Kuroyuriella mikikoi, Evans & Matsumoto, 2015

Evans, Susan E. & Matsumoto, Ryoko, 2015, An assemblage of lizards from the Early Cretaceous of Japan, Palaeontologia Electronica (London, England) 52 (4), pp. 1-36 : 4-11

publication ID	https://doi.org/ 10.26879/519
publication LSID	lsid:zoobank.org:pub:FB40EDBC-4B8B-4E0F-857E-C5AE6CC78C5A
persistent identifier	https://treatment.plazi.org/id/90BE9C80-6176-496E-982A-293725876EEE
taxon LSID	lsid:zoobank.org:act:90BE9C80-6176-496E-982A-293725876EEE
treatment provided by	Felipe
scientific name	Kuroyuriella mikikoi
status	sp. nov.

Treatment

Kuroyuriella mikikoi sp. nov.

Figures 2.1–7, 3.1–9, 4.1–10 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10

zoobank.org/ 90BE9C80-6176-496E-982A-293725876EEE

PALAEO- ELECTRONICA.ORG

Etymology. For Mikiko Yamaguchi to recognise her technical skill in the preparation of the Kaseki-kabe fossil material over many years.

Holotype. SBEI 1510 (Li193), a disarticulated skull on four originally conjoined blocks (A- D) ( Figure 2.1–7 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 ).

Type locality and horizon. Shiramine, Kuwajima district, Hakusan City, Ishikawa Prefecture, Japan. Early Cretaceous Kuwajima Formation, Tetori Group.

Referred material. SBEI 1608 (Li223), an association on two small blocks ( Figure 3.1–9 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 ).

Differential diagnosis. A small lizard that differs from contemporaneous Japanese Sakurasaurus shokawensis (Evans and Manabe, 1998, 2009) and Kuwajimalla kagaensis (Evans and Manabe,

2008); Chinese Jehol Dalinghosaurus longidigitus ( Ji and Ji, 2004; Evans and Wang, 2005), Liushusaurus acanthocaudata ( Evans and Wang, 2010) , and Yabeinosaurus tenuis (Evans et al., 2005; Evans and Wang, 2012; Wang and Evans, 2011); and from other known Jurassic and Early Cretaceous lizards (e.g., Euramerican Paramacellodus spp ., Evans and Chure, 1998; Meyasaurus spp ., Evans and Barbadillo, 1997) in the distinctive posterior margin of the dentary and strongly concave narial margin of maxilla; differs from Yabeinosaurus tenuis in parietal shape and in lacking parietal foramen; differs from Late Cretaceous Mongolian Slavoia darevskii ( Alifanov, 2000a; Gao and Norell, 2000) in having proportionally more gracile jaw, proportionally longer and narrower frontals and parietals, and absence of parietal foramen; resembles the Late Cretaceous Carusia intermedia ( Borsuk-Białynicka, 1985) and Exostinus serratus ( Bhullar, 2010) in having frontals that are anteriorly narrow and posteriorly broad, with strong sub-olfactory processes, in the strong concavity of the maxillary narial margin, and, for Carusia but not Exostinus , in the shape of the dentary coronoid process, but Kuroyuriella differs from both in that the frontals are paired rather than fused and lack coarse tubercular sculpture, there is no sculpture on the maxilla, the parietal is of a different morphology (square and largely excluded from the upper temporal fenestra by postorbitofrontal facets, no parietal foramen) and sculpture pattern (low relief rather than coarse and tubercular); resembles Late Cretaceous genera Parmeosaurus scutatus and Hymenosaurus clarki ( Gao and Norell, 2000) in gracile dentary with straight long axis, but differs from former in having uni- rather than tricuspid teeth, free posterior edge on dentary coronoid process, greater orbital emargination of frontals, deeply concave narial margin of maxilla, and in lacking cranial osteoderms. Hymenosaurus clarki is very poorly preserved, but as described, it differs from Kuroyuriella in having a prefrontal/postfrontal contact that excludes the short wide frontals from the orbital margin and a putative parietal foramen close to the posterior parietal margin.

Material. The holotype skull, SBEI 1510 , was contained within a single block that was fragmented during preparation so that the bones are now spread over four smaller blocks ( Figure 2.1 View FIGURE 2 ) bearing: A, the orbital process of a left maxilla; B, an incomplete left dentary in labial view; C, a right splenial; and D, an association of right maxilla and premaxilla, right and left frontals, left prefrontal and postorbitofrontal, right pterygoid, and right mandible. The bones are mostly disarticulated but associated, suggesting some postmortem decay but little transport .

A second association ( SBEI 1608 ) is attributed to Kuroyuriella mikikoi on the basis of dentary characters (tooth number and shape, curved free posterior edge), and complements the holotype in preserving the parietal, the dentary in lingual view, and more details of the accessory jaw bones (notably coronoids and articular). The specimen is on two blocks, also divided during preparation. Block A bears the left dentary in labial view ( Figure 3.3–4 View FIGURE 3 View FIGURE 4 ), as well as a poorly preserved elongate right postdentary mass and a right quadrate ( Figure 3.7–8 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 ). Block B preserves the parietal and a partial frontal, as well as the right dentary, and postdentary bones including both coronoids, and the left surangular-angular-prearticular-articular association ( Figure 3.1–2, 3.6, 3.9 View FIGURE 3 ). No osteoderms are preserved in association with these specimens, nor are there isolated osteoderms of appropriate size on any of the many matrix samples from this locality .

The individual bones on both blocks are similar in size to comparable bones of extant lizards with an adult skull length of ~ 10 mm, a snout-vent length ( SVL) of 45–50 mm, and a total length of 110–125 mm. For their size, the skull bones are robust and fully ossified with weak sculpturing on the parietal and complex sutures, but given the disarticulation, the skeleton may have been that of a sub-adult.

Description. The holotype (SBEI 1510) preserves a right maxilla (5.8 mm long) in lateral view with 19 functional teeth and spaces for at least two more ( Figures 2.3 View FIGURE 2 , 4.1–2 View FIGURE 4 ). The bone is distinctive in having a strongly concave narial margin, caused partly by the dorsal curvature of the tip of the premaxillary process but also the anterior margin of the vertical facial process. Further posteriorly, the facial process is broken along its base, but the line of breakage clearly demarcates the original anteroposterior length. Posteriorly, the bone tapers into a rather short, suborbital process. The partial left maxilla (not figured) bears a medial shelf that supported the jugal, but the relative contributions of the two bones to the ventral orbital margin cannot be determined. A right premaxilla underlies the left postorbitofrontal ( Figure 2.5 View FIGURE 2 ). Three small teeth are visible with a gap representing a fourth tooth position. Further teeth may be hidden by the overlying element.

Paired frontals with deep subolfactory processes (=cristae cranii) that curve slightly inward and strong midline interdigitations are preserved in ventral aspect ( Figure 2.6–7 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 ). They have separated slightly in the midline, and the right is twisted slightly on its long axis, making the anterior part of the bone appear somewhat narrower than it really is. Anteriorly each bone narrows into a straight edge that underlay the nasal. Posteriorly, both frontals bear ventrolateral and ventromedial parietal facets, suggesting the frontoparietal joint was inflexible rather than mesokinetic. The large prefrontal facet on the right bone is incised posteriorly but becomes shallower anteriorly. It extends roughly halfway along the bone. The prefrontal bone of that side is not preserved, but the left prefrontal has been displaced laterally and is preserved in medial view ( Figure 2.4–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 ). Its frontal process is posteriorly tapered and relatively flat medially. The body is expanded and has a deeply concave medial surface. The posteroventral margin appears to bear a small facet, possibly for a lacrimal bone. Adjacent to the orbital edge of the left frontal in SBEI 1510 is a curved bar of bone with a mediolaterally compressed blade at one end and a thicker bar (rounded cross-section) at the other ( Figure 2.6–7 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 ). This element may be part of a sickle-shaped jugal.

A near-complete left postfrontal or, more probably, postorbitofrontal (based on size and articulations) is preserved between the frontals and the pterygoid ( Figure 2.4–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 ). It is roughly rectangular with a straight lateral margin and a more convex medial margin that would have clasped the frontoparietal suture (and matches the size and shape of the corresponding facet on the parietal of SBEI 1608 (see below) and the smaller shallow facet on the frontal of SBEI 1510. Judging from the arrangement of these bones, the jaw adductor muscles originated from the ventral surface of the parietal and postorbitofrontals.

SBEI 1510 preserves the only palatal element, a right pterygoid ( Figure 2.4–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 ) with an anterior facet for the palatine and a sharply pointed lateral process bearing a narrow slot facet for the ectopterygoid. The posterior quadrate process is incomplete but it is narrow and shallow with a distinct dorsal pit (fossa columellae) for the epipterygoid flanked by a low crest that continues posteriorly along the dorsolateral edge of the bone.

An almost complete parietal is preserved on SBEI 1608 ( Figure 3.5–6 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ). The exposed dorsal surface bears weakly pustulate sculpture anteriorly, but is smooth posteriorly; it lacks any trace of head scale markings. The anterior margin (uppermost in Figure 3.5–6 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ) is irregular rather than straight and a parietal foramen is not evident. The posterior margin bears shallow smooth nuchal shelves for neck muscle attachment, separated by a small median notch, probably for the processus ascendens of the supraoccipital. The postparietal (=supratemporal) processes are broken, revealing bases that are dorso-ventrally shallow in cross-section. On the right side, the parietal margin is partially overlain by a postorbitofrontal ( Figure 3.5–6 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ). On the left, the exposed margin is subdivided into a long anterior concavity that accommodated the postorbitofrontal and a shorter posterior edge that bordered a reduced upper temporal fenestra. Lateral to the right parietal margin, and displaced slightly posteriorly, is a partial right frontal, also lightly sculptured. On superficial examination, this element might be mistaken for an expanded jugal, but it lacks any of the requisite facets or thickenings. SBEI 1608 also preserves a right quadrate in association with the posterior end of the right mandible ( Figure 3.7–8 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 ). It is small (2.3 mm tall) with a short lateral conch supported by a robust curved posterior pillar. The dorsal head is wider than the ventral one, the latter being anteroposteriorly short and weakly divided into medial and lateral condyles.

The lower jaw of Kuroyuriella mikikoi is represented on the holotype (SBEI 1510) by the left dentary and an almost complete right mandible, and on SBEI 1608 by right and left dentaries and parts of the postdentary series ( Figure 4.3–10 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10 ). The left dentary of SBEI 1510 (5.9 mm long, not figured) is preserved in labial view and is broken posterior to the tooth row. It bears 18 teeth with positions for two or three more in replacement and an edentulous region at the symphysial end that originally probably accommodated four or five small teeth. The right dentary (7.4 mm long) is also preserved in labial view. It is slender with a rounded symphysial end and a subtle muscle attachment scar anteroventrally ( Figure 4.7–10 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10 ). The Meckelian fossa, exposed by preparation runs medially and then ventrally. At its posterior end, the dentary bears a small rounded coronoid process with a slightly recurved tip. From this tip, the edge of the dentary curves posteroventrally before extending into a process that meets the surangular. The tip of this surangular process and the posteroventral end of the dentary are damaged, but the bone clearly extended beyond the coronoid process to brace the postdentary bones. The posterodorsal dentary curvature appears to be a distinctive feature of Kuroyuriella mikikoi (resembling the condition in some acontine skinks, Evans, personal observation). A narrow, posteriorly angled coronoid bone projects above the dentary coronoid process. The surangular is relatively shallow throughout. A narrow rod of bone runs above it from the tip of the coronoid lappet. This rod cannot be part of the jaw and may be a displaced hyoid ceratobranchial, or possibly an epipterygoid. The relatively deep angular bears a shallow anterolateral facet for the posteroventral ramus of the dentary. The ventromedial surface of the mandible has been prepared ( Figure 4.10 View FIGURE 4 ). A small mylohyoid foramen perforates the angular just posterior to the level of the coronoid process. The angular met the splenial medially but its posterior limit is uncertain. The right splenial fills the Meckelian fossa posteriorly but terminates some distance from the symphysis, leaving the fossa open for about one-third of its length. The left splenial is preserved in isolation on SBEI 1510C (not figured). It is a relatively short bone with a blunt anterior margin. It fully encloses a small inferior alveolar foramen.

The jaw material of SBEI 1608 complements that of SBEI 1510. An almost complete left dentary ( Figure 4.3–4 View FIGURE 4 ) is preserved in labial view on Block 1608A. There is a posteroventral lappet (missing in SBEI 1510 although there is an impression for it on the angular). The dentary is 5.7 mm long parallel to its alveolar margin and 6.1 mm along the ventral edge. It is shallow with a relatively horizontal inferior margin, a tapering anterior end, and a conspicuous muscle scar along the anteroventral border. There are 21 teeth, with one space for a nonimplanted replacement. The labial surface is perforated by seven large, closely spaced neurovascular foramina. The right dentary ( Figure 4.5–6 View FIGURE 4 View FIGURE 5 View FIGURE 6 ) is in association with postdentary elements of both mandibles including the right and left coronoid, and the left surangular, articular and angular. It is preserved in lingual view and displays a row of 21 closely packed teeth with no gaps for replacements, although several teeth bear small basal replacement pits. The teeth are narrow, straight, and unicuspid. The subdental ridge is shallow anteriorly and becomes more so posteriorly. The Meckelian fossa is medial for most of its length but it becomes ventromedial close to the level of tooth position five and is ventral at the symphysis. A splenial fills the posterior part of the Meckelian fossa for roughly one-third of its length. The coronoid process is preserved behind the last tooth and although there is some damage, part of a curved free posterior edge like that in SBEI 1510 is visible. The right coronoid has been displaced ventrally and is exposed in lateral aspect ( Figure 3.9 View FIGURE 3 ). It bears a small, posteriorly concave coronoid process and widely diverging anteroventral and posteroventral processes, of which the former is longer. The left coronoid is exposed in medial view in articulation with the surangular of that side. The latter element forms the lateral wall of a relatively large adductor fossa, the medial (prearticular) wall of which is largely broken away. The left articular is in situ. Its articular surface slopes posteroventrally but has an almost vertical anterior surface. In surface view ( Figure 4.9 View FIGURE 4 ), it is asymmetrical with an anteroposterior rather than mediolateral long axis. A weak ridge divides it into medial and lateral parts, with the lateral half widest anteriorly and narrowing posteriorly, but the medial half widest posteriorly and tapering anteriorly. The retroarticular process is broken but was narrow at its base.

Affinities. Together, SBEI 1510 and SBEI 1608 , as type and referred specimen, characterise Kuroyuriella mikikoi as a small lizard having paired frontals with deep subolfactory processes; a median parietal without a parietal foramen, with sculpture of low relief, and with lateral shelves that restricted the adductor muscle origins to the ventral surface; upper temporal fenestrae that were at least partially closed by expanded postorbitofrontals; an unsculptured maxilla with a strongly concave narial margin; a large flared prefrontal; and a slender, relatively small pterygoid. In the shallow lower jaw, the teeth are closely packed, cylindrical, and pleurodont with lingual replacement; a subdental ridge is present; the dentary bears a tapering coronoid process that braces the coronoid, and has a posterior extension with a curved free margin; the surangular, angular, and splenial are all present and the surangular is shallow; the adductor fossa is open but not expanded; and the articular surface is asymmetrical .

The restricted upper temporal fenestra of Kuroyuriella mikikoi differs from the condition in iguanians, teiids, borioteiioids, or varanids where the fenestra is open and the jaw muscles extend onto the lateral and/or dorsal surfaces of the parietal. The dentary of Kuroyuriella lacks the closure of the Meckelian fossa found in all crown-group gekkotans, although the fossa remains open in the stem gekkotan Hoburogekko suchanovi ( Daza et al., 2012) . The small, closely packed homodont teeth and lingual replacement of Kuroyuriella mikikoi are unlike the teeth of extant anguimorphs but, again, are found in some fossil taxa (e.g., the Cretaceous Exostinus lancensis ). Paired frontals, a reduced upper temporal fenestra, and a dentary coronoid process are found together in xantusiids, scincids, and cordyliforms (Scincoidea), but unlike many members of the latter two clades, neither SBEI 1510 nor 1608 shows any trace of cranial osteoderms. Skull material of Paramacellodus sp. , a possible scincoid, from the Morrison Formation of North America (Late Jurassic, Evans and Chure, 1998) resembles Kuroyuriella mikikoi in having paired frontals and parietals with thin lateral margins (rather than vertical flanges for adductor muscle attachment) but differs in having separate postorbitals and postfrontals, without the embayment on the side of the parietal or the distinctive jaw morphology.

In order to explore the affinities of Kuroyuriella mikikoi , it was coded into the matrix of Gauthier et al. (2012), as extended by Longrich et al. (2012) (184 characters coded out of 622, 70.4% missing data), and the analyses run as described above (Material and Methods). In both the unconstrained and constrained analyses, with and without character ordering, Kuroyuriella was placed on the squamate stem ( Figures 5.1–2 View FIGURE 5 ). However, with Implied Weighting activated (k=3, 7,15) to reduce the effect of homoplasy, Kuroyuriella moved into Scincoidea, to a position on the stem of Scincidae ( Figures 6.1–2 View FIGURE 6 ). With k=30, Kuroyuriella returned to the stem-squamate position. The consistent placement of Kuroyuriella on the squamate stem is problematic and probably artifactual, but whether the weighted analysis is giving a more accurate placement is uncertain. Of the derived character states possessed by Kuroyuriella , 76 [1] (postorbital partly occludes upper temporal fenestra), 364 [1] (dentary coronoid process extends beyond level of coronoid apex), 367 [2] (coronoid process of dentary overlaps most of anterolateral surface of coronoid), and 369 [2] (dentary terminates well posterior to coronoid apex) provide some support for placement of Kuroyuriella on the stem of scincids, and 129 [1] (prefrontal extends to mid-orbit), 104 [1] (absence of parietal foramen) and 385 [1] (posterior mylohyoid foramen posterior to coronoid apex) would be consistent with that placement. However, given the considerable difference between the results using equal weighting and Implied Weighting, Kuroyuriella remains incertae sedis pending recovery of more complete material.