Signor-Lipps effect
Encyclopedia
The Signor–Lipps effect is a paleontological principle proposed by Philip W. Signor and Jere H. Lipps
which states that, since the fossil
record of organisms is never complete, neither the first nor the last organism in a given taxon
will be recorded as a fossil.
One famous example is the coelacanth
, which was thought to have become extinct in the very late Cretaceous
– until a live specimen was caught in 1938. The animals known as "Burgess Shale type fauna
" are best known from rocks of the Early and Middle Cambrian
periods. However since 2006 a few fossils of similar animals have been found in rocks from the Silurian
, Ordovician
and Early Devonian
periods, in other words up to 100 million years after the Burgess Shale. The particular way
in which such animals have been fossilized may depend on types of ocean chemistry that were present for limited periods of time.
But the Signor–Lipps effect is more important for the difficulties it raises in paleontology:
Jere H. Lipps
Dr. Jere Henry Lipps has been a professor at the University of California at Berkeley in the Department of Integrative Biology since 1988 and Curator of Paleontology at the UC Museum of Paleontology. Lipps is also a past director of the museum and chair of the department...
which states that, since the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record of organisms is never complete, neither the first nor the last organism in a given taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
will be recorded as a fossil.
One famous example is the coelacanth
Coelacanth
Coelacanths are members of an order of fish that includes the oldest living lineage of Sarcopterygii known to date....
, which was thought to have become extinct in the very late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
– until a live specimen was caught in 1938. The animals known as "Burgess Shale type fauna
Burgess shale type fauna
A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are also preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale type fauna" covers assemblages based on taxonomic criteria only.-Extent:The fauna of the middle...
" are best known from rocks of the Early and Middle Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
periods. However since 2006 a few fossils of similar animals have been found in rocks from the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
periods, in other words up to 100 million years after the Burgess Shale. The particular way
Burgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
in which such animals have been fossilized may depend on types of ocean chemistry that were present for limited periods of time.
But the Signor–Lipps effect is more important for the difficulties it raises in paleontology:
- It makes it very difficult to be confident about the timing and speed of mass extinctions, and this makes it difficult to test theories about the causes of mass extinctions. For example the extinction of the dinosaurDinosaurDinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s was long thought to be a gradual process, but evidence collected since the late 1980s suggests it was abrupt, which is consistent with the idea that an asteroid impactChicxulub CraterThe Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...
caused it. - The uncertainty about when a taxon first appeared makes it difficult to be confident about the ancestry of specific generaGenusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. For example if the earliest known fossil of genus X is much earlier than the earliest known fossil of genus Y and genus Y has all the features of genus X plus a few of its own, it is natural to suppose that X is an ancestor of Y. But this hypothesis could be called into question at any time by the finding of a fossil of Y that is earlier than any known fossil of X – unless an even older fossil of genus X is found, and so on.