[Paleontology • 2026] Isodapedon varzealis • A New hyperodapedontine rhynchosaur (Archosauromorpha: Rhynchosauria) from a Cynodont-dominated Site (Upper Triassic) of southern Brazil

 

Isodapedon varzealis  

Schiefelbein, Garcia, Doering & Müller, 2026

DOI: doi.org/10.1098/rsos.260176 

Artwork by Caio Fantini.

 

Abstract

We describe Isodapedon varzealis gen. et sp. nov., a new hyperodapedontine rhynchosaur from the Carnian deposits of the Candelária Sequence, southern Brazil. Phylogenetic analyses place this taxon outside known South American hyperodapedontines, including Macrocephalosaurus mariensis and ‘Hyperodapedon’ species, and support the restriction of the genus Hyperodapedon to its type species, thereby revealing greater diversity within the group than previously recognized. Isodapedon varzealis gen. et sp. nov. is characterized by symmetrical maxillary tooth bearing areas and distinctive dentary morphology, indicating divergent feeding adaptations relative to known rhynchosaurs. Stratigraphic occurrence in beds correlated with the Exaeretodon sub-zone of the Hyperodapedon Assemblage Zone, and its phylogenetic affinities with early diverging hyperodapedontines from Argentina and Scotland, imply a geographically widespread radiation of conservative morphotypes across southwestern Gondwana. This study highlights the complexity of Late Triassic rhynchosaur evolution and underscores the importance of continued palaeontological investigation in refining the taxonomy, biostratigraphy and palaeoecology of this key herbivorous clade.

Keywords: Rhynchosauria, Late Triassic, Candelária Sequence, Archosauromorpha

Subjects:palaeontology

Geological context of the Várzea do Agudo site and provenance of CAPPA/UFSM 0371. (A) Geologic map of the central region of Rio Grande do Sul (southern Brazil) showing the surface distribution of stratigraphic units of the Paraná Basin. (B) Field photograph of the Várzea do Agudo site. (C) Stratigraphic column of the Várzea do Agudo site depicting its fossiliferous content distribution. (D) Life reconstruction of CAPPA/UFSM 0371 by Caio Fantini. (E) Tridimensional model of the partial cranium of CAPPA/UFSM 0371 in right lateral view.

Abbreviations: cs, coarse-sandstone; Fm, formation; fs, fine-sandstone; g, gravel-sandstone; m, mudstone; ms, medium-sandstone. Silhouettes based on artwork by Caio Fantini and Felipe Elias.

Skull and lower jaws of Isodapedon varzealis gen. et sp. nov. (CAPPA/UFSM 0371).
(A) photographs, interpretative illustration and three-dimensional model from surface scan of the skull of CAPPA/UFSM 0371 in right lateral view. (B) Photographs, interpretative illustration and three-dimensional model from surface scan of the skull of CAPPA/UFSM 0371 in left lateral view.

Abbreviations: a, angular; aoc, anguli oris crest; d, dentary; jug, jugal; max, maxilla; mf, mental foramen; pmx l, left premaxilla; pmx r, right premaxilla; sa, surangular; sp, splenial. Black arrow points anteriorly.

Isodapedon varzealis gen. et sp. nov. craniomandibular elements.
 (A) Left skull in lateral view of Isodapedon varzealis gen. et sp. nov. (CAPPA/UFSM 0371) with emphasis on maxillary dentition area (occlusal view). (B) Left lower jaw in medial view with emphasis on lingual dentition area (oblique occlusal view).

Abbreviations: dg, diagonal gaps; lbt, labial teeth; lgt, lingual teeth; ls, longitudinal sulcus; sg, shallow groove; sms, short medial shelf. Black arrow points anteriorly.

Archosauromorpha 

Rhynchosauria  

Rhynchosauridae 

Hyperodapedontinae 

Isodapedon gen. nov.

Type and only species: Isodapedon varzealis sp. nov.

Etymology. The genus name is derived from the Ancient Greek ἴσος (ísos), meaning ‘equal’ or ‘the same’, as a reference to the equally wide tooth-bearing areas of the maxilla.

Isodapedon varzealis sp. nov.

Etymology. The species name refers to the locality of ‘Várzea do Agudo’ in which the specimen was collected.

Diagnosis. The holotype of Isodapedon varzealis is distinguished from all other known rhynchosaurids by the following unique combination of morphological traits (* asterisk denotes autapomorphies): maxilla divided into two tooth-bearing areas by a single longitudinal sulcus; equally wide maxillary lateral and medial tooth-bearing areas; same number (three) of longitudinal rows of teeth in both maxillary lateral and medial tooth-bearing areas; absence of lingual teeth in the medial surface of the maxilla; straight posterior margin of the parietal; sharp sagittal crest extending up to the posterior margin of the parietal; short and rounded ventral process of the squamosal; distinct posterior process in the squamosal; no contact between the lacrimal and palatine; ‘blunt’ and greatly reduced posteroventral process of the dentary; single dorsal blade in the dentary; medial shelf in the medial surface of the posterior portion of the dentary that does not reach half the length of the bone*; medial shelf separated from the dorsal blade of the dentary by a shallow groove; dorsal margin of the posterior portion of the dorsal blade extending markedly further dorsally than the posterior portion of the medial shelf at the level of the posteriormost lingual tooth; and single well-defined row of small, dorsally projected, closely packed lingual teeth in the dentary.

Artistic representation of a Late Triassic landscape of southern Brazil depicting individuals of Isodapedon varzealis gen. et sp. nov. in the foreground and a proterochampsid in the background.

Artwork by Caio Fantini.

 

Jeung Hee Schiefelbein; Maurício Silva Garcia; Mariana Doering and Rodrigo Temp Müller. 2026. A New hyperodapedontine rhynchosaur from a Cynodont-dominated Site (Upper Triassic) of southern Brazil. R Soc Open Sci. 13(4); 260176. DOI: doi.org/10.1098/rsos.260176 [15 Apr 2026]

[Paleontology • 2023] Bentonyx sidensis • Unique Dentition of rhynchosaurs (Archosauromorpha: Rhynchosauria) and their Two-phase Success as Herbivores in the Triassic

Bentonyx sidensis  Langer, Montefeltro, Hone, Whatley & Schultz, 2010

in Sethapanichsakul, Coram & Benton, 2023. 

DOI: 10.1111/pala.12654

Abstract

Rhynchosaurs were key herbivores over much of the world in the Middle and Late Triassic, often dominating their faunas ecologically, and much of their success may relate to their dentition. They show the unique ankylothecodont mode of tooth implantation, with deep roots embedded in the bone of the jaw and low crowns that were rapidly worn down in use. During growth, the main area of oral food processing, located in the middle and posterior portions of the occlusal surfaces of the jaws, moved posteriorly relative to the anterior tips of the jaws, which curved up. As the maxilla and dentary grew by addition of new bone posteriorly, the dental lamina fed in new teeth at the back of the tooth rows. CT scanning of the holotype skull of Bentonyx sidensis from the Middle Triassic of England reveals previously concealed details of the dentition. Together with new dentary material from the same location, this has enabled us to examine the tooth replacement process and elucidate ontogenetic changes in dentition and jaw morphology as the animals aged. There were major changes in rhynchosaur anatomy and function through their evolutionary history, with the early forms of the Middle Triassic dying out before or during the Carnian Pluvial Episode (233–232 Ma), and the subclade Hyperodapedontinae, with broad skulls and adaptations to chop tough vegetation, subsequently diversifying worldwide in a successful ecological expansion until their global extinction 227–225 Ma.

Keywords: dentition, rhynchosaur, tooth replacement, herbivory, Triassic, Carnian Pluvial Episode

Bentonyx sidensis 

 

Thitiwoot Sethapanichsakul, Robert A. Coram and Michael J. Benton. 2023. Unique Dentition of rhynchosaurs and their Two-phase Success as Herbivores in the Triassic. Palaeontology. 66(3); e12654. DOI: 10.1111/pala.12654

[Paleontology • 2023] Beesiiwo cooowuse • A New Rhynchosaur Taxon (Archosauromorpha: Rhynchosauridae: Hyperodapedontinae) from the Popo Agie Formation, Wyoming: Implications for A Northern Pangean Early-Late Triassic (Carnian) Fauna

 

Beesiiwo cooowuse

Fitch, Haas,C’Hair, Ridgley, Ridgley, Oldman, Reynolds & Lovelace, 2023

DOI: 10.3390/d15040544

artwork: Gabriel Ugueto  twitter.com/SerpenIllus

  Abstract

New discoveries in the lower Popo Agie Formation (lower carbonate unit) of central Wyoming necessitated a reevaluation of USNM 494329 from the same unit, the only known hyperodapedontine rhynchosaur in western North America. Well known from Gondwanan deposits, hyperodapedontines appear to be restricted to the Carnian age (Late Triassic), with the exception of Teyumbaita in the earliest Norian age (Late Triassic) of Brazil. Initially assigned to c.f. ‘Hyperodapedon’ sanjuanensis, our phylogenetic analyses reject this hypothesis, in support of a sister relationship between USNM 494329 (Beesiiwo cooowuse, gen. et sp. nov.) and Oryctorhynchus bairdi forming an early-diverging clade that is only distantly related to ‘H.’ sanjuanensis. Five additional specimens recovered from the lower Popo Agie are described. Three are referred to B. cooowuse, and another two are placed closer to Hyperodapedon and the remainder of Hyperodapedontinae. Our analysis demonstrates potential temporal distinction between a grade of earliest-diverging hyperodapedontines (including all Wyoming taxa) and a exclusively Late Carnian, Southern Pangaean hyperodapedontine clade (including ‘H.’ sanjuanensis). We consider the lower Popo Agie Formation to represent the first nonmarine Late Triassic unit of Western North America that can be confidently restricted to the Carnian age.

Keywords: Popo Agie Formation; Late Triassic; rhynchosaur; Hyperodapedontinae; Carnian

(A) The type specimen for Beesiiwo cooowuse (USNM 494329); occlusal view (top left/center) and axial cross section (bottom left/center) of posterior maxilla, and occlusal view of dentary (top/bottom right). Note that the row of teeth medial to L1 and L2 is not M1, but a fragment of the dentary that remains slotted into the longitudinal groove of the maxilla.
(B–D) Occlusal view of three left maxillae from Cottonwood Creek referred to Beesiiwo cooowuse. Note the presence of M1 and absence of any additional medial rows.

All scale bars = 1 cm. Illustration CC by Jim Morrison.

 Systematic Paleontology

Sauria McCartney, 1802, sensu Gauthier et al., 1988

Archosauromorpha von Huene, 1946 sensu Dilkes, 1998 

Rhynchosauridae Cope, 1871 sensu Dilkes, 1998 

Hyperodapedontinae (Chatterjee, 1969 nom. trans. ex Lydekker, 1885 sensu Langer and Schultz, 2000)

Beesiiwo gen. nov.

Etymology: Beesiiwo, from Arapaho “Beesiiwó” (pronounced “ Bah - se - wa - ’ ”, with the apostrophe representing a glottal stop) meaning “big lizard”.

Pronunciation guide: The sound for b is roughly as in English, but more like b before other consonants or at the end of a word; the sound for ee is as in the English ‘bat’ but is held longer; the sound for s is like in English, but always a nonvocailized, never vocalized as in some English s and all English z sounds; the sound for ii is as in English ‘beat’ but held longer; the sound for o is as in English ‘hot; the sound for ‘ is a glottal stop, as in the middle of oh-oh! in English.

 

Beesiiwo cooowuse sp. nov.

Holotype: USNM 494329, fragment of a left maxilla and a left dentary.

 

Etymology: cooowuse, from Arapaho “ co’ oowu se’ ” (pronounced “ ja’ aw-wu sa’ ”, with the apostrophe representing a glottal stop); meaning the “Alcova area”, central WY.

Pronunciation guide: The sound for c is like that of the English j but without any puff of air and more like the English ch before other consonants or at the end of a word; the sound for oo is as in the English ‘got’ but held longer; the sound for w is like in English; the sound for u is as in the English ‘put’; the sound for e is as in English ‘bet’.

Occurrence: The type specimen was collected as clasts within a 0.5–1 m thick microconglomerate in the lower carbonate unit of the Popo Agie Formation near Hole in the Wall, Southern Bighorn Mountains, WY (‘Willow Creek’ of [22]). Referred material (see below) was collected as rounded to partially rounded clasts in a 0.5–2 m thick microconglomerate also referred to the lower carbonate unit of the Popo Agie Formation, Cottonwood Creek near Alcova, WY.

Diagnosis: Diagnosed by the following combination of character-states: maxilla with two lateral tooth rows and one medial tooth row; teeth of the lateral tooth row L1 lacking prominent anterolingual flanges; crest-shaped areas, respectively, medial and lateral to a longitudinal groove on the occlusal surface of the maxilla; dentary without a row of lingual teeth.

 Life reconstruction of Beesiiwo cooowuse with Heptasuchus clarki in the background representing the known fauna from the lower carbonate unit of the Popo Agie Formation.

artwork: Gabriel Ugueto  twitter.com/SerpenIllus

 

Adam J. Fitch, Merle Haas,Wayne C’Hair, Eugene Ridgley, Ben Ridgley, Devin Oldman, Crystal Reynolds and David M. Lovelace. 2023. A New Rhynchosaur Taxon from the Popo Agie Formation, WY: Implications for a Northern Pangean Early-Late Triassic (Carnian) Fauna. Diversity. 15(4), 544. DOI: 10.3390/d15040544

  twitter.com/SerpenIllus/status/1645430371939414017

[Paleontology • 2020] Oryctorhynchus bairdi • A New Rhynchosaur (Reptilia, Archosauromorpha) from the Upper Triassic of Eastern North America

 Oryctorhynchus bairdi

Sues, Fitch & Whatley, 2020

DOI: 10.1080/02724634.2020.1771568 

 facebook.com: Adam Fitch

ABSTRACT

This paper provides a description and analysis of cranial remains of a rhynchosaur from the Upper Triassic Evangeline Member of the Wolfville Formation of Nova Scotia, Canada. This material, primarily comprising jaw elements, represents the first definitive record of Late Triassic rhynchosaurs from eastern North America. All cranial bones can be assigned to Hyperodapedontinae. The dentition of the Nova Scotian hyperodapedontine differs from those of Hyperodapedon and Isalorhynchus. The maxilla apparently did not have more than two lateral rows and two medial rows of teeth. The teeth of the lateral row closest to the single groove dividing the maxillary tooth plate are more than twice as large as those of the medial row closest to the groove. The lateral and medial tooth-bearing portions of the maxilla become increasingly crest-like during ontogeny. The dentary lacks a lingual row of teeth. Based on this combination of features, the hyperodapedontine material from the Evangeline Member is assigned to a new taxon, Oryctorhynchus bairdi, gen. et sp. nov., which is the sister species to an unnamed hyperodapedontine taxon from Wyoming. This clade was recovered as the sister taxon to Hyperodapedon spp. in the phylogenetic analysis. The late Carnian or earliest Norian tetrapod assemblage from the Evangeline Member represents a mixture of faunal elements from a wide paleolatitudinal range, suggesting a more cosmopolitan distribution for continental tetrapods during the early Late Triassic than previously assumed.

SYSTEMATIC PALEONTOLOGY 

REPTILIA Laurenti, 1768 

DIAPSIDA Osborn, 1903, sensu Benton, 1985 

ARCHOSAUROMORPHA Huene, 1946, sensu Benton, 1985 

RHYNCHOSAURIDAE Cope, 1871, sensu Dilkes, 1998

HYPERODAPEDONTINAE Lydekker, 1885, sensu Schultz, Langer, and Montefeltro, 2016 

ORYCTORHYNCHUS BAIRDI, gen. et sp. nov.

Etymology— From Greek oryktos, dug, mined, alluding to the inferred digging habits of rhynchosaurs and the derivation of the name ‘Minas Basin’ from the French ‘les mines,’ and Greek rhynchos, snout. The specific epithet honors the late Donald Baird for his pioneering work on Triassic tetrapods from Nova Scotia.

Hans-Dieter Sues, Adam J. Fitch and Robin L. Whatley. 2020. A New Rhynchosaur (Reptilia, Archosauromorpha) from the Upper Triassic of Eastern North America. Journal of Vertebrate Paleontology. e1771568. DOI: 10.1080/02724634.2020.1771568

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