Non-technical summary
Captorhinidae is a diverse group of small to medium-sized reptiles known from the Pennsylvanian and Permian. Some of its species evolved jaws with multiple rows of teeth that suggest a diet of high-fiber plant material. The authors describe in detail the known material of an early captorhinid with multiple tooth rows from the lower Permian (Cisuralian) of Oklahoma and north-central Texas. Because it differs from other known captorhinid species, it is placed in its own new genus, Sumidadectes.
Introduction
Captorhinidae is a clade of early eureptiles that ranged in time from the Late Carboniferous (Kasimovian-Gzhelian) to the latest Permian (Lopingian). It attained a nearly worldwide distribution by the middle and late Permian, with records from the United States (e.g., Olson, Reference Olson1962), Brazil (Cisneros et al., Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020), China (Reisz et al., Reference Reisz, Liu, Li and Müller2011; Liu, Reference Liu2023), Germany (Fröbisch et al., Reference Fröbisch, Kammerer and Sues2017), India (Kutty, Reference Kutty1972), Morocco (Jalil and Dutuit, Reference Jalil and Dutuit1996), Niger (de Ricqlès and Taquet, Reference de Ricqlès and Taquet1982), Russia (Vjushkov and Chudinov, Reference Vjushkov and Chudinov1957), South Africa (Modesto and Smith, Reference Modesto and Smith2001), Spain (Matamales-Andreu et al., Reference Matamales-Andreu, Roig-Munar, Oms, Galobart and Fortuny2021), Zambia (Gow, Reference Gow2000), and Zimbabwe (Gaffney and McKenna, Reference Gaffney and McKenna1979). The oldest known captorhinids were faunivores or omnivores whereas later forms predominantly subsisted on high-fiber plant material (Reisz and Sues, Reference Reisz, Sues and Sues2000). Traditionally, these taxa were considered part of the group Captorhinomorpha that was regarded as the stem from which all other amniote lineages diverged (e.g., Romer, Reference Romer1966). More recent studies indicated that Captorhinidae is a clade of early-diverging eureptiles and more diverse in terms of craniodental features than previously assumed (Sumida et al., Reference Sumida, Dodick, Metcalf and Albright2010; Reisz et al., Reference Reisz, Liu, Li and Müller2011).
Some captorhinids are among the earliest known amniotes capable of feeding on high-fiber plant matter (Hotton et al., Reference Hotton, Olson, Beerbower, Sumida and Martin1997; Reisz and Sues, Reference Reisz, Sues and Sues2000). This inference is based on their complex dentitions with posterior tooth plates bearing multiple rows of maxillary and dentary teeth, which would have facilitated oral processing of plant material prior to ingestion, and a craniomandibular joint that facilitated fore-and-aft mandibular motion (Hotton et al., Reference Hotton, Olson, Beerbower, Sumida and Martin1997; Reisz and Sues, Reference Reisz, Sues and Sues2000). Captorhinids with multiple tooth rows have long been considered the most derived members of this clade. The phylogenetic analysis of Captorhinidae by de Ricqlès and Taquet (Reference de Ricqlès and Taquet1982) first grouped them together as Moradisaurinae. More recent studies have suggested that multiple rows of teeth evolved more than once among Captorhinidae (Dodick and Modesto, Reference Dodick and Modesto1995). Some phylogenetic analyses (Modesto et al., Reference Modesto, Lamb and Reisz2014; Reisz et al., Reference Reisz, LeBlanc, Sidor, Scott and May2015; Cisneros et al., Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020) recovered the taxon studied in this paper, ‘Captorhinikos’ chozaensis Olson, Reference Olson1954, which has multiple tooth rows, outside the clade Labidosaurus Cope, 1896 + Moradisaurinae. This implies either the loss of all but one tooth row in Labidosaurus or the independent acquisition of multiple tooth rows in ‘Captorhinikos’ chozaensis and Moradisaurinae. More recently, Modesto et al. (Reference Modesto, Richards, Ide and Sidor2019) and Liu (Reference Liu2023) recovered ‘Captorhinikos’ chozaensis as the phylogenetically basalmost moradisaurine.
Olson (Reference Olson1954) briefly described and named a new genus of Captorhinidae, Captorhinikos Olson, Reference Olson1954, with two species, Captorhinikos valensis Olson, Reference Olson1954 (type species) from the upper part of the Vale Formation of Knox County (Texas) and Captorhinikos chozaensis from the lower Choza Formation of Foard County (Texas). Reisz et al. (Reference Reisz, Liu, Li and Müller2011) were the first researchers to include Captorhinikos as a separate operational taxonomic unit (OTU) in their phylogenetic analysis of Captorhinidae, but, according to Modesto et al. (Reference Modesto, Lamb and Reisz2014), their codings were based mostly on ‘Captorhinikos’ chozaensis. Olson's (Reference Olson1954) brief original description of Captorhinikos chozaensis was accompanied by sketches of the holotypic mandible. Vaughn (Reference Vaughn1958) described (but did not illustrate) a partial skull and a number of associated postcranial bones from the Hennessey Formation of Cleveland County (Oklahoma) and referred it to Captorhinikos chozaensis. Subsequently, Olson (Reference Olson1962) reported the discovery of three additional specimens of Captorhinikos chozaensis, including an incomplete skull and much of an articulated postcranial skeleton, from the Hennessey Formation near Norman in Cleveland County (Oklahoma). He also briefly reviewed salient anatomical details of this taxon. Finally, Olson (Reference Olson1970) named a third species of small-bodied captorhinid, Captorhinikos parvus Olson, Reference Olson1970 from the Hennessey Formation of Cleveland County (Oklahoma). Most recently, Cisneros et al. (Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020) identified two natural molds of a hemimandible and a dentary from the lower Permian (Cisuralian) Pedra de Fogo Formation of Piauí State (Brazil) as Captorhinikos sp.
In recent years, phylogenetic analyses of captorhinid eureptiles have consistently found that the genus Captorhinikos, as defined by Olson (Reference Olson1954, Reference Olson1970), is not monophyletic (Modesto et al., Reference Modesto, Lamb and Reisz2014). The three species assigned to this genus by Olson have been recovered in different positions in the tree topologies. Modesto et al. (Reference Modesto, Lamb and Reisz2014) re-evaluated the type species Captorhinikos valensis, arguing that it is quite distinct from what they referred to as ‘Captorhinikos’ chozaensis. Albright et al. (Reference Albright, Sumida and Jung2021) placed ‘Captorhinikos’ parvus in a new genus Rhodotherates Albright, Sumida, and Jung, Reference Albright, Sumida and Jung2021. Here we re-examine ‘Captorhinikos’ chozaensis for comparison with other known captorhinids and assess its phylogenetic relationships. This species was previously documented only in a fairly cursory manner, which has rendered comparisons with other captorhinid taxa difficult.
Geological context
The skeletal remains of ‘Captorhinikos’ chozaensis were recovered from early Permian (Cisuralian) continental sedimentary rocks in Texas and Oklahoma. Cummins (Reference Cummins1908) introduced the name ‘Clear Fork beds’ for continental strata that overlie those of the Wichita Group and underlie those forming the Pease River Group in central Texas. In their geological survey of Runnels County, Beede and Waite (Reference Beede and Waite1918) divided what is now termed the Clear Fork Group (or Formation) of north-central Texas into the Vale Formation, Bullwagon Dolomite, and Choza Formation. They also named the Arroyo Formation but considered it part of the underlying Wichita Group. Later, Sellards et al. (Reference Sellards, Adkins and Plummer1932) formally included the Arroyo Formation in the Wichita Group. This modified stratigraphic division was subsequently adopted by many students of the fossil vertebrates (e.g., Lucas, Reference Lucas2006).
More recent geological surveys have led to a reassessment of the stratigraphic succession. The Clear Fork is now mapped as a formally undivided group on the sheets of the Geologic Atlas of Texas (Nelson et al., Reference Nelson, Hook and Chaney2013). Nelson et al. (Reference Nelson, Hook and Tabor2001) informally divided the Clear Fork Formation into lower, middle, and upper units and listed a number of informal beds and members. The tetrapod fossils reported by Olson and his students came, for the most part, from the middle unit of the Clear Fork Formation, which Olson (Reference Olson1958) termed the middle and upper Vale Formation.
Olson (Reference Olson1967) suggested a correlation between the Hennessey Formation of central Oklahoma and the Choza ‘Formation’ of north-central Texas. This correlation places the former in the Leonardian Series and, on the international geological time scale, in the middle part of the Kungurian Stage (Nelson et al., Reference Nelson, Hook and Chaney2013). Lucas (Reference Lucas2006) regarded the Hennessey tetrapod assemblage as an example of his Redtankian Land Vertebrate Faunachron (LVF), which also includes the tetrapod assemblages from the Clear Fork Formation in north-central Texas.
Materials and methods
All specimens used in this study were originally mechanically prepared. The holotype of ‘Captorhinikos’ chozaensis, FMNH UR 97, includes conjoined mandibular rami, which have been ground down irregularly more posteriorly to expose the tooth rows on each ramus (Fig. 1.1), and a maxilla fragment that was treated in the same manner (Fig. 1.2). An obliquely dorsoventrally crushed skull, FMNH UR 183 (Fig. 1.3, 1.4), was first tentatively identified as Labidosaurikos barkeri Olson, Reference Olson1954 by Seltin (Reference Seltin1959), but Olson (Reference Olson1962) reassigned it to ‘Captorhinikos’ chozaensis. The latter author briefly described three additional specimens (FMNH UR 857, 858, 859) that had been discovered in a sandy shale of the Hennessey Formation exposed in a roadside ditch. The ditch would periodically carry water, which infiltrated the rock and badly damaged the skeletal remains. FMNH UR 857 and 859 were recovered together on a single slab.
Figure 1. (1) Mandible of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., FMNH UR 97, holotype, in occlusal view. (2) Fragment of maxilla of Sumidadectes chozaensis, FMNH UR 97, holotype, in occlusal view. Arrow points anteriorly. (3, 4) Skull of Sumidadectes chozaensis, FMNH UR 183, in dorsal (3) and ventral (4) views. Scale bars = 2 cm (1, 3, 4); 1 cm (2).
USNM V 21275 originally comprised a collection of bone fragments, many of which were subsequently found to fit together during preparation. Its partial skull was reassembled from ~25 pieces (Vaughn, Reference Vaughn1958). It is dorsolaterally crushed and lacks most of the postorbital region of the cranium including the braincase (except for the supraoccipital and partly fused fragments of the basioccipital and exoccipitals; Fig. 2). The left quadratojugal and much of the adjoining squamosal were displaced and pushed over the posterior region of the jugal. As estimated by Olson (Reference Olson1962), the skull was slightly more than 12 cm long. The complete left mandibular ramus is 13.5 cm long. After removal of a section of the right mandibular ramus, part of the right maxilla was ground down to expose part of its five tooth rows. A crust of iron oxide tightly adheres to the bone and thus grinding was originally employed to expose the bone surface. This effort obliterated much of the external surface of most cranial elements and badly damaged parts of the skull such as the tip of the snout. It also made tracing many sutures difficult if not impossible.
Figure 2. Partial skull and mandible of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., USNM V 21275, in dorsolateral (1), left lateral (2), and ventral (3) views. af, articular facet for jaw joint; an, angular; ar, articular; c, coronoid; d, dentary; dc, caniniform tooth of dentary; en, external narial fenestra; f, frontal; j, jugal; l, lacrimal; m, maxilla; mtp, maxillary tooth plate; n, nasal; or, orbit; pa, prearticular; pl, palatine; plp, posterolateral process of articular; pm, premaxilla; po, postorbital; prf, prefrontal; q, quadrate; qj, quadratojugal; sa, surangular; sm, septomaxilla; sp, splenial; sq, squamosal. Gray indicates restored regions on specimen. Scale bar = 2 cm.
Nomenclatural procedure
This work and its nomenclatural act have been registered at Zoobank. The publication is registered under LSID urn:lsid:zoobank.org:pub:0F89869A-1889-4D90-B721-A89BA5F40C4B. Sumidadectes new genus has been registered under LSID urn:zoobank.org:act:4D181F30-8F02-41B9-B16B-74D834CCD36E.
Repositories and institutional abbreviations
FMNH UR, fossil reptile collection, Field Museum of Natural History, Chicago, Illinois, USA; USNM V, Division of Vertebrate Paleontology, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA.
Systematic paleontology
Reptilia Laurenti, Reference Laurenti1768 sensu Modesto and Anderson, Reference Modesto and Anderson2004
Eureptilia Olson, Reference Olson1947 sensu Tsuji and Müller, Reference Tsuji and Müller2009
Captorhinidae Case, Reference Case1911
Moradisaurinae de Ricqlès and Taquet, Reference de Ricqlès and Taquet1982 sensu Modesto et al., Reference Modesto, Lamb and Reisz2014
Sumidadectes new genus
Type species
Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb. (by monotypy).
Diagnosis
Characterized by the following combination of character states: Outline of cranium heart-shaped in dorsal/ventral view. Premaxilla with three teeth, the third of which is smaller than and positioned posterolabial to first two teeth. Maxilla without caniniform tooth. Maxillary tooth plate bearing five tooth rows and forming slight lingual overhang. Dentary with caniniform tooth. Dentary tooth plate bearing four tooth rows.
Etymology
Genus named in honor of Stuart S. Sumida, recognizing his work on Permo-Carboniferous tetrapods including captorhinids, and the Greek dektes, biter.
Occurrence
Cisuralian (Kungurian), Oklahoma and Texas.
Remarks
As discussed in detail in the section on phylogenetic relationships, we segregate ‘Captorhinikos’ chozaensis from the non-monophyletic genus Captorhinikos and propose the new genus for its reception. Captorhinikos is now restricted to the type species Captorhinikos valensis Olson, Reference Olson1954.
Sumidadectes chozaensis (Olson, Reference Olson1954) new combination
Figures 1–6
- Reference Olson1954
Captorhinikos chozaensis Olson, p. 216.
- Reference Modesto, Lamb and Reisz2014
‘Captorhinikos’ chozaensis Modesto et al., p. 299.
Figure 3. (1) Right upper dentition of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., FMNH UR 183, photograph and color-coded tooth distinctions in occlusal view. Teeth outlined in pink are premaxillary teeth; teeth outlined in white are single-row maxillary teeth; teeth outlined in multiple colors represent tooth rows on the maxillary tooth plate. (2) Left lower dentition of Sumidadectes chozaensis, FMNH UR 97, holotype, photograph and color-coded tooth distinctions in occlusal view. Teeth outlined in white are single-row dentary teeth; teeth outlined in multiple colors represent teeth on the dentary tooth plate. Scale bars = 1 cm.
Figure 4. Isolated supraoccipital of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., USNM V 21275, in posterior (1), dorsal (2), anterior (3), and anteroventral (4) views. dlp, dorsolateral process; dmp, dorsomedial process; fm, foramen magnum; mb, impression of membranous semicircular canal; ms, median septum; v, vascular foramen; vs, vascular sulcus. Arrow in (2) points in anterior direction. Scale bar = 1 cm.
Figure 5. Dorsal vertebrae of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., USNM V 21275. (1, 2) Three anterior dorsal vertebrae in left lateral (1) and dorsal (2) views. White outlines delineate apices of neural spines; arrow points to low neural spine. (3) Two posterior dorsal vertebrae in left lateral view. ic, intercentrum. Scale bar = 1 cm.
Figure 6. Hindlimb bones of Sumidadectes chozaensis (Olson, Reference Olson1954) n. comb., USNM V 21275. (1–4) Right femur in dorsal (1), ventral (2), anterior (3), and posterior (4) views. (5) Right fibula. (6, 7) Right astragalus, calcaneum, centrale, and metatarsal in dorsal (6) and ventral (7) views. Green arrow indicates groove for perforating artery; star indicates fracture line in astragalus. as, astragalus; c, centrale 3; ca, calcaneum; fi, articular surface for fibula; ft, fourth trochanter; fta, distal end for contact with proximal tarsals; icf, intercondylar groove; if, intertrochanteric fossa; it, internal trochanter; mt, metatarsal; pa, proximal articular surface; ti, attachment surface for tibiale. Scale bars = 2 cm (1–4); 1 cm (5–7).
Holotype
FMNH UR 97, conjoined mandibular rami (Olson, Reference Olson1954, fig. 86A, B; Fig. 1.1), fragment of the probably left maxillary tooth plate (Fig. 1.2) and an unidentified piece of flat bone. Clear Fork Formation (“middle of lower part of Choza Formation”; Olson, Reference Olson1954, p. 216), Olson's (Reference Olson1948) FA Locality (green nodule site), Foard County, Texas. Cisuralian (Kungurian).
Diagnosis
As for the genus, diagnosed above.
Occurrence
Cisuralian (Kungurian), Oklahoma and north-central Texas.
Description
Cranium
Only FMNH UR 183 and 97 and USNM V 21275 present information on the skull roof. They all are incomplete and were moderately to severely affected by crushing during fossilization. In addition, FMNH UR 857 includes a poorly preserved incomplete skull exposed in ventral view.
All dermal bones of the skull exhibit external ornamentation composed of pits and grooves typical of captorhinids. Preparation damage did not permit a more detailed description of the ornamentation on various regions of the skull. In dorsal view, the skull is heart-shaped in outline with an expanded postorbital region (Olson, Reference Olson1962). It lacks the distinctly offset snout present in more derived captorhinids, e.g., Labidosaurikos meachami Stovall, Reference Stovall1950 (Dodick and Modesto, Reference Dodick and Modesto1995).
The premaxilla is deflected anteroventrally as in most other captorhinids. Each element holds three teeth with posteroventrally extending crowns that are tall, cylindrical, and conical toward the apex. The first and second teeth are large. The third premaxillary tooth is smaller than the first and second and situated labial and slightly posterior to them; in USNM V 21275, the second tooth is slightly taller than the first. The anterodorsal processes of the premaxillae meet the nasals along a deeply interdigitated suture in USNM V 21275. This unusual interdigitation suggests that this sutural contact was subject to considerable compressive forces, which possibly resulted from grubbing with the large premaxillary teeth (Hotton et al., Reference Hotton, Olson, Beerbower, Sumida and Martin1997). The slender, posterodorsally inclined posterior process of the premaxilla is overlapped by an anteroventrally extending process of the maxilla.
The crescent-shaped septomaxilla is visible in both external narial openings of USNM V 21275. However, grinding has obliterated any features.
The maxilla is preserved in FMNH UR 97, 183, and 857 and in USNM V 21275. In FMNH UR 97 and 857, the maxillae are incomplete and badly damaged. The maxilla is long and (in lateral view) dorsoventrally low. Its posterior portion turns slightly laterally below its sutural contact with the jugal. Posteriorly, the maxilla extends back to the approximate level of the center of the orbit. Anteriorly, the bone enters into the posterior margin of the external naris. Dorsally, the maxilla contacts the lacrimal along a posteriorly rising suture that continues to its junction with the jugal. Medially, the maxilla meets the palatine along a straight suture.
Following Dodick and Modesto (Reference Dodick and Modesto1995) and other recent studies, we divide the maxillary dentition into an anterior single-rowed (SR) region and a posterior multiple-rowed (MR) region. In USNM V 21275, the posterior region of the right maxilla was ground down during preparation to expose the tooth rows for specimen identification (Vaughn, Reference Vaughn1958; Fig. 2.3). The teeth are slightly inset from the lateral margin of the maxilla. FMNH UR 183 preserves most of the maxillary dentition on the right side of the snout (Figs. 1.4, 3.1). The SR region has up to five teeth. Each well-preserved tooth is round in cross section at the base and conical at the apex (bullet-shaped). In FMNH UR 183 and USNM V 21275, the first and second tooth are smaller and less massive than the third and fourth; in the former, the fourth tooth is considerably larger than the three other teeth and marks the transition to the MR region. In specimens of Captorhinikos valensis, the third and especially the fourth SR teeth are also large (LeBlanc et al., Reference LeBlanc, Brar, May and Reisz2015). There is no maxillary caniniform tooth. Posteriorly, the MR region of teeth extends to a point just anterior to the posterior margin of the orbit. It forms a tooth plate with five subparallel, anteroposteriorly extending rows of teeth. In all available specimens, the exposed MR region of teeth is incomplete posteriorly. On the nearly complete right maxillary tooth plate of FMNH UR 183 (Fig. 3.1), the outermost row (R1) comprises at least nine teeth or tooth cross sections; the second row (R2, lingual to R1) has 11; the third row (R3) comprises 11; the fourth row (R4) has 12; and the poorly preserved innermost row (R5) comprises at least six. This agrees with the condition in moradisaurine captorhinids, which have maxillary tooth plates with > 40 teeth (Modesto et al., Reference Modesto, Flear, Dilney and Reisz2016). Rows two through five gently arc lingually at midlength and slightly converge anteriorly. This differs from the condition in Labidosaurikos meachami in which all tooth rows terminate at roughly the same level anteriorly (Dodick and Modesto, Reference Dodick and Modesto1995). Anteriorly, rows two through four extend up to the enlarged fourth tooth in the ST row. Where the teeth were not broken or ground down during preparation, the tooth crowns are bullet-shaped with rounded apices. In FMNH UR 183, the teeth at the posterior end of tooth rows one and four are slightly larger than those mesial to them.
The nasals are partly preserved but mostly covered by matrix in USNM V 21275 and more completely in FMNH UR 183. The nasal is gently convex transversely and more or less rectangular in outline.
The lacrimal is preserved in FMNH UR 183 and USNM V 21275. As in other early amniotes, the lacrimal extends anteroposteriorly between the orbit and the external narial opening. Its dorsal contact with the prefrontal forms a short spur along the orbital margin posteriorly as in other captorhinids, e.g., Captorhinus laticeps (Williston, Reference Williston1909) (Heaton, Reference Heaton1979); Olson (Reference Olson1962, fig. 10B, C) misidentified this part as the prefrontal. Ventrally, the lacrimal contacts the maxilla along a straight suture back to the point where the lacrimal, maxilla, and jugal meet. The suture with the jugal extends posterodorsally to the anterior margin of the orbit.
The prefrontal is only partly preserved on the left side in USNM V 21275. Medially, it contacts the frontal along a relatively straight suture, which extends toward the dorsal margin of the orbit posterolaterally. The prefrontal forms the anterodorsal corner of the orbital margin.
The frontal, which is preserved only in USNM V 21275, is rather flat. It participates in the dorsal margin of the orbit. Because most of the frontals are covered by an intractable covering of iron oxide, no additional details can be identified.
The jugal is incompletely preserved in FMNH UR 183 and USNM V 21275. Its lateral surface is mostly flat but curves slightly laterally where the cranium increases in transverse width posteriorly. Anterior to the orbit, the jugal meets the lacrimal along a suture that curves gently dorsally toward the orbit. Behind the orbit, the element considerably increases in dorsoventral depth and contacts the postorbital dorsally along a posterodorsally extending suture. Posteriorly, the jugal meets the squamosal dorsally and the quadratojugal ventrally.
The postorbital is partly preserved in FMNH UR 183 and USNM V 21275; in the latter, only part of the anterior portion of the element has been preserved. It forms the posterodorsal margin of the orbit.
The squamosal is incompletely preserved in FMNH UR 183, and only small portions of the element remain in FMNH UR 857 and USNM V 21275. It is a large bone, but few details of its structure are evident. The squamosal contacts the quadratojugal ventrally and the quadrate posteromedially.
The quadratojugal is incompletely preserved in FMNH UR 183 and USNM V 21275. It is quadrangular in lateral view. Anteriorly, the quadratojugal contacts the jugal. The suture between the quadratojugal and the squamosal appears to be relatively straight but extends somewhat dorsally at the corner between the lateral and occipital surfaces of the cranium.
Only the ventral end of the displaced left quadrate is preserved in USNM V 21275. Its articular surface is divided by a broad trough into steeply medioventrally inclined lateral and medial portions and is shorter anteroposteriorly than the corresponding facet on the articular.
The palate is poorly preserved in FMNH UR 183 and 857 and USNM V 21275. Only the palatine and pterygoid are identifiable. The palatine is partly exposed medial to the left maxilla in USNM V 21275 but shows little detail. The pterygoid is incompletely preserved in FMNH UR 183 and 857. Its transverse flange forms a rather flat, posterolaterally curving, triangular plate of bone. In FMNH UR 183, the transverse flange of the left pterygoid bears a posteromedial patch of denticles whereas the right element (which had been covered with glue) appears to have denticles in the same region as well as in a transverse row. Due to inadequate preservation, it is uncertain whether the pterygoid had a transverse row of teeth; the latter is absent in Gansurhinus spp. and Rothianiscus spp. (Liu, Reference Liu2023). The quadrate flange of the pterygoid is flat and thin.
Braincase
In FMNH UR 857, the braincase was separated from the dermatocranium and displaced during fossilization. Parts of the parabasisphenoid, the stapes, basioccipital, opisthotics, and supraoccipital are exposed. The foramen magnum is visible as a small, ovoid opening.
The parabasisphenoid is partly preserved and its lateral contact with the stapes is evident. More posteriorly, the element meets the basioccipital. Just lateral to the contact with the basioccipital, the parabasisphenoid contacts the opisthotic along a short, straight suture.
The right stapes including the stapedial foramen is visible in FMNH UR 857. It extends posterolaterally toward the quadrate. The stapedial shaft tapers distally.
The basioccipital is preserved in FMNH UR 857 and as part of a small fragment including part of the occipital condyle in USNM V 21275. In the latter, the bone is partly fused to the ventral portions of the exoccipitals, which meet along the midline and exclude the basioccipital from the floor of the cavum cranii. The internal surface of each exoccipital bears two foramina for passage of branches of CN. XII (N. hypoglossus).
Both opisthotics are preserved in FMNH UR 857. The medial portion of the opisthotic is robust. Posteriorly and laterally, it slightly decreases in width just beyond its contact with the supraoccipital.
The isolated, nearly complete supraoccipital of USNM V 21275 (Fig. 4) was loosely attached to the remainder of the cranium in life, as reported for Labidosaurikos meachami (see Dodick and Modesto, Reference Dodick and Modesto1995), although this could be due to the somatically immature nature of this specimen (Sidor et al., Reference Sidor, Ide, Larsson, O'Keefe, Smith, Steyer and Modesto2022). The supraoccipital forms the dorsal margin of the foramen magnum. Its occipital surface bears a pronounced median crest. In lateral view, the bone has a posteroventral extension that roofs the foramen magnum and bears a facet for the opisthotic on either side. The dorsal edge of the supraoccipital forms three processes—a short, (in posterior view) tapering dorsomedial one, and a larger dorsolateral one on either side. The dorsolateral processes do not diverge dorsolaterally as sharply as they do in Captorhinus spp. (Price, Reference Price1935; Fox and Bowman, Reference Fox and Bowman1966). The dorsomedial process is continuous with a vertical median keel on the internal surface of the supraoccipital. On either side of this keel, the bone is transversely concave. Each of the concave surfaces bears a pair of presumably vascular foramina. The anterolateral portions of the supraoccipital contacted the prootics. On the right side, this part of the supraoccipital bears impressions of parts of the membranous labyrinth, as in Captorhinus aguti Cope, Reference Cope1882 (see Modesto, Reference Modesto1998) and, to a lesser extent, in Moradisaurus grandis Taquet, Reference Taquet1969 (Sidor et al., Reference Sidor, Ide, Larsson, O'Keefe, Smith, Steyer and Modesto2022). Posteromedial to these impressions on either side of the foramen magnum, the bone bears a posterodorsally extending sulcus, which terminates in a foramen and probably carried a vein or venous sinus. Fox and Bowman (Reference Fox and Bowman1966, fig. 15) illustrated (but did not mention) a corresponding feature in Captorhinus aguti.
Mandible
The preservation of the mandibular bones is poor in FMNH UR 97 and 857, and some elements in FMNH UR 183 and USNM V 21275 show moderate to severe damage. In ventral view, each mandibular ramus is sigmoidal and becomes thicker and wider transversely at the level of the postorbital region of the cranium. The left mandibular ramus of USNM V 21275 is relatively complete (Fig. 2); its length, measured from the anterior end to the tip of the retroarticular process posteriorly, is 13.5 cm.
The dentary is well preserved in FMNH UR 97 and USNM V 21275. The ground surface on the right ramus of the latter specimen shows that the dentary forms a thin, inverted-C-shaped bony sheath around the Meckelian canal dorsally and laterally. The mandibular symphysis is unfused. The dentary bears a single row of teeth (SR) more anteriorly. The teeth of this row are round in cross section at the base and bullet-shaped with bluntly pointed apices. In USNM V 21275, the third dentary tooth is caniniform and larger than the SR teeth distal to it. The posterior two-thirds of the dentary form an expanded tooth plate bearing four rows of teeth (MR) and increase in dorsoventral depth under the tooth plate. The plate forms an incipient shelf that overhangs the medial surface of the mandibular ramus. The MR section begins anteriorly as two rows, increasing to four rows farther posteriorly. The tooth crowns are bullet-shaped. In FMNH UR 97, both dentaries were irregularly ground down during preparation to determine the number of teeth and tooth rows in the MR section. The posteromedial margin of the tooth plate is incomplete. Some of the tooth cross sections in the SR are difficult to identify due to the extensive use of plaster and glue. The labial tooth row (R1) comprises at least nine tooth cross sections; the second row (R2, lingual to R1) has minimally 12; the third row has 11; and the lingual row (R4) has at least six. The tooth rows gently arc lingually and converge anteriorly. Rows two and three extend anteriorly up to the posterior tooth of the ST series. The cross sections of the teeth slightly increase in diameter posteriorly.
The splenial in FMNH UR 97 and USNM V 21275 forms much of the mandibular symphysis and lingual surface of the anterior half of the lower jaw and wraps around the ventral margin in this portion of the mandibular ramus. Posteriorly, the bone becomes slightly wider posteriorly, terminating at the foramen intermandibulare caudale (the margins of which are damaged on the left mandibular ramus in USNM V 21275). The suture between the dentary and the splenial is fairly straight. The splenial contacts the coronoid posteriorly. Posteriorly, the element is embayed by the anterior margin of the foramen intermandibulare caudale and meets the prearticular. Farther posteriorly and more ventrally, the splenial overlaps the angular.
The coronoid is partly preserved in FMNH UR 97 and USNM V 21275. It has a long anterior process. Posteriorly, the coronoid probably extends onto the lateral surface of the mandibular ramus. Anteriorly, it contacts the splenial.
The prearticular is preserved in FMNH UR 97 and USNM V 21275. Apparently commencing at the posterodorsal margin of the foramen intermandibulare caudale, it contacts the articular posteriorly. Anteriorly, the prearticular forms the posterior and part of the dorsal margin of the foramen intermandibulare caudale. At the posterior end of the foramen, it meets the angular along a slightly jagged suture. Dorsally, the prearticular contacts the coronoid anteriorly, the articular posteriorly, and the surangular between the coronoid and articular. The suture with the articular is relatively straight anteriorly but becomes irregular posteroventrally. Ventrally, the prearticular is overlapped for much of its length by the angular, which decreases in dorsoventral height posteriorly.
The angular is preserved in FMNH UR 97 and USNM V 21275. It extends for much of the length of the lower jaw to the posteroventral corner of the foramen intermandibulare caudale. Anteriorly, starting at the level of this opening, the ventral margin of the mandibular ramus forms a low ridge on the angular that fades toward its posterior end on the retroarticular process. Dorsolaterally, the angular contacts the surangular. Posterolaterally, the angular meets the articular. Posteriorly, the suture terminates at the retroarticular process where the angular tapers between the surangular and articular in lateral view.
The surangular is preserved in FMNH UR 97 and USNM V 21275. It forms the posterodorsal margin of the mandibular ramus. Anteromedially, the surangular meets the coronoid. Medioventrally, it contacts the prearticular along a relatively straight suture. Posteriorly, the surangular covers the lateral surface of the jaw joint.
The articular is well preserved in USNM V 21275. It forms a shelf that is wider mediolaterally than long anteroposteriorly and has a rounded, indented posterior margin in ventral view. The articular is bordered by the surangular laterally, the prearticular medially, and the angular ventrally. The articular surface for the jaw joint faces dorsally and slightly medially and is divided by a low, rounded ridge into a lateral and a medial contact surface for the quadrate. It forms a short projection (‘posterolateral boss’; Heaton, Reference Heaton1979) immediately posterolateral to the articular surface. The retroarticular process is well developed and differs from the short process in Captorhinus laticeps and Labidosaurus hamatus Cope, Reference Cope1896 (see Heaton, Reference Heaton1979, fig. 5).
Postcranial axial skeleton
Olson (Reference Olson1970) estimated a total of 25 presacral vertebrae but noted that the actual number could be slightly higher. Most other known captorhinid taxa have 25 presacrals (Sumida, Reference Sumida1990). USNM V 21275 includes two sets of well-preserved, articulated dorsal vertebrae (Fig. 5). Their neural arches are robust but not as ‘swollen’ above the postzygapophyses as in some other captorhinids such as Captorhinus laticeps (see Dilkes and Reisz, Reference Dilkes and Reisz1986). They do not extend far beyond the posterior margins of the centra posteriorly, unlike in Captorhinus spp. (Sumida, Reference Sumida1990). The widely spaced pre- and postzygapophyses are nearly flat and at most slightly inclined medially. The neural canal is approximately twice as wide transversely as high dorsoventrally. The broad-based transverse processes are well developed and nearly vertically aligned on the anterior dorsal vertebrae. The amphicoelous pleurocentra are hourglass-shaped in sagittal section and wider transversely than tall dorsoventrally, unlike in Captorhinikos valensis in which the two dimensions are equal (Modesto et al., Reference Modesto, Lamb and Reisz2014). The neural spines are mediolaterally wide and have thickened apices on the more anterior dorsal vertebrae (Fig. 5.1, 5.2) whereas those on the more posterior dorsals are longer, taller, and mediolaterally narrow. Sumida (Reference Sumida1990) noted alternation in spine height and structure on the anterior dorsals. In the three articulated anterior dorsal vertebrae of USNM V 21275, the first vertebra has a low, stump-like neural spine whereas the second and third have more distinct neural spines. Small, slightly dorsoventrally flattened intercentra are present between the articular ends of successive pleurocentra. The clearly visible neurocentral sutures on the dorsal vertebrae of USNM V 21275 suggest that the animal was still somatically immature at the time of death.
As in most early amniotes, FMNH UR 857 has two sacral vertebrae. The caudal region of this specimen has 15 preserved vertebrae but, based on other captorhinids (Sumida, Reference Sumida1990), the series is incomplete distally. The neural arches of the caudals are not swollen, and the neural spines on the preserved vertebrae are slender.
A few ribs are preserved in FMNH UR 857 and UR 859. Those exposed in the former specimen are dorsal ribs. None is complete but one of them preserves its holocephalous proximal head. None of the rib shafts appears to be expanded distally. The caudal ribs on the anterior three caudals in FMNH UR 857 curve posterolaterally.
Appendicular skeleton
The right scapulocoracoid is preserved in FMNH UR 857 and the basal portion of the right scapula and fragments of the coracoids in USNM V 21275. In the former, it appears roughly sigmoidal in end view. The scapular fragment of USNM V 21275 has a supraglenoid foramen on the thickened posterior aspect of the element.
FMNH UR 857 preserves both clavicles. The flat and plate-like clavicle meets its antimere medially along interdigitated sutures with the interclavicle.
The more or less T-shaped interclavicle of FMNH UR 857 lacks only the posterior end of its stem. It is long and flat with a broadly expanded anterior portion. The suture between the interclavicle and the clavicles extends slightly posterolaterally on either side, terminating in a ‘spike’ posteriorly. This is also evident on the small fragment preserved in USNM V 21275. The cleithrum is not present in any of the specimens examined in this study.
FMNH UR 857 includes a right humerus. The bone is robust and short. Its proximal and distal articular ends are broadly expanded and flattened. The proximal end is narrower than the distal one, and they are set off from each other at an approximately right angle. The proximal articular end including the head is abraded. The distal articular end bears the capitulum for the radius and the trochlea for the ulna. The entepicondyle is more prominent than the ectepicondyle.
The radius in FMNH UR 100 and 857 is rather slender with a slightly expanded proximal and distal ends. The distal end of the radius forms the articular surface for the radiale. A small notch for contact with the intermedium is present medially.
The proximal end of the ulna in FMNH UR 858 bears a well-developed olecranon process and extensive semilunar notch. The somewhat expanded distal end contacts the intermedium, pisiform, and ulnare.
The right femur is well preserved in USNM V 21275. It is a robust bone (length 5.6 cm) with a short, cylindrical shaft and expanded proximal and distal articular ends (Fig. 6.1–6.4). Its articular ends are unfinished, providing further evidence that USNM V 21275 was somatically immature at the time of death. The large internal trochanter of the femur originates as a robust projection at the anteroventral corner of the proximal articular end and is separated from it by a notch. It extends distally along the anteroventral margin of the femur, connecting with the fourth trochanter distally. The internal trochanter encloses the intertrochanteric fossa with the proximal end on the ventral aspect of the bone. The distal articular condyles extend farther proximally on the ventral surface of the femur and are separated by a deep intercondylar groove on the dorsal aspect of the bone. The larger posterior condyle is longer dorsoventrally.
Although Vaughn (Reference Vaughn1958) listed a complete right tibia for USNM V 21275, only the proximal portions of both tibiae could be located. The proximal region of the tibia forms a robust, expanded proximal articular end, which is partly divided into surfaces for contact with the distal femoral condyles. Its lateral margin is relatively straight, but its medial margin is distinctly curved.
The right fibula in USNM V 21275 (length 4.1 cm) is flattened dorsoventrally and bows away from the tibia, with a distinctly concave medial and nearly straight lateral margin (Fig. 6.5). Its distal end is transversely wider than its proximal one. The angled distal articular surface contacted the calcaneum and astragalus.
The calcaneum of USNM V 21275 is flat and plate-like with gently concave dorsal and ventral surfaces (Fig. 6.6, 6.7). Its articular surfaces for contact with the astragalus and the fibula are set at an angle to each other. It lacks a notch for passage of the perforating artery.
The astragalus in USNM V 21275 is flat and more or less L-shaped in outline (Fig. 6.6, 6.7). It bears distinct proximal facets for contact with the fibula and the calcaneum, respectively, and contacts a centrale (centrale 3; O'Keefe et al., Reference O'Keefe, Sidor, Larsson, Maga and Ide2006) distally. The proximal end of the astragalus meets the fibula. Although its contact with the tibiale is evident, the latter itself is not preserved. The lateral portion of the astragalus bears a deep groove for the perforating artery, forming a notch in the lateral margin of the bone.
O'Keefe et al. (Reference O'Keefe, Sidor, Larsson, Maga and Ide2006) interpreted the astragalus of USNM V 21275 as composed of four rather than three ossifications. They argued that a line across the astragalus traced the original separation between an intermedium and centrale 4. However, microscopic examination shows that this line merely traces an irregular fracture (Fig. 6.6). Thus, we consider the astragalus tripartite, as first demonstrated by Peabody (Reference Peabody1951) for Captorhinus. The left tarsus of FMNH UR 857 has a separate astragalus (with a centrale attached) and intermedium.
The left hind limb of FMNH UR 857 includes a disarticulated tarsus and pes, and several elements from the metatarsus and pedal digits are preserved in USNM V 21275. They include at least two metatarsals, each of which has somewhat expanded proximal and distal ends connected by a slender shaft. A more precise identification of these metatarsals is not possible.
Etymology
Olson (Reference Olson1954) named the species after what he termed the Choza Formation.
Materials
FMNH UR 99, dorsal vertebrae, partial pelvis, femur, and bone fragments; FMNH UR 100, partial skull and postcranial bones; FMNH UR 183, incomplete, crushed skull (Fig. 1.3, 1.4). Clear Fork Formation (Cisuralian: Kungurian), Olson's FA Locality, Foard County, Texas.
FMNH UR 857, poorly preserved skull and much of postcranial skeleton (Olson, Reference Olson1962, figs. 12, 13, 15A); FMNH UR 858, partial vertebral column, parts of the limb girdles, and articulated right forelimb with carpus and much of manus (Olson, Reference Olson1962, fig. 14A, B); FMNH UR 859, articulated partial skull and partial postcranial skeleton; USNM V 21275, incomplete cranium, partial mandible, supraoccipital, and basioccipital with attached fragments of exoccipitals; dorsal vertebrae; incomplete right scapula, coracoid, and procoracoid; partial right ilium; right femur, proximal ends of both tibiae, right fibula, right astragalus and calcaneum; autopodial bones; and several fragments of unidentified bones. A fragment of an ulna, the proximal ends of both humeri, and a right tibia listed by Vaughn (Reference Vaughn1958) as part of USNM V 21275 could not be located. Hennessey Formation (Cisuralian: Kungurian), Cleveland County, Oklahoma.
Remarks
Diagnosing related captorhinid taxa with multiple tooth rows from the Cisuralian of Texas and Oklahoma has been challenging due to the close overall similarity in skeletal structure and poor preservation of and/or preparation damage to specimens. Furthermore, the various holotypes differ in comparable dimensions and some could conceivably represent different developmental stages of a particular taxon. Thus, putative diagnostic features (e.g., number of tooth rows) possibly merely represent ontogenetic differences (Sidor et al., Reference Sidor, Ide, Larsson, O'Keefe, Smith, Steyer and Modesto2022). Other character states, e.g., the degree of divergence of the tooth rows (e.g., Modesto et al., Reference Modesto, Lamb and Reisz2014), are difficult to quantify. Thus, we use a combination of features rather than autapomorphies for diagnosing the new taxon.
Phylogenetic analysis
Olson (Reference Olson1954, p. 216) provided the following diagnosis for ‘Captorhinikos’ chozaensis: “Lower jaw expanded dorso-ventrally below region of multiple rows of dentition. Outer rows of teeth in lower jaw set in from lateral margin of lower jaw. Neural spines of thoracic vertebrae high as compared with those of Captorhinikos valensis.” None of these features is diagnostic at the species level. Olson already cautioned that the relative height of the neural spines possibly reflects a difference in size from the holotype of Captorhinikos valensis. Modesto et al. (Reference Modesto, Lamb and Reisz2014) could not confirm that the latter had neural spines that were short to moderate in height.
Modesto et al. (Reference Modesto, Lamb and Reisz2014, p. 292) diagnosed the type species of Captorhinikos, Captorhinikos valensis, as a “small moradisaurine distinguished by slightly radiating rows of teeth on both maxilla and dentary. Differs from other moradisaurines by the presence of a more densely denticulated pterygoid.” The maxillary and dentary tooth rows of ‘Captorhinikos’ chozaensis also slightly radiate. The transverse flange of the left pterygoid of FMNH UR 183 preserves a patch of denticles along its medial edge and the transverse flange of the right one has denticles along its posterior edge and medial margin although they are largely obscured by glue.
The phylogenetic analysis presented here is based on the character-taxon matrix by Cisneros et al. (Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020), which is a modified version of the matrix compiled by Modesto et al. (Reference Modesto, Lamb and Reisz2014), with the data for Sumidadectes chozaensis checked during the present study (see Appendix). This matrix comprises 21 taxa and 80 characters. Of these characters, 53 (66%) could be scored for Sumidadectes chozaensis. All of the characters in the phylogenetic analysis were treated as unordered and unweighted. Paleothyris acadiana Carroll, Reference Carroll1969 and Protorothyris spp. were used as outgroups in the analysis.
Parsimony analysis in PAUP using the heuristic search algorithm yielded 11 most parsimonious trees. A strict consensus tree was calculated and used for the bootstrap support analysis. The trees have a length of 182 steps, a consistency index (CI) of 0.5659, a homoplasy index (HI) of 0.4341, and a retention index (RI) of 0.7435. Initially, each specimen assigned to ‘Captorhinikos’ chozaensis was coded separately to ascertain whether all of them represented a single taxon.
An earlier study by Reisz et al. (Reference Reisz, Liu, Li and Müller2011) used a composite OTU Captorhinikos that was largely based on ‘Captorhinikos’ chozaensis. Modesto et al. (Reference Modesto, Lamb and Reisz2014) first separately listed character states for both ‘Captorhinikos’ chozaensis and Captorhinikos valensis, and Reisz et al. (Reference Reisz, LeBlanc, Sidor, Scott and May2015) first included ‘Captorhinikos’ chozaensis as a discrete OTU in their phylogenetic analysis of Captorhinidae.
Our analysis confirmed Sumidadectes chozaensis as more derived than Labidosaurus hamatus and as the earliest-diverging moradisaurine (Fig. 7), as previously hypothesized by Modesto et al. (Reference Modesto, Richards, Ide and Sidor2019). This phylogenetic position is supported by five unambiguous synapomorphies: 40 or more teeth on maxillary dental field (7.2); five tooth rows in upper jaw (9.2); marginal teeth conical, not recurved (11.1); dentary with single enlarged tooth anteriorly (57.2.); and presence of dentary lingual shelf (77.1). All moradisaurines more derived than Sumidadectes n. gen. share two synapomorphies: bulbous teeth on marginal dentition (10.1) and presence of double row of teeth extending far anteriorly (12.1). The results of our phylogenetic analysis eliminate the need for hypothesizing convergent acquisition of multiple tooth rows in Sumidadectes n. gen. and Moradisaurinae (Modesto et al., Reference Modesto, Lamb and Reisz2014) or a reversal from herbivory to omnivory in Labidosaurus (Brocklehurst, Reference Brocklehurst2017). First appearing in the fossil record during the late Cisuralian (Kungurian), moradisaurines became a widely distributed, diverse clade of small to medium-sized captorhinids with craniodental features indicative of high-fiber herbivory.
Figure 7. Strict consensus from 11 trees generated by parsimony analysis. Sumidadectes n. gen. is highlighted in bold font. Numerical values at nodes are bootstrap values > 50%. The catalog number MAP PV664 denotes a right hemimandible of an unnamed moradisaurine described by Cisneros et al. (Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020) from the Pedra de Fogo Formation (Cisuralian) of Brazil. Taxa not otherwise appearing in the text are: Captorhinus kierani deBraga, Bevitt, and Reisz, Reference deBraga, Bevitt and Reisz2019; Captorhinus magnus Kissel, Dilkes, and Reisz, Reference Kissel, Dilkes and Reisz2002; Euconcordia Reisz, Haridy, and Müller, Reference Reisz, Haridy and Müller2016; Gansurhinus Reisz et al., Reference Reisz, Liu, Li and Müller2011; Labidosaurikos Stovall, Reference Stovall1950; Labidosauriscus Modesto, Scott, and Reisz, Reference Modesto, Scott and Reisz2018; Moradisaurus Taquet, Reference Taquet1969; Opisthodontosaurus Reisz et al., Reference Reisz, LeBlanc, Sidor, Scott and May2015; Paleothyris Carroll, Reference Carroll1969; Protocaptorhinus Clark and Carroll, Reference Clark and Carroll1973; Protorothyris Price, Reference Price1937; Reiszorhinus Sumida et al., Reference Sumida, Dodick, Metcalf and Albright2010; Rhiodenticulatus Berman and Reisz, Reference Berman and Reisz1986; Romeria prima Clark and Carroll, Reference Clark and Carroll1973; Romeria texana Price, Reference Price1937; Rothianiscus Kuhn, Reference Kuhn1961; Saurorictus Modesto and Smith, Reference Modesto and Smith2001; Thuringothyris Boy and Martens, Reference Boy and Martens1991.
Acknowledgments
This paper was developed from the Master's thesis authored by J.P.J. under the supervision of Stuart S. Sumida (California State University San Bernardino). We thank William F. Simpson and Adrienne Stroup (FMNH) for access to and loans from the collections under their care. Sean Modesto, Paula Mikkelsen, and an anonymous referee provided helpful comments on the manuscript.
Declaration of competing interests
The authors declare none.
Appendix
Character states for Sumidadectes chozaensis for the character-taxon matrix by Cisneros et al. (Reference Cisneros, Angielczyk, Kammerer, Smith, Fröbisch, Marsicano and Richter2020).
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