[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Trimerorhachis (Permian temnospondyl) revised; Entelognathus (Silurian placoderm), earliest modern vertebrate jaw, and more



From: Ben Creisler
bcreisler@gmail.com


A number of recent non-dino papers that may be of interest:
Milner, Andrew R. & Schoch, Rainer R. (2013)
Trimerorhachis (Amphibia: Temnospondyli) from the Lower Permian of
Texas and New Mexico: cranial osteology, taxonomy and biostratigraphy.
Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 270(1) : 91-128
DOI: http://dx.doi.org/10.1127/0077-7749/2013/0360
http://www.ingentaconnect.com/content/schweiz/njbgeol/2013/00000270/00000001/art00007

The morphology and taxonomy of the Early Permian temnospondyl
amphibian genus Trimerorhachis are revised. Using previously
undescribed material from Texas, numerous details of the cranial
osteology of T. insignis Cope, 1878 are redescribed. Emphasis is
placed on the structure of the palate, the sutures of the skull roof,
and on the variation of some phylogenetically important
character-states. The taxonomic revision, which makes particular use
of morphometric characters, supports the four species previously
considered to be valid, and adds a fifth species. T. sandovalensis
Berman & Reisz, 1980 from the Cutler Formation of New Mexico is
probably the most primitive species and does not share any unambiguous
derived character-states with the other species. T. insignis – the
type species – is the only one known from the Nocona and Petrolia
formations of Texas. In the later formations up to the Middle Clear
Fork Group, there are two lineages diverging from the T. insignis
morphology in opposite directions. One, here designated T. greggi sp.
nov. , has larger orbits and a shorter snout and occurs in the Lower,
and probably also the Middle, Clear Fork. The other is T. mesops Cope,
1896, which has smaller orbits and a longer snout and occurs in the
Lower and Middle Clear Fork and possibly the earlier Waggoner Ranch
Formation of Texas. T. rogersi Olson, 1955 is a possible late
population of T. mesops type that occurs in the late Middle Clear
Fork. The simplest model is that there was one short-snouted
Trimerorhachis lineage from the base of the Permian to the top of the
Petrolia Formation (contemporaneous with the long-snouted Neldasaurus
in Archer City Formation beds), followed by two diverging lineages
showing some character displacement from the Waggoner Ranch Formation
upwards, but possibly incipiently present in the Nocona/Petrolia
populations.

====
Entelognathus

Min Zhu, Xiaobo Yu, Per Erik Ahlberg, Brian Choo, Jing Lu, Tuo Qiao,
Qingming Qu, Wenjin Zhao, Liantao Jia, Henning Blom & You’an Zhu
(2013)
A Silurian placoderm with osteichthyan-like marginal jaw bones.
Nature (2013) (advance online publication)
doi:10.1038/nature12617
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12617.html

The gnathostome (jawed vertebrate) crown group comprises two extant
clades with contrasting character complements. Notably, Chondrichthyes
(cartilaginous fish) lack the large dermal bones that characterize
Osteichthyes (bony fish and tetrapods). The polarities of these
differences, and the morphology of the last common ancestor of crown
gnathostomes, are the subject of continuing debate. Here we describe a
three-dimensionally preserved 419-million-year-old placoderm fish from
the Silurian of China that represents the first stem gnathostome with
dermal marginal jaw bones (premaxilla, maxilla and dentary), features
previously restricted to Osteichthyes. A phylogenetic analysis places
the new form near the top of the gnathostome stem group but does not
fully resolve its relationships to other placoderms. The analysis also
assigns all acanthodians to the chondrichthyan stem group. These
results suggest that the last common ancestor of Chondrichthyes and
Osteichthyes had a macromeric dermal skeleton, and provide a new
framework for studying crown gnathostome divergence.

news story:
http://www.nature.com/news/ancient-fish-face-shows-roots-of-modern-jaw-1.13823

===

Tyler R. Lyson, Bhart-Anjan S. Bhullar, Gabe S. Bever, Walter G.
Joyce, Kevin de Queiroz, Arhat Abzhanov &  Jacques A. Gauthier (2013)
Homology of the enigmatic nuchal bone reveals novel reorganization of
the shoulder girdle in the evolution of the turtle shell.
Evolution & Development 15(5): 317–325
DOI: 10.1111/ede.12041
http://onlinelibrary.wiley.com/doi/10.1111/ede.12041/abstract

The turtle shell represents a unique modification of the ancestral
tetrapod body plan. The homologies of its approximately 50 bones have
been the subject of debate for more than 200 years. Although most of
those homologies are now firmly established, the evolutionary origin
of the dorsal median nuchal bone of the carapace remains unresolved.
We propose a novel hypothesis in which the nuchal is derived from the
paired, laterally positioned cleithra—dorsal elements of the ancestral
tetrapod pectoral girdle that are otherwise retained among extant
tetrapods only in frogs. This hypothesis is supported by origin of the
nuchal as paired, mesenchymal condensations likely derived from the
neural crest followed by a unique two-stage pattern of ossification.
Further support is drawn from the establishment of the nuchal as part
of a highly conserved “muscle scaffold” wherein the cleithrum (and its
evolutionary derivatives) serves as the origin of the Musculus
trapezius. Identification of the nuchal as fused cleithra is congruent
with its general spatial relationships to other elements of the
shoulder girdle in the adult morphology of extant turtles, and it is
further supported by patterns of connectivity and transformations
documented by critical fossils from the turtle stem group. The
cleithral derivation of the nuchal implies an anatomical
reorganization of the pectoral girdle in which the dermal portion of
the girdle was transformed from a continuous lateral-ventral arc into
separate dorsal and ventral components. This transformation involved
the reduction and eventual loss of the scapular rami of the clavicles
along with the dorsal and superficial migration of the cleithra, which
then fused with one another and became incorporated into the carapace.


===

Kenneth G. MacLeod, Brian T. Huber, Álvaro Jiménez Berrocoso and Ines
Wendler (2013)
A stable and hot Turonian without glacial delta18O excursions is
indicated by exquisitely preserved Tanzanian foraminifera.
Geology 41 (10): 1083-1086
doi: 10.1130/G34510.1
http://geology.gsapubs.org/content/41/10/1083.abstract

A shift from the icehouse climate in which humans evolved to a Late
Cretaceous–like greenhouse climate is an often-repeated cautionary
prediction of the consequences of continued anthropogenic CO2
emissions. The corollary, that understanding the past might help
predict the future, has justified many Late Cretaceous studies, but
important questions remain about climate stability and sensitivity.
New delta18O measurements of more than 1000 samples of exceptionally
well preserved foraminifera (8 planktic and 11 benthic taxa) from two
sites in Tanzania indicate that hot and remarkably stable conditions
prevailed in the region during the Turonian, including during a
proposed greenhouse glacial event. Planktic taxa have delta18O values
largely between –4.0‰ and –5.0‰, suggesting surface-water temperatures
between 30 and 35 °C. Estimates for seafloor temperatures are between
18 and 25 °C. No parallel shifts in delta18O values are observed among
planktic and benthic taxa, contradicting an often-cited line of
evidence for greenhouse glaciations and supporting an effectively
ice-free Turonian world.

===

M. E. Brookfield, T. J. Algeo, R. Hannigan, J. Williams, and G. M. Bhat (2013)
Shaken and stirred: seismites and tsunamites at the Permian-Triassic
boundary, Guryul Ravine, Kashmir, India.
PALAIOS 28:. 568-582,
doi:10.2110/palo.2012.p12-070r
http://palaios.geoscienceworld.org/content/28/8/568.abstract


The famous Permian-Triassic boundary section at Guryul Ravine in
Kashmir shows repeated strong disturbances in the uppermost 3 m of the
section below the main end-Permian mass extinction horizon. Two
one-meter-thick disturbed beds, with convoluted bedding and fluid
escape structures, are interpreted as seismites. Immediately above,
three lenticular, fining-upward, bioclastic grainstone beds,
interbedded with argillites, are interpreted as tsunamites. In these
beds, hummocky cross-stratification and grading indicate deposition by
waning irregular waves at a minimum water depth of 100 m based on
physical processes and faunas. Bed grain sizes indicate that the waves
needed to move even the coarse sand of the matrix, let alone
associated large pebbles up to 20 cm in diameter, range from
amplitudes of ∼40 m for wave periods of 10 s (the upper limit for
storm waves) to amplitudes of ∼3 m for wave periods of 50 to 1000 s
(typical of large open-ocean tsunamis). Fossil and sedimentary
evidence suggests lengthy intervals between successive tsunami events,
which, together with a lack of geochemical evidence for impact, favors
terrestrial causes. Geochemical proxies show that the Guryul Ravine
environment remained oxic or suboxic throughout the P–Tr transition,
but that anoxia developed regionally at the time of the boundary
crisis. This paper is the first to propose seismites and tsunamites at
the P-Tr boundary, so the geographic extent of these deposits is
unknown, although analogous deposits occur in many sections worldwide
from published reports.