Classification and Phylogeny

Domain:  Eukarya
Kingdom:  Animalia
Phylum:  Chordata
Class:  Reptilia
Order:  Squamata
Family:  Viperidae
Genus:  Crotalus
Crotalus horridus

Domain EukaryaC. horridus is part of this domain primarily because its cells contain membrane-bound organelles, a true nucleus, and genetic material that is comprised of multiple, linear strands of chromosomes.

Kingdom Animalia: C. horridus is considered an animal because it is eukaryotic, multicellular and heterotrophic, meaning it must  consume other organisms in order to acquire its energy.  Also, like other animals, it is motile, its cells lack a cell wall, and its embryo undergoes a blastula stage during development.

Phylum ChordataC. horridus falls under this phylum for having a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, which is a precursor to the thyroid gland, and a post anal-tail at some point in its life cycle.

Class Reptilia:  C. horridus belongs to this class because its skin is covered with scales, it is cold-blooded and it is air-breathing.  Though it doesn't possess four limbs, C. horridus is considered to be a tetrapod due to the fact that it descended from vertebrates that had four limbs, which classifies it as a reptile.  Lastly, the embryos within its eggs are surrounded by a protective  membrane known as an amnion.

Order Squamata:  Members of this order are characterized by the presence of horny scales or shields on the surface of the skin, as well as the possession of specialized quadrate bones that allow them to move the upper jaw relative to the braincase.  This powerful movement  permits animals of this order  to devour other organisms whole.

Family Viperidae:  This family is composed of organisms that possess relatively long, hinged fangs designed for deep penetration and venom injection.

Genus Crotalus:  The genus Crotalus is defined by a group of venomous pit vipers found only in various regions of North and South America.  Currently, there are only twenty-nine identified species, including C. horridus.  Pit vipers are named for the presence of a two facial pits located on either side of the head, long movable fangs and specialized venom glands that secrete deadly venom.  The Genus Crotalus comes from the Latin word crotalum, meaning "bell" or "rattle", and from the Greek word krotalon, meaning "rattle", "castanet", or "little bell".

Species Crotalus horridusC. horridus is one of the few types of rattlesnakes found in the majority of the eastern portion of the United States, including Wisconsin and Minnesota.  The specific epithet,  horridus, comes from a Latin word meaning "dreadful" and is referring to the venom it produces.  It also signifies "rough" or "bristly", referring to its scaly, bumpy skin.


*Classification information found at

The general phylogenetic tree above displays the five main Supergroups within the domain Eukarya.  Basically, eukaryotes are defined by many features that separate them from the prokaryotes, including the presence of an endomembrane system made up of various intracellular organelles, a cytoskeleton consisting of microtubules and microfilaments, the presence of multiple, linear chromosomes contained within a membrane-bound nucleus and many more. By utilizing molecular data, scientists were able to reconstruct phylogenies that resolved any confusion regarding the differences between the microbial eukaryotes, and the multicellular eukaryotes.  This early molecular data was based on small subunit ribosomal RNA gene sequences that aided in constructing a ladder that started with basal lineages and ended with multicellular groups, such as the animals, plants and fungi. Within the Eukarya domain, five main Supergroups have emerged.  The differences between the Supergroups are based on   molecular data, morphological characteristics and  inter-Supergroup relationships concerning cell structure and composition.  Each Supergroup contains  various lineages leading to a distinct kingdom or other major grouping, such as  the animals or fungi.  One of these Supergroups, the Unikonta, houses the animals.  As shown above, the animals are members of a more specific grouping within the Unikonta known as the Opisthokonts. This grouping is characterized by two features uncommon to all other eukaryotes, which includes flagellated cells containing a single flagellum situated at the posterior end of the cell, as well as flat, mitochondrial cristae found within the cells themselves.  Due to these defining characteristics, animals are most closely related to a group of unicellular protists known as the choanoflagellates. Since C. horridus is an animal, it possess these key features common to all Opisthokonts, and therefore resides in the Unikonta Supergroup .  Other familiar members of the Unikonta, specifically the Opisthokonts, include the true Fungi, the parasitic Ichthyosporea, and a group of amoebae called the Nucleariidae.

    This phylogenetic tree is much more refined than the first tree presented and contains several features that classify the animals into more specific groupings.  The tree directly above is based on paleontological evidence and evolutionary relationships among the various clades that stem from a major group of animals known as the Amniotes.  According to the fossil record, which has documented the origin and evolution of the Amniotes, relationships between fossils have suggested that the Amniotes first diverged into two lineages.  The first clade,  the synapsids (Synapsida), is comprised of the mammals and their extinct relatives, while the other clade, the reptiles (Reptilia) houses a clade of its own known as the diapsids, which includes all of the living reptiles that we are familiar with today, as well as the birds.  The diapsids (Diapsida) encompass all of the living lizards, crocodiles, birds, Sphenodon,  and turtles that exist today and all of their extinct relatives, like the dinosaurs. 

In the tree shown directly above, the diapsids have been divided further into two clades: the lepidosaurs (Lepidosauromorpha) and the archosaurs (Archosauromorpha).  Lepidosaurs consist of the lizards, snakes and Shenodon (Tuataras), while the archosaurs occupy the birds and crocodiles.  Besides these two groups, the tree also shows many of their extinct relatives, which are indicated by a yellow cross.  This tree is based on paleontological evidence as well as morphological differences, particularly skeletal.  The diaspsids were given their name primarily because they contain two fenestrae, or holes, within the temporal region of the skull, either in the upper or lower region.  Some diapsids, like the lizards, have lost the lower fenestrae only, while others, such as the snakes, have lost both.  This is one of the skeletal differences that separates the lepidosaurs from the archosaurs.  Within the subclass Lepidosauromorpha, there exists a more specific group known as the sqaumates (order Squamata), which includes the lizards and snakes only.  These sqaumates share a recent common ancestor with Sphenodon (Tuataras), hence all three are lepidosaurs and are part of the Diapsida lineage.  As stated above, C. horridus is a type of snake (rattlesnake) and is therefore an avid member of the order Squamata and all of the clades that precede it.

*To view more phylogenic trees visit Tree of

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