Category Archives: News

Body-size transitions and trait evolution in therapsids and archosauromorphs

(Notice the clever use of the word ‘dinosaur’ in the title, even if dinosaurs are only a small part of the study)

Rise of dinosaurs reveals major body-size transitions are driven by passive processes of trait evolution

Roland B. Sookias, Richard J. Butler and Roger B. J. Benson

A major macroevolutionary question concerns how long-term patterns of body-size evolution are underpinned by smaller scale processes along lineages. One outstanding long-term transition is the replacement of basal therapsids (stem-group mammals) by archosauromorphs, including dinosaurs, as the dominant large-bodied terrestrial fauna during the Triassic (approx. 252– 201 million years ago). This landmark event preceded more than 150 million years of archosauromorph dominance. We analyse a new body-size
dataset of more than 400 therapsid and archosauromorph species spanning the Late Permian– Middle Jurassic. Maximum-likelihood analyses indicate that Cope’s rule (an active within-lineage trend of body-size increase) is extremely rare, despite conspicuous patterns of body-size turnover, and contrary to proposals that Cope’s rule is central to vertebrate evolution. Instead, passive processes predominate in taxonomically and ecomorphologically more inclusive clades, with stasis common in less inclusive clades. Body-size limits are clade-dependent, suggesting intrinsic, biological factors are more important than the external environment. This clade-dependence is exemplified by maximum size of Middle–early Late Triassic archosauromorph predators exceeding that of contemporary herbivores, breaking a widelyaccepted ‘rule’ that herbivore maximum size greatly exceeds carnivore maximum size. Archosauromorph
and dinosaur dominance occurred via opportunistic replacement of therapsids following extinction, but were facilitated by higher archosauromorph growth rates.

Keywords: evolutionary trends; body size; Cope’s rule; Archosauromorpha; Therapsida; Permo-Triassic

http://rspb.royalsocietypublishing.org/content/early/2012/01/31/rspb.2011.2441

__________________________________
Laura

Robust estimates of extinction time in the geological record

Abstract
The rate at which a once-abundant population declines in density prior to local or global extinction can strongly influence the precision of statistical estimates of extinction time. Here we report the development of a new, robust method of inference which accounts for these potential biases and uncertainties, and test it against known simulated data and dated Pleistocene fossil remains (mammoths, horses and Neanderthals). Our method is a Gaussian-resampled, inverse-weighted McInerny et al. (GRIWM) approach which weights observations inversely according to their temporal distance from the last observation of a species’ confirmed occurrence, and for dates with associated radiometric errors, is able to sample individual dates from an underlying fossilization probability distribution. We show that this leads to less biased estimates of the ‘true’ extinction date. In general, our method provides a flexible tool for hypothesis testing, including inferring the probability that the extinctions of pairs or groups of species overlap, and for more robustly evaluating the relative likelihood of different extinction drivers such as climate perturbation and human exploitation.
Fulltext:

Determining the natural length of the current interglacial

abstract:
No glacial inception is projected to occur at the current atmospheric CO2 concentrations of 390 ppmv (ref. 1). Indeed, model experiments suggest that in the current orbital configuration—which is characterized by a weak minimum in summer insolation—glacial inception would require CO2 concentrations below preindustrial levels of 280 ppmv (refs 2, 3, 4). However, the precise CO2threshold4, 5, 6 as well as the timing of the hypothetical next glaciation7 remain unclear. Past interglacials can be used to draw analogies with the present, provided their duration is known. Here we propose that the minimum age of a glacial inception is constrained by the onset of bipolar-seesaw climate variability, which requires ice-sheets large enough to produce iceberg discharges that disrupt the ocean circulation. We identify the bipolar seesaw in ice-core and North Atlantic marine records by the appearance of a distinct phasing of interhemispheric climate and hydrographic changes and ice-rafted debris. The glacial inception during Marine Isotope sub-Stage 19c, a close analogue for the present interglacial, occurred near the summer insolation minimum, suggesting that the interglacial was not prolonged by subdued radiative forcing7. Assuming that ice growth mainly responds to insolation and CO2 forcing, this analogy suggests that the end of the current interglacial would occur within the next 1500 years, if atmospheric CO2 concentrations did not exceed 240±5 ppmv.

Let’s dry up a remnant ocean

http://www.nature.com/nature/journal/v480/n7377/full/nature10651.html

Garcia-Castellanos, D. & Villasenor, A., 2011: Messinian salinity crisis
regulated by competing tectonics and erosion at the Gibraltar arc.
–Nature: Vol. 480, #7377, pp. 359-363 [doi: 10.1038/nature10651]

The Messinian salinity crisis1, 2 (5.96 to 5.33 million years ago) was
caused by reduced water inflow from the Atlantic Ocean to the Mediterranean
Sea resulting in widespread salt precipitation and a decrease in
Mediterranean sea level of about 1.5 kilometres due to evaporation3. The
reduced connectivity between the Atlantic and the Mediterranean at the time
of the salinity crisis is thought to have resulted from tectonic uplift of
the Gibraltar arc seaway and global sea-level changes, both of which control
the inflow of water required to compensate for the hydrological deficit of
the Mediterranean1, 4. However, the different timescales on which tectonic
uplift and changes in sea level occur are difficult to reconcile with the
long duration of the shallow connection between the Mediterranean and the
Atlantic5 needed to explain the large amount of salt precipitated. Here we
use numerical modelling to show that seaway erosion caused by the Atlantic
inflow could sustain such a shallow connection between the Atlantic and the
Mediterranean by counteracting tectonic uplift. The erosion and uplift rates
required are consistent with previous mountain erosion studies, with the
present altitude of marine sediments in the Gibraltar arc6, 7 and with
geodynamic models suggesting a lithospheric slab tear underneath the
region8, 9, 10. The moderate Mediterranean sea-level drawdown during the
early stages of the Messinian salinity crisis3, 5 can be explained by an
uplift of a few millimetres per year counteracted by similar rates of
erosion due to Atlantic inflow. Our findings suggest that the competition
between uplift and erosion can result in harmonic coupling between erosion
and the Mediterranean sea level, providing an alternative mechanism for the
cyclicity observed in early salt precipitation deposits and calling into
question previous ideas regarding the timing of the events that occurred
during the Messinian salinity crisis1.

– Mikko

Deinonychus

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0028964

The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds

Denver W. Fowler, Elizabeth A. Freedman, John B. Scannella, Robert E. Kambic

Abstract
Most non-avian theropod dinosaurs are characterized by fearsome serrated teeth and sharp recurved claws. Interpretation of theropod predatory ecology is typically based on functional morphological analysis of these and other physical features. The notorious hypertrophied ‘killing claw’ on pedal digit (D) II of the maniraptoran theropod Deinonychus (Paraves: Dromaeosauridae) is hypothesized to have been a predatory adaptation for slashing or climbing, leading to the suggestion that Deinonychus and other dromaeosaurids were cursorial predators specialized for actively attacking and killing prey several times larger than themselves. However, this hypothesis is problematic as extant animals that possess similarly hypertrophied claws do not use them to slash or climb up prey. Here we offer an alternative interpretation: that the hypertrophied D-II claw of dromaeosaurids was functionally analogous to the enlarged talon also found on D-II of extant Accipitridae (hawks and eagles; one family of the birds commonly known as “raptors”). Here, the talon is used to maintain grip on prey of subequal body size to the predator, while the victim is pinned down by the body weight of the raptor and dismembered by the beak. The foot of Deinonychus exhibits morphology consistent with a grasping function, supportive of the prey immobilisation behavior model. Opposite morphological trends within Deinonychosauria (Dromaeosauridae + Troodontidae) are indicative of ecological separation. Placed in context of avian evolution, the grasping foot of Deinonychus and other terrestrial predatory paravians is hypothesized to have been an exaptation for the grasping foot of arboreal perching birds. Here we also describe “stability flapping”, a novel behaviour executed for positioning and stability during the initial stages of prey immobilisation, which may have been pivotal to the evolution of the flapping stroke. These findings overhaul our perception of predatory dinosaurs and highlight the role of exaptation in the evolution of novel structures and behaviours.

– Jacqueline

Microraptor

http://www.pnas.org/content/108/49/19662.abstract?etoc

Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds

Jingmai O’Connor, Zhonghe Zhou, and Xing Xu

Abstract

Preserved indicators of diet are extremely rare in the fossil record; even more so is unequivocal direct evidence for predator–prey relationships. Here, we report on a unique specimen of the small nonavian theropod Microraptor gui from the Early Cretaceous Jehol biota, China, which has the remains of an adult enantiornithine bird preserved in its abdomen, most likely not scavenged, but captured and consumed by the dinosaur. We provide direct evidence for the dietary preferences of Microraptor and a nonavian dinosaur feeding on a bird. Further, because Jehol enantiornithines were distinctly arboreal, in contrast to their cursorial ornithurine counterparts, this fossil suggests that Microraptor hunted in trees thereby supporting inferences that this taxon was also an arborealist, and provides further support for the arboreality of basal dromaeosaurids.

– Jacqueline