Phylogenetic bracketing
Encyclopedia
Phylogenetic bracketing is a method of inference
used in biological sciences
. It is to infer the likelihood of unknown traits in organism
s based on their position in a phylogenetic tree. One of the main applications of phylogenetic bracketing is on extinct animals, known only from fossils. The method is often used for understanding traits that do not fossilize well, such as soft tissue anatomy, physiology
and behaviour. By considering the closest and second closest well known (usually extant) organisms, traits can be asserted with a fair degree of certainty, though the method is extremely sensitive to problems from convergent evolution
.
, a theropod dinosaur
, is bracketed by bird
s and crocodile
s. A feature found in both birds and crocodiles would likely be present in Tyrannosaurus, such as the capability to lay an amniotic egg, whereas a feature both birds and crocodiles lack, such as hair
, would probably not be present in Tyrannosaurus. Sometimes this approach is used for the reconstruction of ecological traits as well.
While the method has it's greatest value when using extant species for bracketing, the method itself do not not require that both bracketing groups have extant members, nor that the species or group to be bracketed is extinct. The only real requisite is that the two bracketing species/groups are more well known with regard to the trait in question, than the species to be bracketed is.
mammals are mostly known only from fragmentary remains, usually the dentary only. Still, the general habit and build of these animals can be asserted with quite some confidence. The advanced mammal-like reptiles like Thrinaxodon
is known from fairly complete specimen, and comparably sized modern shrew
s and marsupial mice
offer an extant group. Combining the two gives a reasonable complete picture of the Jurassic mammals, indicate they were small, carnivorous dwellers of the undergrowth.
been known for many taxa, based on the shape of more well known labyrinthodont finds.
s (like a bird). This is less certain, and depends on at what stage feathers evolved. From phyolgeny alone, it is equally possible Tyrannosaurus had scute
s (like a crocodile). In contrast, concluding that Tyrannosaurus had hair
would be a type III inference, and considered unlikely. Type III inferences are only warranted if there is some positive evidence that the extinct creature possessed the trait.
Inference
Inference is the act or process of deriving logical conclusions from premises known or assumed to be true. The conclusion drawn is also called an idiomatic. The laws of valid inference are studied in the field of logic.Human inference Inference is the act or process of deriving logical conclusions...
used in biological sciences
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
. It is to infer the likelihood of unknown traits in organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s based on their position in a phylogenetic tree. One of the main applications of phylogenetic bracketing is on extinct animals, known only from fossils. The method is often used for understanding traits that do not fossilize well, such as soft tissue anatomy, physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
and behaviour. By considering the closest and second closest well known (usually extant) organisms, traits can be asserted with a fair degree of certainty, though the method is extremely sensitive to problems from convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
Extant phylogenetic bracketing (EPB)
Normally, phylogenetic bracketing is done by comparing an extinct animal to its nearest living relatives. For example, TyrannosaurusTyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...
, a theropod dinosaur
Dinosaur
Dinosaurs 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...
, is bracketed by bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s and crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
s. A feature found in both birds and crocodiles would likely be present in Tyrannosaurus, such as the capability to lay an amniotic egg, whereas a feature both birds and crocodiles lack, such as hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
, would probably not be present in Tyrannosaurus. Sometimes this approach is used for the reconstruction of ecological traits as well.
While the method has it's greatest value when using extant species for bracketing, the method itself do not not require that both bracketing groups have extant members, nor that the species or group to be bracketed is extinct. The only real requisite is that the two bracketing species/groups are more well known with regard to the trait in question, than the species to be bracketed is.
Example of bracketing with one extinct and one extant group
JurassicJurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
mammals are mostly known only from fragmentary remains, usually the dentary only. Still, the general habit and build of these animals can be asserted with quite some confidence. The advanced mammal-like reptiles like Thrinaxodon
Thrinaxodon
Thrinaxodon was a cynodont, an ermine-sized therapsid. Pits on the skull suggest that Thrinaxodon may have had whiskers, and by extension a protective covering of fur. There are suggestions that it was warm-blooded...
is known from fairly complete specimen, and comparably sized modern shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
s and marsupial mice
Dasyuridae
Dasyuridae is a family of marsupials native to Australia and New Guinea, including 61 species divided into 15 genera. Many are small and mouse-like, giving them the misnomer marsupial mice, but the group also includes the cat-sized quolls, as well as the Tasmanian Devil...
offer an extant group. Combining the two gives a reasonable complete picture of the Jurassic mammals, indicate they were small, carnivorous dwellers of the undergrowth.
Example of bracketing with only extinct groups
A fragmentary fossil with a known phylogeny can be compared to more complete fossil specimen to give an idea about general build and habit. The body of labyrinthodonts can for usually be interfered to be broad and squat with a sideways compressed tail, despite only the skullSkull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
been known for many taxa, based on the shape of more well known labyrinthodont finds.
Types of interference
There are three types of interferences that can be drawn based on phyolgeny. The above example, concluding that Tyrannosaurus could lay amniotic eggs, is the standard type, and is called a type I inference. A type II inference is made where one of the extant relatives has the trait and the other does not. In the case of the Tyrannosaurus, a type II interference could be to conclude that it had featherFeather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
s (like a bird). This is less certain, and depends on at what stage feathers evolved. From phyolgeny alone, it is equally possible Tyrannosaurus had scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...
s (like a crocodile). In contrast, concluding that Tyrannosaurus had hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
would be a type III inference, and considered unlikely. Type III inferences are only warranted if there is some positive evidence that the extinct creature possessed the trait.