Dawn of the synapsids

The synapsids are a clade of amniotes that comprise mammals (including us humans), their ancestral reptilian groups (often incorrectly referred to as ‘mammal-like reptiles’) and other extinct relatives. Mammals are not descended from reptiles, but both classes of vertebrate shared a common ancestor. The synapsids are distinguished by having one temporal fenestra low behind each orbit in the skull and are the second most diverse clade of amniotes after the diapsids. The earliest synapsids of all constitute a family known as the Ophiacodontidae: medium-sized animals that thrived throughout the Late Carboniferous and Early Permian, which were also some of the largest terrestrial carnivores at the time.

They were an extraordinary group with unusually large skulls in proportion to their body size, being very narrow, high and elongate. Their jaws were densely packed with many small, sharp teeth that were curved slightly and started to show some differentiation in size. They also had comparatively short, sturdy limbs as well as very large pectoral girdles, presumably to provide muscle attachments for holding up the disproportionately big head.

The undisputed basalmost ophiacodontid is represented by Archaeothyris florensis (Reisz, 1972) known from fragmentary material found in Nova Scotia, Canada, dating back to the late Carboniferous (c. 306 Ma during the Westphalian stage/Mid Pennsylvanian) and is also the oldest confirmed synapsid. It resembled a modern lizard to some extent, but already had the high, elongate skull characteristic of the group. At approximately 0.5 m long, it co-existed with other primitive reptile taxa like Palaeothyris in swampy forests consisting of tree-like club mosses (lycopsids) and ruled by giant arthropods, insects and amphibians. It had strong jaws equipped with sharp, pointed teeth that were roughly the same shape, but also possessed an enlarged pair of canines at the front of the maxilla, indicating a mixed carnivorous diet (van Tuinen & Hadley, 2004).

Figure 1: My reconstruction of Archaeothyris florensis.

The best-studied and best-known genus of all is Ophiacodon (Romer, 1925; Romer & Price, 1940), numerous skeletons of which have been found in Early Permian strata from Colorado, Kansas, New Mexico, Ohio, Oklahoma, Texas and Utah, so is represented by a thorough fossil record. Six species have been described (determined by stratigraphic position and size), although it’s not 100% clear how many of these species are truly valid.

The type species O. mirus was initially described in 1878 by Othniel Charles Marsh on the basis of some vertebrae and a mandible during the “Bone Wars”, a period of intense rivalry between Edward Drinker Cope to discover and name more new fossil species. Marsh evidently wanted to outcompete Cope, who had an in-press article naming and describing the reptile, but it was deficient and written so promptly that the genus Ophiacodon was neglected by the scientific community for over 3 decades. Meanwhile, Cope published an article shortly after Marsh published his, describing three new species in a differently named genus. On the basis of individual vertebrae, these were named Theropleura retroversa, Theropleura triangulata and Theropleura uniformis. Later, the holotype specimen of Ophiacodon was re-examined and a description was made of a brand-new intact skeleton (Williston & Case, 1913). Even later, Ophiacodon and Theropleura were revealed to be synonymous, the former which had priority as it was named first, even though the species names were still kept (Romer & Price, 1940).

Figure 2: My reconstruction of Ophiacodon mirus.

These are the six species of Ophiacodon identified to this day:

 

O. hilli - known from a partial skeleton found in Kansas.

O. major - known from incomplete material found in Texas.

O. mirus - the type species, known from a few skeletons found in Oklahoma and New Mexico, including one that is almost complete.

O. navajonicus - known from incomplete postcranial remains found in New Mexico.

O. retroversus - known from numerous material found in Oklahoma and Texas, which includes an almost complete skeleton

O. uniformis - known from a few incomplete skeletons found in Oklahoma and Texas.

However, it may be that differences in size (which range from 5 - 10 ft long) only reflect various growth stages instead of different species (Brinkman, 1988). At first, Ophiacodon was thought to have been a semi-aquatic predator but is now considered to have been entirely terrestrial, as recent studies have disproved the alleged aquatic adaptations that the animal may have had (Felice & Angielczyk, 2014).

There are many other known ophiacodontids which have been described from relatively incomplete material, such as the Late Carboniferous taxa Stereorhachis dominans from France and Echinerpeton intermedium and Clepsydrops sp. from North America, as well as the Early Permian taxa Baldwinonus trux and Sterophallodon ciscoensis from North America. Protoclepsydrops haplous (Carrol, 1964), known from Joggins, Nova Scotia, may have been an ophiacodontid too, preceding even Archaeothyris florensis, but due to the paucity of fossil material it is hard to know for certain.

One other family of early carnivorous synapsids appearing in the Late Carboniferous were the even more primitive-looking varanopids. Like the ophiacodontids, they too had skulls which were narrow, deep and elongated with the orbits set far back, but had a more slender mandible with a specialised marginal dentition, as well as some extended parietal bones on the skull roof above the orbits. (Reisz & Dilkes, 2003). Other characteristics of the group also include a covering of extensive dermal osteoderms unlike other clades of synapsids, which is especially evident in Heleosaurus (Botha-Brink and Modesto, 2009). The varanopids greatly resembled monitor lizards and like their modern namesake, may have lived similar lifestyles and ecological niches; they have a stratigraphically wide-ranging, cosmopolitan distribution extending to the Mid Permian.

The type genus Varanops brevirostris was as big as a large monitor lizard (at ~ 4 ft long) with sharp, flattened, strongly-curved teeth, clearly those of a flesh-eating predator. It is known from a few individuals which include almost complete skeletons from the Garber Formation of Oklahoma and the Arroyo Formation of Texas. One of these skeletons even provides evidence of its ecology, where bite marks along with a tooth stuck between the ulna and radius shows that the body of this individual was scavenged on by a dissophoroid temnospondyl amphibian (Reisz & Tsuji, 2006).

Figure 3: My  reconstruction of Varanops brevirostris.

The largest known varanopid of all is Watongia meieri, which judging from fragmentary remains found at the Chickasha Formation (Mid Permian in age) of Oklahoma is estimated to be approximately 2 - 2.5 m long, so may have been the apex predator of its time and place. It is believed to be a varanopid and not a gorgonopsid as first thought, judging from characteristics such as large, lateral protuberances on the postorbital bone as well as marginal, backward-curving teeth unserrated on the anterior and posterior edges. It may also have had a disproportionately large head in comparison to its body (Reisz & Laurin, 2004).

 

Figure 4: My reconstruction of Watongia meieri, based on a study by Reisz & Laurin (2004). Note the bony protuberance on the postorbital (behind the eye).

One of the most primitive varanopids is Mycterosaurus longiceps, a small, agile carnivorous/insectivorous synapsid known from a partial skeleton found at the Waggoner Ranch Formation dating back to the Early Permian (Artinskian). As well as having maxillary teeth distinct from those of other varanopids, it possessed many small palatal teeth too. It also had vertebrae comparable to Varanops: the dorsal neural spine is broad in lateral view, with vertical posterior and anterior edges. The dorsal centra are considerably longer than the lumbar centra (Romer & Price, 1940). In addition, a highly mobile ankle joint present in both Varanops and Mycterosaurus suggests that they had a semi-digitigrade stance.

Figure 5: My reconstruction of Mycterosaurus longiceps.

References:

Botha-Brink, J., Modesto, S. P. 2009. Anatomy and relationships of the Middle Permian varanopid Heleosaurus scholtzi based on a social aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Palaeontology, 29 (2), 389-400.

Brinkman, D. 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology, 8 (2), 172-180.

Carroll, R. L. 1964. The earliest reptiles. Zoological Journal of the Linnean Society, 45 (304), 61–83.

Felice, R. N., Angielczyk, K. D. 2014. Was Ophiacodon (Synapsida, Eupelycosauria) a swimmer? A Test Using Vertebral Dimensions. Early evolutionary history of the Synapsida, Springer Netherlands, pp. 25-51.

Reisz, R. R. 1972. Pelycosaurian Reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology, 144 (2), 27-60.

Reisz, R. R., Dilkes, D. W. 2003. Archaeovenator hamiltonensis, a new varanopid (Synapsida: Eupelycosauria) from the Upper Carboniferous of Kansas. Canadian Journal of Earth Sciences, 40 (4), 667-678.

Reisz, R. R., Laurin, M. 2004. A reevaluation of the enigmatic Permian synapsid Watongia and of its stratigraphic significance. Canadian Journal of Earth Sciences, 41 (4), 377-386.

Reisz, R. R. Tsuji, L. A. 2006. An articulated skeleton of Varanops with bite marks: the oldest known evidence of scavenging among terrestrial vertebrates. Journal of Vertebrate Palaeontology, 26 (4), 1021-1023.

Romer. A, S. 1925. An ophiacodont reptile from the Permian of Kansas. Journal of Geology, 33 (2), 173-182.

Romer, A. S., Price, L. I. 1940. Review of the Pelycosauria. Geological Society of America Special Papers, 28, 1-538.

van Tuinen, M., Hadly., E. A. 2004. Error in estimation of rate and time inferred from the early amniote fossil record and avian molecular clocks. Journal of Molecular Evolution, 59, 267-276.

Williston, S. W., Case, E. C. 1913. Description of a nearly complete skeleton of Ophiacodon Marsh. Carnegie Institution of Washington Publication, 181, 37-59.