Turtles are an old and complex group
of animals with a confusing taxonomic history. The turtles often called
"freshwater turtles" of the family Emydidae have been particularly
troublesome. As it happens morphology, mtDNA, and nuclear DNA produce
conflicting results for their relationships. In a new paper, Uwe Fritz and colleagues review the
situation and make a recommendation on what names should be applied to some of
the North American pond turtles, formerly placed in the genus Clemmys.
Duméril (1806) established the genus Emys for virtually all freshwater
turtles known at the time. His genus contained more than 90 species which are
now known to be scattered in multiple families representing many distinct
turtle lineages (Chelidae, Chelydridae, Dermatemydidae, Emydidae, Geoemydidae,
Kinosternidae, Pelomedusidae, Platysternidae, Podocnemididae, and
Testudinidae). Boulenger (1889) limited the genus Emys to two species, the
European Pond Turtle, Emys orbicularis and the North American Blanding's
Turtle, Emys blandingii. This arrangement
remained until 1957 when Loveridge and Williams transferred the Blanding's
Turtle to the genus Emydoidea.
Blanding's Turtles have a unique skull, neck and thoracic rib morphology which
more closely resembles the Chicken Turtle, Deirochelys
reticularia. Some other species previously in Emys were moved to Ritgen's
genus Clemmys established in 1828. From the early 19th century Clemmys contained Old and New World
freshwater turtles that were considered unspecilaized species lacking distinct
morphological. The exception was Louis Agassiz, in 1857 he considered each of
the New World species assigned to Clemmys
as a representative of a distinct genera (Actinemys marmorata, Calemys muhlenbergii, Glyptemys insculpta, Nanemys guttata). Sam McDowell's 1964
osteological study revising the ‘aquatic Testudinidae’, Restricted Clemmys to the four Nearctic
species Clemmys guttata, C.
insculpta, C. marmorata and C. muhlenbergii, while the remaining Old
World species were transferred to the genera Mauremys and Sacalia.
McDowell (1964) realized Old World and New World freshwater turtles represent highly
distinct groups and placed all Old World species plus the extraterritorial
Neotropical genus Rhinoclemmys in the subfamily Batagurinae and the New World
species plus the Palaearctic genus Emys
in the Emydinae. These two subfamilies constituted, along with land tortoises
(Testudininae), the family Testudinidae in McDowell’s (1964) classification.
This arrangement is the one that is retained to the present, except each of these
groups is now treated as a full family and the name Geoemydidae replaced
Bataguridae because of name priority. McDowell recognized the close
relationship of the four Nearctic Clemmys
species, with the box turtles of the genus Terrapene and the Old World Pond Turtle, Emys orbicularis. He placed all of them in the ‘Emys complex’. However he did not
include the Blanding's Turtle, Emydoidea
blandingii. Instead he placed it with the distinct Chicken Turtle in the ‘Deirochelys complex’ (Emydoidea blandingii + Deirochelys reticularia). In 1974
Bramble pointed out, the morphology of structures associated with the plastral
hinge of Emydoidea argues rather for
a close relationship of Emydoidea
with Emys and Terrapene, and not with
Deirochelys. The plastral hinge of Emys, Emydoidea and Terrapene consists
of ligamentous tissue that allows for almost complete closure of the shell, a
trait better developed in Terrapene. Based upon this, Gaffney and
Meylan concluded that Emys, Emydoidea and Terrapene represent a monophyletic group within the subfamily
Emydinae (as opposed to the subfamily Deirochelyinae within the family
Emydidae). The three genera share not only a plastral hinge, but also a divided
scapula, a unique character among living turtles. The morphological similarity
of these structures of Emydoidea, Emys and Terrapene was unique enough that Bramble (1974) concluded the
plastral hinge could not have evolved more than once. The four Clemmys species lacked not only the
plastral hinge, but also all of the complicated morphological structures
associated with this character, and were considered to have a basal
phylogenetic position within Emydinae, an assumption already assumed by
McDowell and Bramble. Thus, Gaffney and
Meylan placed all other emydid genera (Chrysemys,
Deirochelys, Graptemys, Malaclemys, Pseudemys, Trachemys) in another subfamily (Deirochelyinae) within the
Emydidae. By the mid 1990's mitochondrial DNA was revolutionizing how we looked
at evolutionary relationships and Bickham and colleagues presented data that Clemmys is paraphyletic with respect to
all other genera of the subfamily Emydinae (Emys,
Emydoidea, Terrapene), and that the Spotted Turtle (guttata) was sister to all other emydines. Thus, the Wood Turtle, C. insculpta and the Bog Turtle, C. muhlenbergii, formed the sister group
to a major clade divided into a subclade with the European Pond Turtle, Emys orbicularis, Emydoidea
blandingii, and C. marmorata, and another subclade with all studied Terrapene
species as its sister group. Bickham and colleagues used evidence from morphology,
behavior and life history, to show the hinged taxa nested within Clemmys species. This prompted Burke et
al. in 1996 to expanding the genus Emys
to include all emydine species except C.
insculpta and C. muhlenbergii.
To add to the confusion nuclear genomic
data produced conflicting results, depending on which genes were used. The
Spotted Turtle, Clemmys guttata,
showed up as the sister to ((Emydoidea + Emys) + Actinemys) + Terrapene or
as the sister to Actinemys marmorata
and these two species together are the sister group of (Emydoidea + Emys). Box
turtles, Terrapene then appear to be
the sister to (Actinemys marmorata + Clemmys guttata) + (Emydoidea + Emys). The
contradictory branching patterns depends upon the selected loci and suggest a
lineage sorting problem. Ignoring the unclear phylogenetic position of Actinemys marmorata, one recently
proposed classification scheme placed Actinemys
marmorata, Emydoidea blandingii, Emys orbicularis, and Emys trinacris (the Sicilian Pond Turtle) in one genus (Emys), while another classification
scheme treats Actinemys, Emydoidea, and Emys as distinct genera. Fritz et al.
consider the inclusion of Actinemys
in the same taxon as Emydoidea + Emys as unacceptable under a
phylogenetic classification framework because of evidence for the non-monophyly
of this clade. The genra Actinemys, Emydoidea, and Emys are morphologically distinct, and their differences exceed the
differences that typically occur among species of the same genus. Thus they
recommend continued usage of the distinct genera Actinemys, Emydoidea and Emys. To find the full text of this
paper follow the link below.
Citation
Labels: chelonians, nomenclature, systematics