The authors have declared that no competing interests exist.
Analyzed the data: RRR JF. Wrote the paper: RRR JF.
The origin and early evolution of amniotes (fully terrestrial vertebrates) led to major changes in the structure and hierarchy of terrestrial ecosystems. The first appearance of herbivores played a pivotal role in this transformation. After an early bifurcation into Reptilia and Synapsida (including mammals) 315 Ma, synapsids dominated Paleozoic terrestrial vertebrate communities, with the herbivorous caseids representing the largest vertebrates on land.
The origin and early evolution of herbivory represents a major evolutionary event in terrestrial vertebrate history because its acquisition allowed tetrapods to become primary consumers, and access directly the vast, largely untapped resources provided by the primary producers on land, the cellulose rich leaves and stems of terrestrial plants. Herbivory in its broadest term refers to a form of consumption in which organisms eat principally autotrophs. Among vertebrates this feeding strategy can be subdivided into many categories, including folivory, frugivory, granivory, but among early terrestrial vertebrates, it is feeding on leaves, stems, roots, and rhizomes that are most relevant because it has the potential to be reflected in the skeletal anatomy
Even though the earliest tetrapods are Late Devonian (late Frasnian, 375 Ma) in age, they effectively ventured onto land only during the Early Carboniferous (Visean, 345-328 Ma)
It is generally recognized that two clades of early synapsids include herbivores, the caseids and edaphosaurs
Caseids belong to the most basal clade of Synapsida, the Caseasauria, which also includes the small carnivorous eothyridids (
Here we describe a new small caseid (
Photograph and interpretive drawing of holotype KUVP 9616b, from the late Pennsylvanian of Kansas, USA. Preserved parts of the skeleton include the posterior 1/3rd of the skull and mandible, the vertebral column with ribs, pelvic girdle and right hind limb. Abbreviations: as, astragalus; ca, calcaneum; f, femur; fi, fibula; il, ilium; is, ischium; pu, pubis; s1, first sacral rib; s2, second sacral rib; t, tibia; roman numerals indicate the digit number in the pes.
Photograph and interpretive drawing of cranial anatomy of KUVP 9616b. Part of the skull table and cheeks, parts of occiput and braincase, and the posterior portion of the right mandible are preserved. Abbreviations: an, angular; ar, articular; atn, atlantal neural arch; ax, axis; d, dentary; ex, exoccipital; h, hyoid; j, jugal; m, maxilla; op, opisthotic; p, parietal, po, postorbital; pp, postparietal; pt, pterygoid; q, quadrate; qj, quadratojugal; s, stapes; sa, surangular; so, supraoccipital; sq, squamosal; st, supratemporal; t, tabular.
Interpretive illustration of pelvic girdle and right hind limb of KUVP 9616b, drawn from reverse side through clear plastic embedding of specimen. Abbreviations: as, astragalus; ca, calcaneum; f, femur; fi, fibula; il, ilium; is, ischium; pu, pubis; t, tibia.
Estimates of maximum body size of included taxa are placed within three weight categories in order to indicate the general trends, less than 10(green), 10–100 kg (blue), and more than 100 kg (red). We restricted our size ranges to the above bins because several taxa are known only from fragmentary remains or juveniles. Only those herbivorous taxa that were sufficiently complete to permit size estimates were included. See Material and Methods for details on body mass estimates. The taxa shown as open, colored boxes of various shades of green are all considered to be herbivores. The taxa shown as crosshatched boxes are all considered to be either carnivorous, or omnivorous.
The holotype of
Approximate weight estimates as indicators of overall size, as shown in
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “
Synapsida Osborn, 1903
Caseasauria Williston, 1912
Caseidae Williston, 1911
urn:lsid:zoobank.org:act:D81C4A21-EC45-42FF-B4D7-ED79A5130BD8; 0C30568F-331D-40C6-A4C3-D9CBD0058B97.
The generic name refers to the fact that this is the oldest known member of the clade; the specific epithet is an homage to the late Dr. Larry Martin, the original collector of the holotypic specimen.
KUVP 9616b (Dyke Museum of Natural History, University of Kansas), a partial skull and postcranial skeleton of a juvenile individual, including the posterior one-third of the skull and right mandible, nearly complete vertebral column including cervical, dorsal, sacral and caudal vertebrae, pelvic girdle and nearly complete right hind limb (
No permits were required for the described study, which complied with all relevant regulations.
Hamilton Quarry near Hamilton, Greenwood County, Kansas, USA; Calhoun Shale, Shawnee Group, Virgilian Series (Stephanian of Europe), Upper Pennsylvanian.
A small caseid with the following autapomorphies: small, blade-like neural spines in dorsal and anterior caudal vertebrae; first sacral rib with very broad distal head, three times the size of that of the second sacral rib. Differs from other caseids in having unexpanded rib cage, last two dorsal ribs more slender than the other dorsals.
The cranial morphology of
The lower jaw is only partially preserved and exposed in lateral view. The posterior part of the dentary is exposed in lateral view, its dorsal edge being covered by the maxilla. It has a long posterior process that extends along the dorsal edge of the lower jaw, and covers part of the surangular in the region of the low coronoid eminence. In this regard,
There are 27 presacral and two sacral vertebrae in
In contrast to the condition in other caseids, the dorsal ribs of
The shoulder girdle and forelimb are not preserved, but the pelvic girdle and the right hind limb are present. The ilium has the typical caseid morphology of a tall iliac blade with vertical grooves and ridges distally. The blade is centered directly above the acetabulum, with modest anterior and slightly larger posterior processes, similar to that seen in
A phylogenetic analysis using PAUP* 4.0b10
In order to test the affinities of
The discovery of
Herbivory, as defined in this paper, is one of the most important innovations in terrestrial vertebrate evolution, and requires a number of anatomical and physiological attributes for efficient breakdown of the cellulose rich walls of terrestrial plants
In the case of caseids, herbivory is indicated by the presence of a massive rib cage in the thoracic and dorsal regions, and the expanded trunk extends posteriorly to the pelvic girdle, with large ribs fused to the lumbar vertebrae. This osteological evidence of herbivory is present in all caseids, except for
The two localities that have yielded these early caseids are the only ones known to preserve upland faunas during the Early Permian, quite distinct from the typical, widely distributed deltaic, floodplain depositional environments that characterize the vast majority of the Early Permian fossil vertebrate localities of Pangaea, and that are completely devoid of caseid remains. Caseids become common elements of the lowland environments only in the latter part of the Early Permian, during the Kungurian. This occurrence pattern suggests that the early history of caseid evolution, and their initial acquisition of herbivory may have occurred in upland environments. In strong contrast, the early Permian lowland localities contain the remains of two other groups of herbivorous tetrapods, the diadectids and edaphosaurids.
The oldest known herbivorous edaphosaur is
Two other groups of terrestrial vertebrates also acquire herbivory in this initial stage of amniote diversification, diadectid cotylosaurs and captorhinid reptiles. Late Pennsylvanian and Early Permian diadectids also show convincing evidence of dental and skeletal adaptations for herbivory. These enigmatic Paleozoic forms are part of Diadectomorpha, a sister group to crown Amniota. A preliminary phylogeny of diadectids indicates that
The fossil record of captorhinids, the first group of reptiles to diversify extensively in the Paleozoic, extends from the late Pennsylvanian into the Late Permian. Despite an extensive, rich fossil record, evidence of herbivory appears late in the history of this group, in the uppermost part of the Kungurian
An enigmatic group of early parareptiles, the bolosaurids, also show evidence of herbivory. The fossil record of these poorly known small parareptiles extends from the earliest Permian of New Mexico to the Middle Permian of China
The overall pattern provided by the fossil record of synapsids, diadectids, and captorhinids (and possibly bolosaurids) allows us to conclude that herbivory evolved independently in these clades for the first time in the late Pennsylvanian at a time when the terrestrial flora was undergoing some major changes
Direct comparisons between time-calibrated phylogenies (
A somewhat similar pattern is also present in some of the carnivorous clades of synapsids, with early or basal members of each clade being smaller than the younger, more derived species, but carnivorous clades do not show the dramatic increases in size seen in the herbivorous clades. For example, among varanopids, a clade of agile predators that have a similar temporal range as the caseids, we see an increase in size from less than 10 kg in
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We gratefully acknowledge Diane Scott and Nikola Wong Ken for the preparation, photography, and illustrations of the skeleton. The initial acid preparation of the skeleton was performed by Akiko Shinya. We also thank David Mazierski with assistance for the preparation of Figure 4. Useful discussions with Nicolas Campione, Richard Kissel, and Kirstin Brink helped in determining body mass estimates for the diadectids, caseids and edaphosaurs.