Academic Bun fight: Is Nanotyrannus Really a Juvenile Tyrannosaurus?

Damnit, the groove is real and only in Nanotyrannus!

When a groove is not a groove: Clarification of the appearance of the dentary groove in tyrannosauroid theropods and the distinction between Nanotyrannus and Tyrannosaurus. Reply to Comment on: “Distribution of the dentary groove of theropod dinosaurs: implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al., 1988”

Authors:

Schmerge et al

Abstract:

The occurrence of a lateral groove in the dentary of theropod dinosaurs was argued to be a key diagnostic character for establishing the validity of Nanotyrannus lancensis as a unique taxon separate from Tyrannosaurus rex by Schmerge and Rothschild (2016). The validity of this distinction has been challenged in a comment paper by Brusatte et al. (2016). The main criticisms raised in this comment address the methodology of the original study, the distribution of the dentary groove in theropods, the possibility of ontogenetic variability in the occurrence of the dentary groove, and the application of phylogenetic analysis to studying character distributions. In this reply, we clarify the definition of the theropod dentary groove, elucidate the difference between a true dentary groove and the appearance of a false “pseudo-groove”, justify our original methodology with a discussion of the errors involved in identifying grooves by Brusatte et al. (in press), and support our original findings with descriptions of additional specimens. Investigation of additional specimens of Nanotyrannus, as well as critical examination of Tyrannosaurus specimens presented by Brusatte et al. (2016), reaffirm the result of our original study that Nanotyrannus can be differentiated from Tyrannosaurus based on the depth of its dentary groove, independent of ontogenetic stage. Despite any possible ontogenetic variation in the appearance of the dentary groove that can be interpreted, all specimens of Nanotyrannus possess distinct grooves whereas Tyrannosaurus lacks a groove. The most parsimonious explanation for the different appearance of these grooves is that Nanotyrannus does not represent a juvenile Tyrannosaurus.

gimme a break.

Dentary groove morphology does not distinguish ‘Nanotyrannus’ as a valid taxon of tyrannosauroid dinosaur. Comment on: “Distribution of the dentary groove of theropod dinosaurs: Implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al., 1988”

Authors:

Brusatte et al

Abstract:

There has been considerable debate about whether the controversial tyrannosauroid dinosaur ‘Nanotyrannus lancensis’ from the uppermost Cretaceous of North America is a valid taxon or a juvenile of the contemporaneous Tyrannosaurus rex. In a recent Cretaceous Research article, Schmerge and Rothschild (2016) brought a new piece of evidence to this discussion: the morphology of the dentary groove, a depression on the lateral surface of the dentary that houses neurovascular foramina. They argued that an alleged ‘Nanotyrannus’ specimen, which possesses a groove, cannot be referable to Tyrannosaurus rex, which they considered as lacking the groove, and they hypothesized that ‘Nanotyrannus’ is closely related to albertosaurine tyrannosauroids, which also are said to possess the groove. However, we show that the groove is a widespread feature of tyrannosauroids that is present in T. rex and many other specimens, and that it is an ontogenetically variable feature that changes from a sharp, deeply-impressed groove to a shallower sulcus as an individual matures. As a result, the presence or absence of a dentary groove does not clarify the validity of ‘Nanotyrannus’ or its phylogenetic position among tyrannosauroids. We consider it most parsimonious that ‘Nanotyrannus’ specimens belong to juvenile T. rex.

There Were Distinct Major Papionin Primate Clades in the Pliocene/Pleistocene South Africa

Cercopithecoid humeri from Taung support the distinction of major papionin clades in the South African fossil record

Authors:

Gilbert et al

Abstract:

Associated cercopithecoid postcrania are rare in the Plio-Pleistocene fossil record, particularly in the case of South African karst cave sites. However, as clear postcranial differences between major papionin clades have been documented, it should be possible to assign isolated papionin postcrania to the Cercocebus/Mandrillus and Papio/Lophocebus/Theropithecus groups wherever sufficient anatomy is preserved. Here, we demonstrate that two partial humeri preserved at Taung, UCMP 56693 and UCMP 125898, are most likely attributable to the Cercocebus/Mandrillus and Papio/Lophocebus/Theropithecus clades, respectively. Univariate analyses (ANOVAs and t-tests) and multivariate analyses (discriminant function analyses) of humeral features, combined with a phylogenetic analysis of 24 humeral characters, all support our assessment. Given that the overwhelming number of craniodental specimens at Taung are attributable to two papionin taxa, Procercocebus antiquus (a member of the Cercocebus/Mandrillus clade) and Papio izodi (a purported fossil species of the modern genus Papio), we assign UCMP 56693 to Pr. antiquus and UCMP 125868 to P. izodi with a high degree of confidence. Implications for cercopithecoid evolution and biogeography are discussed, with a particular emphasis on these two fossil taxa.

Cladistic analysis of Caseid Synapsids

Cladistic analysis of Caseidae (Caseasauria, Synapsida): using the gap-weighting method to include taxa based on incomplete specimens

Authors:

Romano et al

Abstract:

Occupying the role of primary consumer and having an early–middle Permian age range, caseids (Caseasauria, Synapsida) are fundamental to the interpretation of the early history of terrestrial vertebrate ecosystems. Despite this importance, no comprehensive, species-level phylogenetic study of Caseidae has yet been performed. Herein, we present a phylogenetic analysis of the group, using gap weighting to include poorly known taxa. Besides the description and comments on the resultant topologies, some more general issues concerning cladistic methodologies are briefly addressed. This study highlights the importance of a total-evidence approach, including as many within-group taxa and characters as possible. Continuously varying characters, in the form of indices derived from measurement of individual skeletal elements, proved to be highly important, adding significantly to the resolution of, and support for, recovered trees. The utility of the postcranial skeleton in understanding relationships among basal synapsids is highlighted.

How Theropod Dinosaurs Became Birds

Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds

Authors:

Lee et al

Abstract:

Recent discoveries have highlighted the dramatic evolutionary transformation of massive, ground-dwelling theropod dinosaurs into light, volant birds. Here, we apply Bayesian approaches (originally developed for inferring geographic spread and rates of molecular evolution in viruses) in a different context: to infer size changes and rates of anatomical innovation (across up to 1549 skeletal characters) in fossils. These approaches identify two drivers underlying the dinosaur-bird transition. The theropod lineage directly ancestral to birds undergoes sustained miniaturization across 50 million years and at least 12 consecutive branches (internodes) and evolves skeletal adaptations four times faster than other dinosaurs. The distinct, prolonged phase of miniaturization along the bird stem would have facilitated the evolution of many novelties associated with small body size, such as reorientation of body mass, increased aerial ability, and paedomorphic skulls with reduced snouts but enlarged eyes and brains.

One of the authirs, Darren Naish, has a blog post on the paper.

A New Eryopid Temnospondyl From Early Permian Germany

An Eryops-like interclavicle from the Early Permian of the Saar-Nahe Basin, and a discussion of temnospondyl interclavicle characters

Authors:

Witzmann et al

Abstract:

A large, isolated temnospondyl interclavicle from Rockenhausen, Rhineland-Palatinate, is described that is derived from the Early Permian Meisenheim Formation of the Saar-Nahe Basin, south-western Germany. The element resembles closely the interclavicle of the North American genus Eryops in the following characters that justify its assignment to an eryopid temnospondyl: (1) The broad-ovate shape with a narrow, convex anterior margin and slightly convex or straight anterolateral margins, and the length-to-width ratio of approximately 0.7; (2) the ventral surface with a raised triangular field and the irregular dermal sculpture consisting of transversely oriented ridges, furrows and pits; (3) the anteroposteriorly shortened clavicular facets. The interclavicle from Rockenhausen represents the first unambiguous remain of an eryopid in the Saar-Nahe Basin. In contrast to dissorophoid and stereospondylomorph temnospondyls that constitute autochthonous faunal elements in the lakes of the Saar-Nahe Basin, eryopids seem to be allochthonous and might have been washed in from more distant habitats. Comparison of the morphologies and proportions of temnospondyl interclavicles suggests that the plesiomorphic state for temnospondyls is a proportionally small interclavicle equal in length and width or slightly longer than wide. Interclavicles that are broader than long are a derived character of zatracheids, certain dissorophoids (branchiosaurids) and Eryops and the specimen from the Saar-Nahe Basin, but evolved convergently in these groups. The much enlarged, elongate interclavicles appear to be a synapomorphy of stereospondylomorphs.

Order of the Stick Does Paleo Humor

Go read.

Mammal Morphology Has Roots in the Massively Creative Destruction of Permian Triassic Mass Extinction

The radiation of cynodonts and the ground plan of mammalian morphological diversity

Authors:

1. Marcello Ruta (a)
2. Jennifer Botha-Brink (b,c)
3. Stephen A. Mitchell (d)
4. Michael J. Benton (d)

Affiliations:

a. School of Life Sciences, University of Lincoln, Lincoln LN2 2LG, UK

b. Karoo Palaeontology, National Museum, PO Box 266, Bloemfontein 9300, South Africa

c. Department of Zoology and Entomology, University of the Free State, Bloemfontein 9300, South Africa

d. School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK

Abstract:

Cynodont therapsids diversified extensively after the Permo-Triassic mass extinction event, and gave rise to mammals in the Jurassic. We use an enlarged and revised dataset of discrete skeletal characters to build a new phylogeny for all main cynodont clades from the Late Permian to the Early Jurassic, and we analyse models of morphological diversification in the group. Basal taxa and epicynodonts are paraphyletic relative to eucynodonts, and the latter are divided into cynognathians and probainognathians, with tritylodonts and mammals forming sister groups. Disparity analyses reveal a heterogeneous distribution of cynodonts in a morphospace derived from cladistic characters. Pairwise morphological distances are weakly correlated with phylogenetic distances. Comparisons of disparity by groups and through time are non-significant, especially after the data are rarefied. A disparity peak occurs in the Early/Middle Triassic, after which period the mean disparity fluctuates little. Cynognathians were characterized by high evolutionary rates and high diversity early in their history, whereas probainognathian rates were low. Community structure may have been instrumental in imposing different rates on the two clades.

Are Modern Amphibians Really Lepospondyls?

The origin(s) of extant amphibians: a review with emphasis on the “lepospondyl hypothesis”

Authors:

1. David MARJANOVIĆ (a,b)
2. Michel LAURIN (a)

Affiliations:

a. UMR 7207, CNRS/MNHN/UPMC/Collège de France, Département Histoire de la Terre, case postale 48, 57 rue Cuvier, F-75231 Paris cedex 05, France

b. Abteilung Forschung, Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, D-10115 Berlin, Germany

Abstract:

The origins of the extant amphibians (frogs, salamanders, caecilians) remain controversial after over a century of debate. Three groups of hypotheses persist in the current literature: the “temnospondyl hypothesis” (TH) which roots Lissamphibia Haeckel, 1866 (the smallest clade composed of the extant amphibians) within the Paleozoic temnospondyls, the “lepospondyl hypothesis” (LH) which postulates a monophyletic Lissamphibia nested within the Paleozoic lepospondyls, and the “polyphyly hypothesis” (PH), according to which the frogs and the salamanders are temnospondyls while the caecilians are lepospondyls. The discovery of the Middle Jurassic to Pliocene albanerpetontids, which are very similar to the extant amphibians, has complicated rather than resolved this situation. We present a review of recent publications and theses in this field, several of which show more support for the LH than for the TH and considerably more than for the PH. In addition, we show that there is no particular attraction between long-bodied lissamphibians (caecilians) and long-bodied lepospondyls (such as the lysorophians): when they are removed from two published matrices, reanalyses nonetheless find the LH. In one case the LH is found even when all salamanders are removed as well. We furthermore propose that the complex of characters called the salamander mode of autopodium development is (in its less extreme forms) plesiomorphic for limbed vertebrates, so the apparent presence of this mode of development in temnospondyls cannot support the TH or the PH. Still, a consensus will not be reached soon, despite the increasing range of data and types of analysis that are used (morphological, molecular and combined phylogenetics, development biology, molecular divergence dating, paleontological supertree dating, combined dating, and calculation of confidence intervals on first appearances in the fossil record). We present examples of pertinent character state distributions and explore a large gap in the fossil record of small stegocephalians

Warning: the title link is to a PDF.

New Phylogeny for South American Cretaceous Endemic Mammals (or Meridiolestida finds a new home)

A new phylogeny for basal Trechnotheria and Cladotheria and affinities of South American endemic Late Cretaceous mammals

Authors:

1. Alexander O. Averianov (a,b)
2. Thomas Martin (c)
3. Alexey V. Lopatin (d)

Affiliations:

a. Zoological Institute of the Russian Academy of Sciences, Universitetskaya nab. 1, 199034, Saint Petersburg, Russia

b. Department of Paleontology, Geological Faculty, Saint Petersburg State University, 16 Liniya VO 29, 199178, Saint Petersburg, Russia

c. Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn, Nussallee 8, 53115, Bonn, Germany

d. Borissiak Paleontological Institute of the Russian Academy of Sciences, Profsouznaya str. 123, 117997, Moscow, Russia

Abstract:

The endemic South American mammals Meridiolestida, considered previously as dryolestoid cladotherians, are found to be non-cladotherian trechnotherians related to spalacotheriid symmetrodontans based on a parsimony analysis of 137 morphological characters among 44 taxa. Spalacotheriidae is the sister taxon to Meridiolestida, and the latter clade is derived from a primitive spalacolestine that migrated to South America from North America at the beginning of the Late Cretaceous. Meridiolestida survived until the early Paleocene (Peligrotherium) and early Miocene (Necrolestes) in South America, and their extinction is probably linked to the increasing competition with metatherian and eutherian tribosphenic mammals. The clade Meridiolestida plus Spalacotheriidae is the sister taxon to Cladotheria and forms a new clade Alethinotheria. Alethinotheria and its sister taxon Zhangheotheria, new clade (Zhangheotheriidae plus basal taxa), comprise Trechnotheria. Cladotheria is divided into Zatheria (plus stem taxa, including Amphitherium) and Dryolestida, including Dryolestidae and a paraphyletic array of basal dryolestidans (formerly classified as “Paurodontidae”). The South American Vincelestes and Groebertherium are basal dryolestidans.

Xenopermian: Alas, a Teaser, Alternate Anomodont Phylogeny

This week was also brutal though I did move the ball forward on finishing the post, but just could not get it done for today.  There are three clades left living in the Xenopermian of anomodonts.  They are the descendents of Tiarajudens, the dicynodonts and Venjukoviamorphs (suminids and friends).

Enjoy.