Evolution of the mammal-like reptiles

Abstract

Mammals, along with the biologically remarkably similar birds, are the vertebrates that are most completely adapted to the physiological rigours of the terrestrial environment. Whilst all the terrestrial dwelling tetrapods can operate in the absence of the buoyancy effect of water, and can use the gaseous oxygen available, mammals have in addition evolved a highly sophisticated ability to regulate precisely the internal temperature and chemical composition of their bodies in the face of the extremes of fluctuating temperature and the dehydrating conditions of dry land. From this perspective, the origin of mammalian biology may be said to have commenced with the emergence of primitive tetrapods onto land around 370 Ma, in the Upper Devonian. Until the 1990s, the only Devonian tetrapod at all well known was Ichthyostega from east Greenland, as described by Jarvik (e.g. Jarvik 1980, 1996). Famous for its combination of primitive fishlike characters such as the lateral line canals, bony rays supporting a tail fin, and remnants of the opercular bones, with fully tetrapod characters such as the loss of the gills and opercular cover, robust ribcage, and of course large feet with digits, Ichthyostega provided more or less all the fossil information there was relating to the transition from a hypothetical rhipidistian fish to a tetrapod. Subsequently, however, an ever-increasing number of other Upper Devonian tetrapods have been described, and the emerging picture of the origin of vertebrate terrestriality has become more complicated and surprising (Ahlberg and Milner 1994; Clack 2002). The earliest forms are Upper Frasnian in age, and include Elginerpeton from the Scottish locality of Scat Craig (Ahlberg 1995, 1998). So far known only from a few bones of the limbs and jaws, Elginerpeton adds little detail to the understanding of the evolution of tetrapods except to demonstrate that the process had commenced at least 10 million years prior to the existence of Ichthyostega. The next oldest tetrapods are Fammenian in age and include Ichthyostega, and a second east Greenland form, Acanthostega, which has been described in great detail (Coates and Clack 1990, 1991; Coates 1996).

Nomenclature, comparative anatomy, and evolution of the reflected lamina of the angular in non-mammalian synapsids

“…This hypothesis is also consistent with our findings that a trend towards the simplification of angular topography is seen in both dicynodonts and cynodonts (Fig. 12), two clades in which the pterygoideus musculature was likely reduced over time (Kemp, 1982;King et al, 1989).…”

Enigmatic humerus from the mid-Permian of South Africa bridges the anatomical gap between “pelycosaurs” and therapsids

“…This suggests that important changes to muscle leverage occurred first, whilst still relying on a fundamentally screw-like articular morphology to help guide and constrain joint motion (Bishop et al, 2022). It is predicted that, were the scapulocoracoid of this animal to be found, the glenoid would retain a decidedly screw-like structure, rather than the simple hemi-sellar morphology that typifies known therapsid groups (Brocklehurst et al, 2022b; Kemp, 1982). Intriguingly, an isolated scapulocoracoid of ‘ Brithopus priscus ,’ from the mid-Permian of Russia, appears to possess a somewhat screw-like glenoid (Efremov, 1954:fig.…”

“…Due to logistical constraints, this was undertaken prior to the remaining small amount of matrix being mechanically prepared away from the ventral surface (Movie S1). For purposes of comparison and illustration, this study also created digital models of the humerus of MCZ VPRA-3357 Dimetrodon limbatus (Bishop et al, 2023) and UMZC T.883, an indeterminate Tanzanian gorgonopsian (Kemp, 1982). The latter was generated using a GoSCAN 20 laser surface scanner (Creaform, U.S.A.).…”

New Skull Material of Taeniolabis taoensis (Multituberculata, Taeniolabididae) from the Early Paleocene (Danian) of the Denver Basin, Colorado

“…An internarial bar, formed by a complete internarial process (variously also called the internasal, prenasal, dorsal, or ascending process) dividing the external nasal aperture into left and right halves is found in tetrapods ancestrally and, despite being rarely preserved in fossils, is known to have been retained in a variety of non-mammaliamorph cynodonts (e.g., Beishanodon , Dadadon , Galesaurus , Menadon , Riograndia , tritylodontids) as well as in some early-branching mammaliamorphs such as Sinoconodon , Morganucodon , Hadrocodium , Haldanodon , Necrolestes , and probably Docodon (Kemp 1982 ; Hopson and Barghusen 1986 ; Rowe 1986 , 1988 ; Sues 1986 ; Lillegraven and Krusat 1991 ; Flynn et al 2000 ; Wible and Rougier 2000 , 2017 ; Bonaparte et al 2001 ; Luo et al 2001 ; Kammerer et al 2008 ; Gao et al 2010 ; Rougier et al 2015 ; Pusch et al 2019 ). Rowe ( 1988 : 251) declared the internarial process to be “absent in adult Monotremata, Multituberculata, and Theria, rendering the external nares confluent in postnatal ontogeny.”…”