Cladistically, it is defined as
Crocodylus niloticus (the
Nile crocodile) and all
crocodylians more closely related to
C. niloticus than to either
Alligator mississippiensis (the
American alligator) or
Gavialis gangeticus (the
gharial). This is a
stem-based definition for
crocodiles, and is more inclusive than the
crown group Crocodylidae. As a crown group, Crocodylidae only includes the
last common ancestor of all
extant (living) crocodiles and their descendants (living or
extinct), whereas Crocodyloidea, as a stem group, also includes more
basal extinct crocodile ancestors that are more closely related to living crocodiles than to
alligators or
gavialids. When considering only living taxa (
neontology), this makes Crocodyloidea and Crocodylidae
synonymous, and only Crocodylidae is used. Thus, Crocodyloidea is only used in the context of
paleontology. Traditionally, crocodiles and alligators were considered more closely related and grouped together in the clade
Brevirostres, to the exclusion of the
gharials. This classification was based on
morphological studies primarily focused on analyzing skeletal traits of living and extinct fossil species. However, recent molecular studies using
DNA sequencing have rejected Brevirostres upon finding the crocodiles and gavialids to be more closely related than the alligators. The new clade
Longirostres was named by Harshman
et al. in 2003. A 2018
tip dating study by Lee & Yates simultaneously using
morphological, molecular (
DNA sequencing), and
stratigraphic (
fossil age) data established the inter-relationships within
Crocodilia, which was expanded upon in 2021 by Hekkala
et al. using
paleogenomics by extracting DNA from the extinct
Voay. The below
cladogram shows the results of the latest study, and how Crocodyloidea may only contain one additional
taxon beyond Crocodylidae: }} ==References==