cd3K8mFPomg

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