Basal Synapsids Research Articles

Current and historical perspectives on the completeness of the fossil record of pelycosaurian-grade synapsids

Understanding of the early evolution of tetrapods may have been influenced by the effects of geological and anthropogenic sampling biases, as suggested by previous studies. Here we make a thorough assessment of how these and other biases affect the record of pelycosaurian-grade synapsids, a group of tetrapods that dominated the terrestrial realm during the late Carboniferous and early Permian. Modified versions of the character and skeletal completeness metrics are used to assess the completeness of the known specimens, while a recently published supertree of basal synapsids is tested for congruence between the stratigraphic and phylogenetic hypotheses. A historical analysis is also applied to the record in order to show how the biases in the study of this group over the past centuries may have influenced the current record. The results show that the completeness of basal synapsid specimens shows very little variation throughout the Early Permian. A peak in skeletal and character completeness metric values in the Middle Permian (Wordian) is attributed to exceptional preservation of the Mezen fauna. A significant negative correlation is found between the skeletal completeness metric and a taxic diversity curve of pelycosaurs, implying a tendency to name many species based on poor material. The lack of correlation between the character completeness metric and diversity is attributed to the history of discovery in the group: the majority of pelycosaur species were named between the 1930s and 1960s, when character based assignments were secondary to size, location and stratigraphy. A strong correspondence between the phylogeny and stratigraphy implies a reliable phylogenetic hypothesis, but the low relative completeness index score suggests that a great deal of the fossil record is missing. While the relative completeness index has improved over the past century, the rate of improvement has slowed in the last two decades, and the slowing rate of discovery of new species suggests that the limit of what the fossil record of pelycosaurs has to offer is being approached. A consistent shape of diversity curves over the past century implies that the same biases which affected the record in the past are still affecting it now. This study highlights the need for sampling correction methods to be employed in studies of early synapsids.

The Role of Foraging Mode in the Origin of Therapsids: Implications for the Origin of Mammalian Endothermy

The question of the adaptive basis for the origin of mammalian endothermy remains unresolved despite a great deal of research effort. Controversy continues over which physiological adaptations were of greatest importance in starting ectothermic nonmammalian synapsids of the Late Paleozoic on the path that culminated in modern endothermic mammals. Models of the selective basis for the origin of endothermy fall into two main categories: “thermoregulation first” and “aerobic capacity first.” Studies of lizards show a dichotomy between a low-energy “sit-and-wait” (SW) foraging mode in Iguania and a more energy-intensive “widely foraging” (WF) mode in Autarchoglossa. It is proposed that in the transition from basal synapsids (“pelycosaurs”) to therapsids, a shift from the primitive SW mode to the WF mode put the ancestors of mammals on the path to increased aerobic capacity and the ability to sustain high levels of foraging activity. Selection for increased energy expenditure disproportionately increased the amount of food energy consumed, thus improving foraging efficiency. A shift from reliance on anaerobic muscle metabolism for short but rapid dashes to capture prey to a reliance on aerobic metabolism for active searching for prey necessitated improvements of the cardiovascular system and lungs for increased aerobic capacity and greater stamina. Over time, therapsids became locked into high food requirements, which selected for improvements in aerobic metabolism, locomotor and food-processing ability, and neurosensory/behavioral specializations. Evidence of a link between maximum activity metabolism and resting (basal) metabolism in anurans and rodents suggests that further increases in aerobic activity metabolism required an increased basal metabolic rate, which led to high body temperatures and, ultimately, homeothermy. Therapsids show adaptations for increased activity, greater food-getting and food-processing ability, and higher metabolic rates than basal synapsids (“pelycosaurs”). It is argued that the “foraging mode” model is preferable to the “parental care” model of Farmer and the “correlated progression” model of Kemp for understanding the origin of mammalian endothermy.

Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies

Basal synapsids (‘pelycosaurs’) form the basalmost portion of the mammalian stem lineage and document the transition from primitive ‘reptile-like’ basal amniotes to derived, mammal-like therapsids. They dominated terrestrial ecosystems of the latest Carboniferous and Early Permian (∼300–271 million years ago), producing large-bodied terrestrial animals (3–6.5 metres long), high-fibre herbivores, and macropredators for the first time in vertebrate history, alongside an array of smaller-bodied forms. Despite numerous recent discoveries and reassessments of fossils collected over the past 250 years, and despite their importance for understanding the early diversification of terrestrial vertebrates, a comprehensive assessment of global relationships among basal synapsids has not been undertaken. A new phylogenetic dataset comprising 45 taxa (plus four outgroups and four therapsids) and 239 characters (147 cranial; 92 postcranial) reveals considerable uncertainty in the relationships of higher clades of basal synapsids. Although cranial data support the current consensus that Caseasauria is the most basal clade, postcranial data and the full dataset suggest that a clade of Ophiacodontidae + Varanopidae occupies this position. Although relationships within higher clades are well supported, relationships among those clades are poorly supported. The likely source of this uncertainty lies in the exceptionally poor early record of the group, which renders determinations of the plesiomorphic condition of higher clades speculative, although cranial data are generally represented by shorter ghost lineages and should perhaps be favoured. The new dataset suggests well-supported phylogenetic placements for several taxa of historically uncertain affinities: Trichasaurus is a caseid; Lupeosaurus is an edaphosaurid; and Basicranodon and Ruthiromia are varanopids.

The last “pelycosaur”: a varanopid synapsid from the Pristerognathus Assemblage Zone, Middle Permian of South Africa

We report on a partial varanopid skull and mandible from the Pristerognathus Assemblage Zone of the Beaufort Group, in the South African Karoo Basin, which is probably latest Middle Permian (Capitanian) in age. This mycterosaurine is not only the youngest known varanopid from the Southern Hemisphere, but it is also the youngest known "pelycosaur" (i.e., non-therapsid synapsid). Like all other members of this clade of hypercarnivores, the teeth are strongly flattened, recurved, and have finely serrated cutting edges. The anterior dentary teeth form a caniniform region, and the splenial features a foramen intermandibularis oralis, the first ever to be described in a "pelycosaur." The last varanopids were the smallest carnivores of latest Middle Permian continental faunas. Occupation of the small carnivore guild appears to have allowed varanopids to achieve a nearly cosmopolitan distribution throughout the Middle Permian, between the great Early Permian radiation of basal synapsids and the spectacular diversification of therapsid synapsids in the Late Permian and Early Triassic.

Comparative osteohistology of hyperelongate neural spines in the Edaphosauridae (Amniota: Synapsida)

Abstract: The Permo‐Carboniferous Edaphosauridae is an extinct family of omnivorous and herbivorous basal synapsids in which a dorsal sail, comprised of a series of hyperelongate vertebral spinous processes (‘neural spines’), evolved convergently to their sphenacodontid contemporaries. Descriptive and quantitative microstructural analysis of these hypertrophied neural spines allows inferences of sail growth, allometry and structural properties and permits systematic comparisons with other groups (i.e. sphenacodontids). In Edaphosaurus and Ianthasaurus, neural spine microstructure is characterized by a lamellar‐zonal primary bone complex within the cortex, distal cross‐sectional bone density ranging from approximately 0.50 to 0.75, low cortical porosity ranging from 1.0 to 5.0 per cent, relative bone wall thickness ranging from 10 to 25 per cent and presence of a central cavity, fully developed with smooth endosteal margins in both genera. Tubercle growth, however, appears to have been slower and incremental in the specimen of Ianthasaurus examined here. Microstructural similarities further support the position of Lupeosaurus within Edaphosauridae as hypothesized by earlier authors, though most parsimoniously in a basal position (as a central cavity is only incipiently developed and tubercles are lacking). Histological properties of hyperelongate neural spines in Lupeosaurus and other edaphosaurids do not meet previously established requirements of the thermoregulatory hypothesis, either calling into question the sail’s thermoregulatory function in this group or indicating a reconsideration of those putative thermoregulatory adaptations and the purported soft‐tissue correlates of the sail. Conversely, microstructural patterns exhibit variation that is consistent with an intra‐ or interspecific display function. Given the diverse microstructural properties at the family and genus‐level, histological study of hyperelongate neural spines may also aid in recognizing taxonomic compositions of Permo‐Carboniferous microvertebrate assemblages in which only fragmentary materials are known.

Revisional notes on the Permian tetrapod ichnofauna from the Tiddas Basin, central Morocco

During the mid-1980s, structural geological field work in the up to 300 m thick red-bed sequence of the Permian Tiddas Basin, central Morocco, revealed the first occurrence of Palaeozoic tetrapod footprints in NW Africa. Preservational ambiguity of part of the few published trace fossils and resulting taxonomic inconsistency necessitate a long-needed reanalysis of the type locality. Based on 110 recently collected specimens the revised tetrapod ichnofauna of the study area comprises tracks of Batrachichnus Woodworth, 1900 — Limnopus Marsh, 1894, Amphisauropus Haubold, 1970, Dimetropus Romer and Price, 1940, Varanopus Moodie, 1929 — Hyloidichnus Gilmore, 1927, and Dromopus Marsh, 1894 which can be referred to small and medium-sized temnospondyl, seymouriamorph, basal synapsid (‘pelycosaur’), captorhinomorph, and araeoscelid trackmakers. This assemblage is interpreted to represent a late Early Permian (Artinskian–Kungurian) tetrapod ichnofauna. Due to the relative abundance of footprints assigned to semiaquatic tetrapods and sedimentological evidence for pronounced humid palaeoclimatic conditions we propose correlation of the track-bearing strata with the late Artinskian/early Kungurian wet phase. Regarding the quality, quantity, and peculiar diversity of its fossils, including the earliest record of seymouriamorph and advanced captorhinomorph tracks from the African continent, the Tiddas Basin could be of great importance for the reconstruction of late Early Permian terrestrial ecosystems in proximity to the interface of Euramerican and Gondwanan biotic provinces.

The roles of herbivory and omnivory in early dinosaur evolution

ABSTRACTHerbivorous and omnivorous dinosaurs were rare during the Carnian stage of the Late Triassic. By contrast, the succeeding Norian stage witnessed the rapid diversification of sauropodomorphs and the rise of the clade to ecological dominance. Ornithischians, by contrast, remained relatively rare components of dinosaur assemblages until much later in the Mesozoic. The causes underlying the differential success of ornithischians and sauropodomorphs remain unclear, but might be related to trophic specialisation. Sauropodomorphs replaced an established herbivore guild consisting of rhynchosaurs, aetosaurs and basal synapsids, but this faunal turnover appears to have been opportunistic and cannot be easily attributed to either competitive interactions or responses to floral change. Consideration of diversity patterns and relative abundance suggests that the ability to eat plants might have been a major factor promoting sauropodomorph success, but that it was less important in the early evolution of Ornithischia. On the basis of current evidence it is difficult to determine the diet of the ancestral dinosaur and scenarios in which omnivory or carnivory represent the basal condition appear equally likely.

Eothyris and Oedaleops: do these Early Permian synapsids from Texas and New Mexico form a clade?

ABSTRACT The monospecific genera Eothyris (Petrolia Formation, Leonardian, Archer County, Texas) and Oedaleops (Abo/Cutler Formation, Wolfcampian, Rio Arriba County, New Mexico), known solely from cranial remains, are confidently assigned to the monophyletic Caseasauria based on cranial and dental characters. In addition, Eothyris and Oedaleops comprise the monophyletic Eothyrididae based on nine cranial and dental characters. In contrast to the medium to large sized herbivorous caseids, the small eothyridids exhibit dental features that indicate that they were faunivores. The presence of well-developed caniniform teeth suggests that they were predators. Both Eothyris parkeyi Romer and Oedaleops campi Langston exhibit a suite of plesiomorphic cranial characters that identify them as basal synapsids. Thus, the eothyridids are better representatives of the primitive synapsid cranial morphotype than the often-used ophiacodontids. Although they appear relatively late in synapsid evolution, both eothyridids are significantly older than all other caseasaurs, forcing the establishment of a long unrecorded lineage for caseids.

A Dicynodont-Theropod Association in the Latest Triassic of Poland

It is generally accepted that during the Triassic the composi− tion of tetrapod faunas underwent a series of fundamental transformations, mainly as a result of diversification of archosaurs and decline of therapsids (Benton 1994, 2004, 2006). The last herbivorous basal synapsids, dicynodonts, disappeared from the record in the early Norian of the Americas, about 220 Ma (Langer et al. 2007), being un− known from the Late Triassic of Europe. Here, we report a partially articulated skeleton and isolated bones of a giant rhino−size dicynodont in the Upper Triassic fluvial sedi− ments at Lisowice (Lipie Śląskie clay−pit) in southern Po− land. Paleobotanical data indicate an early Rhaetian age for the fauna (Dzik et al. 2008; Niedźwiedzki and Sulej 2008). The dicynodont bones are associated with bones of carnivo− rous dinosaurs, pterosaurs, as well as capitosaur and plagio− saur amphibians. Dicynodonts were represented in the Ger− manic Basin throughout the Late Triassic, as proven by findings of smaller dicynodonts in older deposits in the same area, associated there with temnospondyl amphibians. It ap− pears, thus, that the fossil record of tetrapod succession in the Late Triassic was strongly controlled by ecological fac− tors and biased by uneven representation of particular envi− ronments. The Lisowice assemblage proves that faunas dominated by dicynodonts did not entirely disappear at least until the end of the Triassic.

Evolution of bone microanatomy of the tetrapod tibia and its use in palaeobiological inference

Bone microanatomy appears to track changes in various physiological or ecological properties of the individual or the taxon. Analyses of sections of the tibia of 99 taxa show a highly significant (P <or= 0.005) relationship between long-bone microanatomy and habitat. Randomization tests reveal a highly significant (P <or= 0.005) phylogenetic signal on several compactness profile parameters and lifestyle. Discriminant analyses yield an inference model which has a success rate of 63% when lifestyle is coded into three states (aquatic, amphibious and terrestrial) or 83% for a binary model (aquatic vs. amphibious to terrestrial). Lifestyle is inferred to have been terrestrial for the stem-tetrapod Discosauriscus (Early Permian), the basal synapsid Dimetrodon (Early Permian), the dicynodont therapsid Dicynodon (Late Permian), an unindentified gorgonopsian (Late Permian); the parareptile Pareiasaurus (Middle or Late Permian) is modelled as being aquatic, but was more likely amphibious.

The morphology of the terminal phalanges in Permo-Carboniferous synapsids: an evolutionary perspective

Morphological features of the terminal phalanges of extinct tetrapods can be used to infer whether or not keratinous claws were present even though these structures are not preserved in the fossil record. Such features as dense vascularization grooves and foramina, and a general claw-like morphology, are present in some of the earliest fully terrestrial tetrapods, the Permo-Carboniferous synapsids. Early synapsids are represented by a rich fossil record that preserves the detailed anatomy of the terminal phalanges and allows for an examination of the early evolution of these structures in a well-resolved phylogenetic context. The pattern of change in the morphology of the terminal phalanges of five basal synapsids, Cotylorhynchus romeri, Varanops sp., Edaphosaurus boanerges, Haptodus garnettensis, and Dimetrodon limbatus, reveals a clear trend from a broad, flat, and spatulate morphology in the basal taxa to a tall, narrow, and curved structure. This trend in overall shape change does not reflect changes in feeding behaviour. The size and shape of the flexor tubercle appears to be a factor of size and function, rather than possessing a phylogenetically informative signal. The osteological features used to infer the presence of a keratinous sheath in the synapsids are also observed in the non-amniote taxon Diadectes absitus. This indicates that claws were not an amniote innovation and that they instead originated somewhere outside the crown group Amniota.

The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis

The replacement of the basal synapsid pelycosaurs by the more 'mammal-like' therapsids in the Permian was an important event in the history of tetrapods because it initiated the eventual transition to the mammals. It is also an example of taxon replacement in the fossil record that is unusually amenable to explanation, based on a combination of analysis of the biological significance of the inferred character changes, with the stratigraphic, palaeogeographic and palaeoecological circumstances of the time. An hypothesis is presented in which the origin of the therapsids resulted from a correlated progression of character evolution leading to higher levels of metabolic activity and homeostatic regulation of the body. It was a response to the availability of a seasonally arid, savanna-like biome. The subsequent explosive radiation of therapsids was associated with habitat expansion made possible by the Mid-Permian development of geographical continuity between that biome and the temperate biomes. The final extinction of the pelycosaurs was a case of incumbent replacement by the new therapsid lineages.

Confusing dinosaurs with mammals: tetrapod phylogenetics and anatomical terminology in the world of homology.

At present, three different systems of anatomical nomenclature are available to researchers describing new tetrapod taxa: a nonstandardized traditional system erected in part by Sir Richard Owen and subsequently elaborated by Alfred Romer; a standardized system created for avians, the Nomina Anatomica Avium (NAA); and a standardized system for extant (crown-group) mammals, the Nomina Anatomica Veterinaria (NAV). Conserved homologous structures widely distributed within the Tetrapoda are often granted different names in each system. The recent shift toward a phylogenetic system based on homology requires a concomitant shift toward a single nomenclatural system also based on both evolutionary and functional morphological homology. Standardized terms employed in the NAA and NAV should be perpetuated as far as possible basally in their respective phylogenies. Thus, NAA terms apply to nonavian archosaurs (or even all diapsids) and NAV terms apply to noncrown-group mammals and more basal synapsids. Taxa equally distant from both avians and crown-group mammals may maintain the traditional nonstandardized terminology until a universal anatomical nomenclature for all tetrapods is constructed.

Earliest evidence of cynodont burrowing.

A 251 million year old partial burrow cast containing an articulated skeleton of the mammal-like carnivore Thrinaxodon liorhinus is the oldest evidence for burrowing by a cynodont synapsid. The burrow cast comes from terrestrial flood plain sediments close to the Permian-Triassic boundary in the Karoo of South Africa. Together with those of the later cynodont Trirachodon, the Thrinaxodon burrow cast indicates that burrow-making was broadly distributed in basal synapsids and has a long history in non-mammalian synapsid evolution. A reconstruction of its appendicular skeleton in cross-section indicates that Thrinaxodon was able to adopt a facultatively mammalian stance within its burrow shaft. Burrows of cynodont design are more common in Triassic rocks than previously realized, and suggest that burrowing may represent an adaptive response by cynodonts to the environmental conditions associated with the mass extinction event that punctuated the end of the preceding Permian period. The widespread occurrence of burrowing among extant mammals implies that the ancient synapsid ability to burrow conferred a strong adaptive value in the evolution of Mammalia.

Archaeovenator hamiltonensis, a new varanopid (Synapsida: Eupelycosauria) from the Upper Carboniferous of Kansas

Study of a nearly complete skeleton of a small amniote, preserved as part and counterpart in Upper Carboniferous limestone slabs of the Hamilton Quarry, Kansas, indicates that it is a new genus and species of varanopid synapsid. Archaeovenator hamiltonensis gen., and sp. nov. is the oldest known varanopid. Phylogenetic analysis indicates that it is the sister taxon to all other known varanopids. Discovery of this new varanopid is important because it improves our knowledge of the terrestrial fauna of the Hamilton quarry, expands the known diversity of Late Carboniferous basal synapsids, and extends the fossil record of Varanopidae deeper within the Stephanian. Varanopidae has the longest fossil record (Late Carboniferous – upper Middle Permian) of all known clades of basal synapsids.

A second varanopseid skull from the Upper Permian of South Africa: implications for Late Permian ‘pelycosaur’ evolution

Late Permian terrestrial faunas of South Africa and Russia are dominated taxonomically and ecologically by therapsid synapsids. On the basis of a single specimen from the Upper Permian of South Africa, the varanopseid Elliotsmithia longiceps is the sole basal synapsid (‘pelycosaur’) known from Gondwana. Recent fieldwork in the Upper Permian of South Africa has produced a second varanopseid specimen that is referrable to Elliotsmithia. Data from both this specimen and the holotype suggest that Elliotsmithia forms a clade with Mycterosaurus from the Lower Permian of North America and Mesenosaurus from the Upper Permian of Eastern Europe. That postulate is supported by the three most parsimonious trees discovered in a new analysis of varanopseid phylogeny. However, the available data cannot resolve the interrelationships of these three genera. The new phylogenetic results contrast with earlier work identifying Elliotsmithia as the basal member of a clade that includes the North American taxa Aerosaurus, Varanops, and Varanodon. The new trees reduce the stratigraphic debt required by the latter scenario, and the one with the least stratigraphic debt identifies Elliotsmithia and Mesenosaurus as sister taxa. Two new taxa are erected, Mycterosaurinae and Varanodontinae, for the two varanopseid subclades.

A Phylogenetic Perspective on Locomotory Strategies in Early Amniotes1

Past approaches to understanding the evolution of locomotory strategies among Paleozoic amniotes (“primitive reptiles” of previous parlance) have been influenced by preservational bias: early occurrences of some amniote taxa were used to polarize the acquisition or development of locomotory structures among the earliest amniotes. Using a phylogeny representing the current consensus in the literature, we investigate the major locomotory strategies that have been posited for Paleozoic amniotes (basal synapsids on one hand and early reptiles on the other) by optimizing the major locomotory styles identified for these taxa onto the consensus tree, in order to present an overview of the pattern of evolution of locomotory strategies inherited and adopted by various amniote lineages.

A new species of Dimetrodon (Synapsida: Sphenacodontidae) from the Lower Permian of Germany records first occurrence of genus outside of North America

A new species of the sphenacodontid synapsid Dimetrodon, D. teutonis, is described on the basis of a single, adult specimen consisting of a large portion of the presacral vertebral column. The holotype was collected from the Lower Permian Tambach Formation, lowermost formational unit of the Upper Rotliegend, of the Bromacker quarry locality in the midregion of the Thuringian Forest near Gotha, central Germany. This is the first record of the genus outside of North America and, therefore, provides not only additional biological evidence of a continuous Euramerican landmass during the Early Permian, but also the absence of any major physical or biological barrier to faunal interchange of terrestrial vertebrates. An estimated weight of 14 kg for D. teutonis is half that of the smallest, previously recognized species, D. natalis. Sphenacodontid phylogeny indicates that the diminutive size of D. teutonis represents an autapomorphy and is in general accord with the absence of large-sized, basal synapsid predators at this truly terrestrial upland locality. It is speculated that the diminutive size of D. teutonis was probably an adaptation to a truly terrestrial, relatively uplands existence like that represented by the Bromacker locality. Here it subsisted on small vertebrates (and possibly large invertebrates) of the Bromacker assemblage, in which the dominant members in both size and abundance were herbivorous diadectids, and it was unlikely to encounter large predators.