Echimyidae is the family^[2] of neotropical spiny rats and their fossil relatives.^[3] This is the most species-rich family of hystricognath rodents.^[4] It is probably also the most ecologically diverse, with members ranging from fully arboreal to terrestrial to fossorial to semiaquatic habits.^[4] They presently exist mainly in South America; three members of the family also range into Central America, and the hutias are found in the West Indies in the Caribbean. Species of the extinct subfamily Heteropsomyinae formerly lived on Cuba, Hispaniola, and Puerto Rico in the Antilles.

Quick Facts Scientific classification, Type genus ...

Echimyidae Temporal range: Late Oligocene–Recent PreꞒ Ꞓ O S D C P T J K Pg N
Several members of the Echimyidae. From top-left, clockwise: coypu, Ferreira's spiny tree-rat, Atlantic spiny rat, Desmarest's hutia.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Rodentia
Suborder:	Hystricomorpha
Infraorder:	Hystricognathi
Parvorder:	Caviomorpha
Superfamily:	Octodontoidea
Family:	Echimyidae Gray, 1825
Type genus
Echimys F. Cuvier, 1809
Subfamilies
Capromyinae Echimyinae Euryzygomatomyinae Carterodontinae †Adelphomyinae †Eumysopinae †Heteropsomyinae

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In general form, most spiny rats resemble rats, although they are more closely related to guinea pigs and chinchillas. Most species have stiff, pointed hairs, or spines, that presumably serve for protection from predators.

Many echimyids can break off their tails when attacked. This action may confuse predators long enough for the spiny rat to escape. Unlike the tails of some species of lizards, however, the tails of spiny rats do not regenerate. Therefore, the tactic can only be used once in an individual's lifetime.

Most spiny rats are rare and poorly known, but a few are extremely abundant. Various species are respectively terrestrial, arboreal, or fossorial. In general, the arboreal forms are most rat-like in appearance, whilst the burrowing species are more gopher-like, with stocky bodies and short tails. Most species do poorly in conditions of high heat and aridity and are restricted to regions with abundant water. They are almost exclusively herbivorous.

The current taxonomic content of the family Echimyidae has been reshaped over time, and its organization into coherent units stems from two realizations. The first is that cladistic approaches applied to morphological characters showed that many traits used to define taxa were plesiomorphic or homoplastic. The second realization came from the advent of phylogenetic analyses of DNA sequence and protein sequence data with probability methods — maximum likelihood and Bayesian inference — leading to the identification of robust clades and the recognition of higher categorical ranks (see Phylogeny section).^[5][6] The following table recapitulates and compares the taxonomic content of taxa recognized on molecular and traditional basis: the two families Capromyidae and Myocastoridae, the five subfamilies Echimyinae, Euryzygomatomyinae, Capromyinae, Dactylomyinae, Eumysopinae, and the four tribes Echimyini, Myocastorini, Capromyini, and Plagiodontini.

The phylogenetic tree of the Echimyidae shows a major split between the subfamily Echimyinae and an assemblage containing the Euryzygomatomyinae, Carterodon, and the Capromyidae. The first major clade contains a majority of arboreal genera (e.g., Phyllomys, Dactylomys, and Mesomys), a few terrestrial taxa (e.g., Proechimys), and a subaquatic one (Myocastor). The second major clade includes fossorial genera (e.g., Euryzygomatomys or Carterodon), a terrestrial one (Trinomys), and members inhabiting the Caribbean islands (Capromyidae).