Robert R. Reisz

CHAPTER 2 – A NEW PERSPECTIVE ON TETRAPOD PHYLOGENY

This chapter provides an in-depth explanation of the origin of amniotes. It follows articles from various authors, especially those that compare hypotheses on the origin of lissamphibians where Temnospondyls and Lepospondyls were considered. Several investigations have been made on the closest relatives of amniotes like the Anthracosaurs, Aiadectomorpha, and the Seymouriamorphs that can provide insights to the origin and determines which early terrestrial Choanates are relatives of amniotes, Lissamphibians, and the crown group Tetrepoda. Some conclusions indicate that Lissanphibubians are part of Temnospongyli and have other relatives like Microsauria, Colosteidae, Nectridea, and Ichthyostegidae. Diadectormorpha is considered to be the sister of Amniota and that Seymouriamorpha, Anthracosauroidaeae, Crassigyrinus, and Loxommatoidea were related to this clade and thus all known terrestrial Paleozoic Choanates except Ichthyostegids were part of lissamphibians and amniotes. Various method matrices have been used to come up with the different results and conclusive discussions of the comparisons of previous phylogenies, decay index and robustness of the clades, the origin of Lissamphibians, the status of Anthracosaurs, status of the crown group of terrestrial Choanates, and the evolution of selected characters.

A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders

The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli or Lepospondyli, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls. Recent molecular analyses are also controversial, with estimations for the batrachian (frog–salamander) divergence significantly older than the palaeontological evidence supports. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians and caudatans from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.

The first articulated skeleton of Dendrerpeton acadianum (Temnospondyli, Dendrerpetontidae) from the Lower Pennsylvanian locality of Joggins, Nova Scotia, and a review of its relationships

ABSTRACT The discovery of the first articulated, well-preserved specimen of Dendrerpeton acadianum makes it possible to describe this taxon completely for the first time, and reassess its phylogenetic relationships. Large, laterally facing orbits, large, rounded squamosal embayments, absence of lateral line sulci, a short presacral column of only 24 vertebrae that is less than twice the length of the skull, and large stout limbs all suggest a terrestrial lifestyle distinct from the aquatic and semiaquatic adaptations of most contemporary Carboniferous amphibians. Although Dendrerpeton bears a general resemblance to the highly terrestrial dissorophoid temnospondyls and even shares with them a large squamosal embayment supported ventrally by the quadratojugal and a quadrate process, phylogenetic analysis does not support a sister-group relationship. Rather, Dendrerpeton is best considered the primitive sistergroup of a clade of temnospondyls including trimerorhachoids, “eryopoids” (Eryops, Parioxys, and Scl...

The origin and early evolutionary history of amniotes

Recent phylogenetic analyses of Paleozoic tetrapods have yielded startling new insights into the origin and early evolutionary history of amniotes. The origin of this successful group involves evolutionary innovations that are associated with the development of the cleidoic egg and related reproductive strategies, and are therefore not represented directly in the fossil record. Despite this obvious difficulty, recent studies have been able to distinguish Paleozoic amniotes from their anamniotic tetrapod relatives to determine major patterns of interrelationships.

Owenetta kitchingorum, sp. nov., a small parareptile (Procolophonia: Owenettidae) from the Lower Triassic of South Africa

Abstract The parareptile Owenetta rubidgei is known from several skulls, all found in Upper Permian sediments of South Africa. The anatomy of a new species of Owenetta, O. kitchingorum, from the Early Triassic is described here on the basis of three nearly complete skeletons. This new species is distinguished from O. rubidgei on the basis of three dental and three cranial features. Postcranial features cannot be used in the diagnosis because the postcranial anatomy of O. rubidgei is unknown. The small parareptile Barasaurus from the Late Permian of Madagascar and Owenetta are united in the clade Owenettidae, as the sister-taxon of Procolophonidae. Controversies surrounding parareptilian phylogeny are discussed as they relate to Owenetta.

ANATOMY AND RELATIONSHIPS OF LAMBEOSAURUS MAGNICRISTATUS, A CRESTED HADROSAURID DINOSAUR (ORNITHISCHIA) FROM THE DINOSAUR PARK FORMATION, ALBERTA

Abstract The first detailed description of the lambeosaurine Lambeosaurus magnicristatus (Ornithischia: Hadrosauridae) confirms that it is a distinct taxon characterized by its comparatively enormous cranial crest, formed predominantly by the caudodorsal process of the premaxilla, and an acute crest-snout angle. The holotype of L. magnicristatus occurs stratigraphically higher than all other Dinosaur Park Formation lambeosaurines at the Dinosaur Provincial Park locality. The only referred specimen was collected over 170 kilometers southeast of the type locality. Correlation of its host stratum with the well-known Dinosaur Park section reveals that L. magnicristatus has no biostratigraphic overlap with L. lambei and suggests that it replaces L. lambei on a regional scale in southern Alberta at the end of ‘Dinosaur Park time.’ Species-level phylogenetic analysis of Lambeosaurinae corroborates the monophyly of Lambeosaurus. The genus is characterized by five apomorphies, including a procumbent crest, complete enclosure of the ophthalmic canal of the laterosphenoid, the presence of a flange on the caudodorsal process of the premaxilla that overlaps the nasal in the rostral region of the crest, caudal extension of the premaxilla such that it forms the caudal margin of the crest, and a unique joint between the rostral nasal and the caudodorsal process of the premaxilla. Lambeosaurine phylogeny indicates that the development of a hypertrophied cranial crest evolved independently at least three times within the clade, suggesting that the crest enlargement is a recurring evolutionary trend within Lambeosaurinae.

The anatomy and relationships of the Lower Permian reptile Araeoscelis

ABSTRACT Two recently discovered skeletons of the enigmatic Lower Permian reptile Araeoscelis provide important, additional information on its osteology and permit a reassessment of its phylogenetic relationships. A combination of primitive and derived characters shared by Araeoscelis and Petrolacosaurus justifies their inclusion in the suborder Araeoscelidia as members of separate families and indicates that they represent the earliest, most primitive known stage in the adaptive radiation of the diapsid reptiles. Araeoscelis exhibits a suite of unusual cranial features resulting in a massive, sturdily constructed skull, which is interpreted as an adaptation to a specialized diet that probably included invertebrates protected by heavy exoskeletons. This theory is also used to explain the absence of a lower temporal fenestra in Araeoscelis as the result of secondary closure. Some cranial features are noted that support this hypothesis.

Palaeontology: Turtle origins out to sea

Various aspects of turtle evolution are the subject of vigorous debate among vertebrate palaeontologists. A newly described fossil species, the oldest yet discovered, adds grist to the mill.

The Early Permian reptile Acleistorhinus pteroticus and its phylogenetic position

ABSTRACT Restudy of Acleistorhinus pteroticus indicates that this Early Permian amniote from North America is the oldest known member of Parareptilia. Despite its stratigraphic position, Acleistorhinus is not a basal parareptile, and a phylogenetic analysis of parareptile interrelationships demonstrates that Acleistorhinus is a sister taxon to the Russian clade Lanthanosuchidae. This hypothesis of relationships is supported by eleven synapomorphies. The presence of an Early Permian parareptile in North American sediments provides strong support for the recent hypotheses of amniote phylogeny that propose an extensive evolutionary radiation for this group of reptiles.

The Permian mammal-like herbivore Diictodon, the oldest known example of sexually dimorphic armament

Dicynodonts, a highly successful group of Palaeozoic tetrapods, were herbivores with keratinous beaks, and were frequently equipped with large, neomorphic tusks. Diictodon is a particularly abundant dicynodont genus, allowing statistical investigation of its palaeobiology. Anatomical, morphometric and distributional analyses provide evidence of sexual dimorphism, based on the presence or absence of formidable tusks. Tusk occurrence is also correlated with the presence of a cranial boss on the skull roof and, possibly, with greater cranial size. This earliest well–documented example of dimorphic armament suggests that sexual dimorphism, and the complex social behaviour that accompanies it, have long been characteristic of the synapsid lineage.