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Development
and constraint among southern placental mammals (Afrotheria &
Xenarthra)
Asher
RJ, Lin KH, Kardjilov N, Hautier LJ. 2011. Variability and constraint
in the mammalian vertebral column. Journal of Evolutionary Biology
24(5): 1080-1090.
We
examined variation in vertebral counts within 42 species of mammals,
representing monotremes, marsupials and major clades of placentals.
These data show that xenarthrans and afrotherians have, on average, a
high proportion of individuals with meristic deviations from species’
median series counts. Monotremes, xenarthrans, afrotherians and
primates show relatively high variation in thoracolumbar vertebral
count. Among the clades sampled in our dataset, rodents are the least
variable, with several species not showing any deviations from median
vertebral counts, or vertebral anomalies such as asymmetric ribs or
transitional vertebrae. Most mammals show significant correlations
between sacral position and length of the rib cage; only a few show a
correlation between sacral position and number of sternebrae. The
former result is consistent with the hypothesis that adult axial
skeletal structures patterned by distinct mesodermal tissues are
modular and covary; the latter is not. Variable levels of correlation
among these structures may indicate that the boundaries of prim ⁄
abaxial mesodermal precursors of the axial skeleton are not uniform
across species. We do not find evidence for a higher frequency of
vertebral anomalies in our sample of embryos or neonates than in
post-natal individuals of any species, contrary to the hypothesis that
stabilizing selection plays a major role in vertebral patterning.
Hautier
LJ, Weisbecker V, Sánchez-Villagra MR, Goswami A, Asher RJ. 2010.
Skeletal development in sloths and the evolution of mammalian vertebral
patterning. Proceedings of the
National Academy of Sciences USA 107(44):18903-18908.
Mammals show a very low level of variation in vertebral count,
particularly in the neck. Phenotypes exhibited at various stages during
the development of the axial skeleton may play a key role in testing
mechanisms recently proposed to explain this conservatism. Here, we
provide osteogenetic data that identify developmental criteria with
which to recognize cervical vs. noncervical vertebrae in mammals.
Except for sloths, all mammals show the late ossification of the
caudal-most centra in the neck after other centra and neural arches. In
sloths with 8–10 ribless neck vertebrae, the caudal-most neck centra
ossify early, matching the pattern observed in cranial thoracic
vertebrae of other mammals. Accordingly, we interpret the ribless neck
vertebrae of three-toed sloths caudal to V7 as thoracic based on our
developmental criterion. Applied to the unusual vertebral phenotype of
long-necked sloths, these data support the interpretation that elements
of the axial skeleton with origins from distinct mesodermal tissues
have repatterned over the course of evolution.
Asher
RJ, Lehmann T. 2008. Dental eruption in afrotherian mammals. BMC Biology
6:14.
Here, we suggest relatively late eruption of the permanent
dentition as
a shared characteristic of afrotherian mammals. This characteristic and
other features (such as vertebral anomalies and testicondy) recall the
phenotype of a human genetic pathology cleidocranial dysplasia),
correlations with which have not been explored previously in the
context of character evolution within the recently established
phylogeny of living mammalian clades. Although data on the absolute
timing of eruption in sengis, golden
moles and tenrecs are still unknown, craniometric comparisons for
ontogenetic series of these taxa show that considerable skull growth
takes place prior to the complete eruption of the permanent cheek
teeth. Specimens showing less than half (sengis, golden moles) or
two-thirds (tenrecs, hyraxes) of their permanent cheek teeth reach or
exceed the median jaw length of conspecifics with a complete dentition.
With few exceptions, afrotherians are closer to median adult jaw length
with fewer erupted, permanent cheek teeth than comparable stages of
non-afrotherians. Manatees (but not dugongs), elephants and hyraxes
with known age data show eruption of permanent teeth late in ontogeny
relative to other mammals. While the occurrence of delayed eruption,
vertebral anomalies and other potential afrotherian synapomorphies
resemble some symptoms of a human genetic pathology, these
characteristics do not appear to covary significantly among mammalian
clades.
Asher
RJ, Bennett N, Lehmann T. 2009. The new framework for understanding
placental mammal evolution. Bioessays
31(8): 853-864.
An unprecedented level of confidence has recently crystallized around a
new hypothesis of how living placental mammals share a pattern of
common descent. The major groups are afrotheres (e.g., aardvarks,
elephants), xenarthrans (e.g., anteaters, sloths), laurasiatheres
(e.g., horses, shrews), and euarchontoglires (e.g., humans, rodents).
Compared with previous hypotheses this tree is remarkably stable;
however, some uncertainty persists about the location of the placental
root, and (for example) the position of bats within laurasiatheres, of
sea cows and aardvarks within afrotheres, and of dermopterans within
euarchontoglires. A variety of names for subclades within the new
placental mammal tree have been proposed, not all of which follow
conventions regarding priority and stability. More importantly, the new
phylogenetic framework enables the formulation of new hypotheses and
testing thereof, for example regarding the possible developmental
dichotomy that seems to distinguish members of the newly identified
southern and northern radiations of living placental mammals.
Evolution
of
endemic African mammals
Asher
RJ, Maree S, Bronner G, Bennett NC, Bloomer P, Czechowski P, Meyer M,
Hofreiter M. 2010. A phylogenetic estimate for golden moles (Mammalia,
Afrotheria, Chrysochloridae). BMC Evolutionary
Biology 10:69.
Golden moles (Chrysochloridae) are small,
subterranean, afrotherian mammals from South Africa and neighboring
regions. Of the 21 species now recognized, some are relatively common,
whereas others
are rare
and endangered. Here, we use a combined analysis of partial sequences
of the nuclear GHR gene and morphological characters to derive a
phylogeny of species in the family Chrysochloridae. We
elevate Huetia to generic
status to include the species leucorhinus and
confirm the use of the Linnean binomial Carpitalpa arendsi, which
belongs within Amblysominae along with
Amblysomus and Neamblysomus.
A
second group, Chrysochlorinae, includes
Chrysochloris, Cryptochloris,
Huetia, Eremitalpa, Chrysospalax, and Calcochloris. Bayesian methods
make chrysochlorines paraphyletic by placing the root within them,
coinciding with root positions favored by a majority of
randomly-generated outgroup taxa. Maximum Parsimony (MP) places the
root either between chrysochlorines and amblysomines (with Chlorotalpa
as sister taxon to amblysomines), or at Chlorotalpa, with the former
two groups reconstructed as monophyletic in all optimal MP trees. Our
optimal
topologies support a division of chrysochlorids into amblysomines and
chrysochlorines, with Chlorotalpa
intermediate between the two.
Furthermore, evolution of the chrysochlorid malleus exhibits homoplasy.
The elongate malleus has evolved just once in the Cryptochloris-
Chrysochloris group; other changes in shape have occurred at
multiple
nodes, regardless of how the root is resolved
Asher RJ, Avery
DM. 2010. New golden moles (Afrotheria, Chrysochloridae) from the
Pliocene of South Africa. Paleontologica Electronica
13(1):3A.
We describe new material of fossil golden moles (Chrysochloridae)
from
the early Pliocene
site of Langebaanweg, South Africa. Based
on size and morphology, at least three species are represented in
this assemblage,
two of which are represented by material of sufficient quality to name.
Based on
relative abundance, humeral and mandibular types can be associated with
other material.
Craniodentally, the most common Langebaanweg species closely resembles the extant
Cape golden mole, Chrysochloris
asiatica, but differs in showing a relatively narrow
distal humerus, proportionally similar to that of the extant Eremitalpa granti. A second, rarer species is
represented by two well-preserved mandibles that exhibit a stout,
enlarged lower second incisor, a robust mandibular corpus, and is
associated with a less common
humeral type that resembles living Chrysochloris.
At least one
additional species is represented by a small number of relatively large
humeri, femora,
and scapular fragments. Because it lacks any craniodental
representation, it is not named in this
paper. We tentatively suggest that the relatively narrow distal margin
of the humerus
of the new, C. asiatica-like
species may have been adapted to a habitat similar to that of
the modern E. granti, a
"sand-swimming" golden mole currently known from
northwestern
South Africa and southern Namibia.
Asher
RJ, Hofreiter M. 2006. Tenrecid phylogeny and the noninvasive
extraction of nuclear DNA. Systematic
Biology 55(2):181-194
Due in part to scarcity of material, no published study has yet
cladistically addressed the systematics of living and fossil Tenrecidae
(Mammalia, Afrotheria). Using a noninvasive technique for sampling
nuclear DNA from museum specimens, we investigate the evolution of the
Tenrecidae and assess the extent to which tenrecids fit patterns of
relationships proposed for other terrestrial mammals on Madagascar.
Application of several tree-reconstruction techniques on sequences of
the nuclear growth hormone receptor gene and morphological data for all
recognized tenrecid genera supports monophyly of Malagasy tenrecids to
the exclusion of the two living African genera. However, both parsimony
and Bayesian methods favor a close relationship between fossil African
tenrecs and the Malagasy Geogale, supporting the hypothesis of island
paraphyly, but not polyphyly. More generally, the noninvasive
extraction technique can be applied with minimal risk to rare/unique
specimens and, by better utilizing museum collections for genetic work,
can greatly mitigate field expenses and disturbance of natural
populations.
Diversification
of therian mammals
Wible
JR , Rougier GW, Novacek MJ, Asher RJ. 2009. The eutherian
mammal
Maelestes gobiensis from the
Late Cretaceous of Mongolia and the
phylogeny of Cretaceous Eutheria. Bulletin
of the American Museum of
Natural History 327:1-123.
Maelestes gobiensis
Wible et al. 2007 is amended to include an incomplete skull, left
dentary, atlas, axis, last cervical and first 11 thoracic vertebrae, 11
partial ribs, incomplete scapula, clavicle, humerus, and proximal
radius and ulna. An astragalus on a separate block was referred to
Maelestes by Wible et al. (2007), but it is too large to belong to this
taxon and is removed from the isotype. Several corrections and updates
are made to the phylogenetic analysis of Wible et al. (2007). The
original analysis and the one in this report include 408 morphological
characters (127 dental, 212 cranial, and 69 postcranial) in Maelestes along with 68 other taxa.
Maelestes is identified as a
member of Cimolestidae sensu Kielan-Jaworowska et al. (2004) along with
the slightly younger and poorer known North American taxa Batodon Marsh, 1892, and Cimolestes Marsh, 1889.
Cimolestidae, in turn, is grouped with Asioryctitheria sensu Archibald
and Averianov (2006), which includes monophyletic Mongolian and
Uzbekistani clades. In contrast to some previous analyses, but in
common with Wible et al. (2007), no Cretaceous eutherians are
identified as members of any placental group.
Sánchez-Villagra
MR, Ladevèze S, Horovitz I, Argot C, Hooker JJ, Macrini TE, Martin T,
Moore-Fay S, deMuizon C, Schmelzle T, Asher RJ. 2007. Exceptionally
preserved North American Paleogene metatherians: adaptations and
discovery of a major gap in the opossum fossil record. Biology
Letters 3(3):318-322.
We studied new exceptionally well-preserved partial skeletons of
the Early Oligocene fossil Herpetotherium
from the White River Formation in Wyoming, which allowed us to test the
relationships of this taxon and examine its adaptations.
Herpetotheriidae, with a fossil record extending from the Cretaceous to
the Miocene, has traditionally been allied with opossums (Didelphidae)
based on fragmentary material, mainly dentitions. Analysis of the new
material reveals that several aspects of the cranial and postcranial
anatomy, some of which suggests a terrestrial lifestyle, distinguish
Herpetotherium from opossums. We found that Herpetotherium is the sister group
to the crown group Marsupialia and is not a stem didelphid. Combination
of the new palaeontological data with molecular divergence estimates,
suggests the presence of a long undocumented gap in the fossil record
of opossums extending some 45 Myr from the Early Miocene
to the Cretaceous.
Asher
RJ, Meng J, Wible JR, McKenna MC, Rougier GW, Dashzeveg D, Novacek MJ.
2005. Stem Lagomorpha and the Antiquity of Glires. Science 307 (5712): 1091-1094.
We
describe several fossils referable to Gomphos
elkema from deposits close to the Paleocene-Eocene boundary at
Tsagan Khushu, Mongolia. Gomphos
shares a suite of cranioskeletal characters with extant rabbits, hares,
and pikas but retains a primitive dentition and jaw compared to its
modern relatives. Phylogenetic analysis supports the position of Gomphos as a stem lagomorph and
excludes Cretaceous taxa from the crown radiation of placental mammals.
Our results support the hypothesis that rodents and lagomorphs radiated
during the Cenozoic and diverged from other placental mammals close to
the Cretaceous-Tertiary boundary.