A megabias, or a taphonomic megabias, is a large-scale pattern in the quality of the fossil record that affects paleobiologic analysis at provincial to global levels and at timescales usually exceeding ten million years. [1] It can result from major shifts in intrinsic and extrinsic properties of organisms, including morphology and behaviour in relation to other organisms, or shifts in the global environment, which can cause secular or long-term cyclic changes in preservation. [1]
The fossil record exhibits bias at many different levels. At the most basic level, there is a global bias towards biomineralizing organisms [Note 1] , because biomineralized body parts are more resistant to decay and degradation. Due to the principle of uniformitarianism, [3] there is a basic assumption in geology that the formation of rocks has occurred by the same naturalistic processes throughout history, and thus that the reach of such biases remains stable over time. A megabias is a direct contradiction of this, whereby changes occur in large scale paleobiologic patterns. This includes:
It is generally assumed that the quality of the fossil record decreases globally and across all taxa with increasing age, because more time is available for the diagenesis and destruction of both fossils and enclosing rocks, [4] and thus the term "megabias" is usually used to refer to global trends in preservation. However, it has been noted that the fossil record of some taxa actually improves with greater age. [4] Examples such as this, and other related paleobiological trends, clearly indicate the action of a megabias, but only within one particular taxon. Hence, it is necessary to define four classes of megabias related to the reach of the bias, first defined by Kowalewski and Flessa.
A change in the quality of a single taxon's record. It may happen whenever evolutionary, environmental, or geologic trends affect the taxon's fossilization potential. [4]
This occurs when the fossilization potential varies from group to group. Among-taxon megabiases include both static and dynamic distortions. A static megabias is constant through time. For example, flatworms have always had a poor record relative to mollusks because mollusks as a taxon are almost always biomineralizing, whereas flatworms are almost never biomineralizing.
A dynamic megabias occurs when the fossilization potential of one group relative to that of another group changes through time. For example, lingulide brachiopods had a lower fossilization potential than articulate brachiopods, but only in the early Paleozoic. [5] This has changed through time, governed by changes in bathymetry and lateral shifts in lithofacies associated with basin evolution. By the Middle Ordovician, articulates had diversified to become important components of all marine environments except the deep basin; inarticulates were most important in offshore and basinal settings. By moving away from the main taphonomic window[ clarification needed ], the inarticulates sacrificed their fossilization potential whilst not necessarily decreasing their biological diversity.
A global megabias is one that occurs over the whole world, though not necessarily in all depositional environments. There are numerous documented examples of this, many of which concern the Cambrian explosion and the Ediacaran fauna, both divisive subjects within the paleobiological community.
Many fossils, such as the Cambrian Small Shelly Fauna (SSF), are preserved through secondary phosphatization. Such preservation can boost the diversity estimates in three ways: [6]
The number of phosphatic facies deposits decreases significantly from the early- to mid-Cambrian. Most probably, this is the cause for the pattern of SSF diversity decline after the Botomian extinction. Whilst there may well have been a significant extinction worldwide of small shelly fossils, a significant decrease in the abundance of phosphatized facies may have caused the decline in SF diversity to appear much more severe than it really was.
It has been noted after the last 20 years that Exceptional Faunas, a particular class of Konservat-Lagerstätten exemplified by Burgess shale deposits, are statistically overabundant [7] considering their age and style of preservation. This problem is also known as the "Post-Cambrian closure of the deep-water slope-basin taphonomic window". It seems that the cause of this may have been ecological, at least in the root cause; a post-Cambrian changes in the amount of bioturbation in deeper-water low oxygen environments increased sedimentary porosity and thus enhanced microbial activity and accelerated rates of decay. [7] These higher rates of decay after the Cambrian meant that many soft bodied organisms were destroyed before the opportunity for preservation arose.
The Maotianshan Shales are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation, famous for their Konservat Lagerstätten, deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales form one of some forty Cambrian fossil locations worldwide exhibiting exquisite preservation of rarely preserved, non-mineralized soft tissue, comparable to the fossils of the Burgess Shale of British Columbia, Canada. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China.
An exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. Some large exoskeletons are known as "shells". Examples of exoskeletons within animals include the arthropod exoskeleton shared by chelicerates, myriapods, crustaceans, and insects, as well as the shell of certain sponges and the mollusc shell shared by snails, clams, tusk shells, chitons, and nautilus. Some animals, such as the turtle, have both an endoskeleton and an exoskeleton.
Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term taphonomy was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov to describe the study of the transition of remains, parts, or products of organisms from the biosphere to the lithosphere.
A Lagerstätte or fossil bed is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying the decomposition of both gross and fine biological features until long after a durable impression was created in the surrounding matrix. Lagerstätten span geological time from the Neoproterozoic era to the present. Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Ordovician Soom Shale, the Silurian Waukesha Biota, the Devonian Hunsrück Slates and Gogo Formation, the Carboniferous Mazon Creek, the Triassic Madygen Formation, the Jurassic Posidonia Shale and Solnhofen Limestone, the Cretaceous Yixian, Santana, and Agua Nueva formations, the Eocene Green River Formation, the Miocene Foulden Maar and Ashfall Fossil Beds, the Pliocene Gray Fossil Site, the Pleistocene Naracoorte Caves, the La Brea Tar Pits, and the Tanis Fossil Site.
Phosphatization, or phosphatic fossilization, refers to the process of fossilization where organic matter is replaced by abundant calcium-phosphate minerals. It has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the scanning electron microscope.
The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätte. It is one of two in the world containing Redlichiidan trilobites. The Emu Bay Shale is dated as Cambrian Series 2, Stage 4, correlated with the upper Botomian Stage of the Lower Cambrian.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
The Chancelloriids are an extinct family of superficially sponge-like animals common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile, radially symmetrical hollow bag-like organisms with a soft skin armored with star-shaped calcareous sclerites from which radiate sharp spines.
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 69 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion. Additional sites with a similar form of preservation are known from the Ediacaran and Ordovician periods.
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 million years ago in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.
The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.
The fossils of the Burgess Shale, like the Burgess Shale itself, are fossils that formed around 505 million years ago in the mid-Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 65,000 specimens in a series of field trips up to the alpine site from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to reveal new species, and statistical analysis suggests that additional discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.
The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil Lagerstätte. It was quarried by Charles Walcott from 1911–1917, and was the source of 95% of the fossils he collected during this time; tens of thousands of soft-bodied fossils representing over 150 genera have been recovered from the Phyllopod bed alone.
Small carbonaceous fossils (SCFs) are sub-millimetric organic remains of organisms preserved in sedimentary strata.
The Waukesha Biota is an important fossil site located in Waukesha and Franklin, Milwaukee County within the state of Wisconsin. This biota is preserved in certain strata within the Brandon Bridge Formation, which dates to the early Silurian period. It is known for the exceptional preservation of soft-bodied organisms, including many species found nowhere else in rocks of similar age. The site's discovery was announced in 1985, leading to a plethora of discoveries. This biota is one of the few well studied Lagerstättes from the Silurian, making it important in our understanding of the period's biodiversity. Some of the species are not easily classified into known animal groups, showing that much research remains to be done on this site. Other taxa that are normally common in Silurian deposits are rare here, but trilobites are quite common.
The Winneshiek Shale is a Middle Ordovician (Darriwilian-age) geological formation in Iowa. The formation is restricted to the Decorah crater, an impact crater near Decorah, Iowa. Despite only being discovered in 2005, the Winneshiek Shale is already renowned for the exceptional preservation of its fossils. The shale preserves a unique ecosystem, the Winneshiek biota, which is among the most remarkable Ordovician lagerstätten in the United States. Fossils include the oldest known eurypterid, Pentecopterus, as well as giant conodonts such as Iowagnathus and Archeognathus.
Susannah M. Porter is an American paleontologist and geobiologist who studies the early evolution of eukaryotes, the early Cambrian fossil record of animals, and the evolution of skeletal biomineralization. She is currently a professor at the University of California, Santa Barbara. Porter is a Fellow of the Paleontological Society. She has received national recognition awards from the Geological Society of America.
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