Early Cretaceous troodontine troodontid (Dinosauria: Theropoda) from the Ohyamashimo Formation of Japan reveals the early evolution of Troodontinae

Abstract

A new troodontid dinosaur, Hypnovenator matsubaraetoheorum gen. et sp. nov., is described based on an articulated postcranial skeleton recovered from the fluvial deposits of the Albian Ohyamashimo Formation of the Sasayama Group in Tambasasayama City, Hyogo Prefecture, Japan. Hypnovenator is distinguished from other troodontids by four autapomorphies and a combination of additional features. Our phylogenetic analysis positions Hypnovenator as the oldest and one of the most basal troodontines, forming a clade with Gobivenator mongoliensis. The discovery of Hypnovenator suggests that small-bodied maniraptorans with a sleeping posture were common not only in environments with volcanic and eolian events or alluvial systems but also in fluvial systems. Geometric morphometric analysis of manual ungual phalanges shows that manual ungual phalanges I and III of Hypnovenator exhibit considerable morphological variation but are functionally similar, which differs from those of non-troodontine troodontids, reflecting the transition of manual motion within Troodontinae. Hypnovenator also has mosaic features in the pes related to cursoriality. This study reveals that asymmetrical arctometatarsus occurred by the Albian, and some morphological changes, such as shorter digit IV than digit III and non-ungual phalanges of digits III with roller joints and digit IV with weakly ginglymoid articulation, arose during the early Late Cretaceous.

Full text

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Early Cretaceous troodontine
troodontid (Dinosauria: Theropoda)
from the Ohyamashimo Formation
of Japan reveals the early evolution
of Troodontinae
Katsuhiro Kubota
1,2,3*, Yoshitsugu Kobayashi
3 & Tadahiro Ikeda
1,2
A new troodontid dinosaur, Hypnovenator matsubaraetoheorum gen. et sp. nov., is described based
on an articulated postcranial skeleton recovered from the uvial deposits of the Albian Ohyamashimo
Formation of the Sasayama Group in Tambasasayama City, Hyogo Prefecture, Japan. Hypnovenator
is distinguished from other troodontids by four autapomorphies and a combination of additional
features. Our phylogenetic analysis positions Hypnovenator as the oldest and one of the most basal
troodontines, forming a clade with Gobivenator mongoliensis. The discovery of Hypnovenator suggests
that small-bodied maniraptorans with a sleeping posture were common not only in environments with
volcanic and eolian events or alluvial systems but also in uvial systems. Geometric morphometric
analysis of manual ungual phalanges shows that manual ungual phalanges I and III of Hypnovenator
exhibit considerable morphological variation but are functionally similar, which diers from those
of non-troodontine troodontids, reecting the transition of manual motion within Troodontinae.
Hypnovenator also has mosaic features in the pes related to cursoriality. This study reveals that
asymmetrical arctometatarsus occurred by the Albian, and some morphological changes, such as
shorter digit IV than digit III and non-ungual phalanges of digits III with roller joints and digit IV with
weakly ginglymoid articulation, arose during the early Late Cretaceous.
Keywords Arctometatarsus, Geometric morphometric analysis, Sleeping posture, eropoda, Troodontidae,
Troodontinae
Troodontidae is a clade of small-bodied and gracile theropod dinosaurs1. Although the phylogenetic posi-
tion of Troodontidae is traditionally considered a clade with Dromaeosauridae, forming Deinonychosauria1–9,
Troodontidae is also regarded as a sister clade to Avialae10,11. Anchiornis from the Late Jurassic of China is
problematic in its phylogeny and is included in Troodontidae2,4–7,11 or Avialae10,12,13. ese active discussions
signicantly improve our understanding of the phylogeny and osteology of non-avian theropod and greatly
inuence our comprehension of early avialan evolution8,14. Since the discovery of the rst troodontid Troodon
in the Upper Cretaceous of Canada15, troodontid materials have been discovered from the Middle Jurassic to
Upper Cretaceous of Asia, Europe, and North America1,9,16. However, troodontid specimens with articulation
are extremely rare. Although well-preserved and articulated basal troodontid specimens have been found in the
Barremian deposits of China over the last 20 years6,7,17–20, diagnosed troodontids from the middle Cretaceous are
represented by only two taxa, Sinornithoides21 and Urbacodon22. Sinornithoides from China comprises a nearly
complete skeleton with a sleeping posture, whereas Urbacodon from Uzbekistan consists only of a dentary with
some teeth. Two non-named troodontid specimens, MPC-D 100/4423 and 100/1405, are fragmentary. Recent
phylogenetic studies have identied relatively stable clades such as Sinovenatorinae and Troodontinae. Jinfen-
gopteryginae is another potential clade16 but has been unstable in other studies10.
In September 2010, a partial theropod skeleton including the forelimb and knee was discovered in crushing
rocks from the Ohyamashimo Formation during the construction of a public park at Nishikosa in Tambasasayama
OPEN
1Museum of Nature and Human Activities, Hyogo, Sanda, Hyogo 669-1546, Japan. 2Institute of Natural and
Environmental Sciences, University of Hyogo, Sanda, Hyogo 669-1546, Japan. 3Hokkaido University Museum,
Hokkaido University, Sapporo, Hokkaido 060-0810, Japan. *email: kubota@hitohaku.jp
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City, Hyogo Prefecture (Fig.1). is discovery was made by Mrs. Kaoru Matsubara and Takaharu Ohe, members
of an amateur group “Research Group on the Sasayama Group (Sasayama-sougun wo shiraberu kai)”. In July
2011, an articulated theropod heel was collected from the same locality during an excavation organized by the
Museum of Nature and Human Activities, Hyogo. ese specimens were identied as a troodontid theropod24,
which is an only conrmed occurrence of a troodontid in Japan (Supplementary Text S1). Here we describe the
Nishikosa material, test its phylogenetic position within troodontids, quantify the shapes of the ungual phalanx,
and discuss the implications for the manual and pedal evolution of Troodontidae.
Figure1. Locality maps and geology in Tambasasayama and Tamba cities, Hyogo Prefecture, Japan. (A) Map
of Japan showing the locations of Tambasasayama (darkblue) and Tamba cities (light blue) in Hyogo Prefecture.
(B) Distribution of the Sasayama Group in Tambasasayama and Tamba cities. (C) Stratigraphic sections of
the Sasayama Group, adapted from Hayashi etal.25. U–Pb ages in the lower parts of the Ohyamashimo and
Sawada formations were obtained by Kusuhashi etal.26. e routes for the stratigraphic sections are shown in
(D). estratigraphic positions of thelocalities (A) Kamitaki, (B) Kawashiro Tunnel, (C) Nishikosa (where
Hypnovenator matsubaraetoheorum gen. et sp. nov. was recovered), and (D) Miyada are indicated by red circles.
(D) Geological map of the Sasayama Group, fossil localities, and routes for stratigraphic sections. is map is
aer Yoshikawa27.is gure was created using Adobe Illustrator 28.3 (https:// www. adobe. com/).
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Institutional abbreviations
MNHAH, Museum of Nature and Human Activities, Hyogo, Sanda, Hyogo, Japan; MPC, Institute of Paleontol-
ogy, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia; SDUST, Vertebrate Palaeontological Collection of
College of Earth Science and Engineering, Shandong University of Science and Technology (Qingdao, China).
Results
Geological setting
e Ohyamashimo Formation, the lower unit of the Sasayama Group, in Tambasasayama and Tamba cities of
Hyogo Prefecture (Fig.1A,B), consists of sandstones, mudstones, and conglomerates, representing uvial deposits
under a semi-arid to subhumid climate25. is formation has yielded numerous dinosaur remains and eggshells28.
Isolated teeth from some localities include tyrannosauroids, therizinosaurs, dromaeosaurids, sauropods, iguano-
dontians, and ankylosaurs24,29, while the skeletons of a sauropod (Tambatitanis) and neoceratopsian are found
in Kamitaki30 and at three localities (Kawashiro Tunnel31, Miyada32, and Nishikosa24), respectively (Fig.1D).
Nishikosa (35° 04ʹ 30″ N, 135° 09ʹ 35″ E) is located at the western edge of the Sasayama Basin. Unfortunately,
despite several excavations, the theropod-bearing horizon has not been relocated at Nishikosa.
Nishikosa is placed approximately 1km southeast of the eastern end of Route 1 for the stratigraphic section
(Fig.1D), where the lower part of the Ohyamashimo Formation is exposed25. e beds around the localities
show NW–SE at the strike and 30°NE at the dip27. Nishikosa sits about 20–60m higher in altitude than the
eastern end of Route 1, indicating that the theropod-bearing horizon at Nishikosa can be temporally correlated
above the upper limit of the stratigraphic section in Route 1, likely within the middle part of the Ohyamashimo
Formation (Fig.1C). U–Pb ages of zircons indicate 112.1 ± 0.4Ma for the lowermost part of the Ohyamashimo
Formation and 106.4 ± 0.4Ma for the lower part of the overlying Sawada Formation26. us, the middle part of
the Ohyamashimo Formation is assigned to be theearly to middle Albian in age.
Systematic paleontology
eropoda Marsh33.
Coelurosauria von Huene34.
Troodontidae Gilmore35.
Hypnovenator matsubaraetoheorum gen. et sp. nov.
ZooBank ID
urn:lsid:zoobank.org:pub:BF77721B-211E-4190-B012-3669BD1221AA (for this publication), urn:lsid:zoobank.
org:act:AF64F61F-8854-42E2-8F7C-6645242534CB (for the new genus), and urn:lsid:zoobank.org:act:C3398111-
FCE7-4624-AE03-38170349345D (for the new species).
Etymology
e genus name derives from “hypno”, refers to “sleep” in ancient Greek, and “venator”, means “hunter” in Latin.
e specic name, “matsubaraetoheorum”, honors Mrs. Kaoru Matsubara and Takaharu Ohe, who are the rst
discoverers of a block including a part of Hypnovenator matsubaraetoheorum holotype specimen.
Holotype
MNHAH D1033340, consisting of two caudal vertebrae, two dorsal ribs, thirty-eight gastralia, a chevron, le
humerus, le radius, le ulna, le carpal, le metacarpals I to III, the distal end of right metacarpal I, le manual
phalanges I-1, I-2, II-1, II-3, and III-1 to 4, the distal part of le femur, le tibia and bula missing the mid-sha,
the distal end of right tibia, le astragalus, the distal end of right astragalus, the proximal parts of le metatar-
sals II to V, the proximal ends of right metatarsals II and IV, right pedal phalanges II-3, III-1 to 4, and IV-1 to 5
(Figs.2, 3; Supplementary Figs.S1–S13 and TableS1).
Locality and horizon
Hyogo Prefectural Tamba Namikimichi Central Park at Nishikosa, Tambasasayama City, Hyogo Prefecture, Japan;
theearly to middle Albian (112.1–106.4 Ma26) Ohyamashimo Formation of the Sasayama Group.
Diagnosis
A troodontid with the following unique characters: a pair of proximodistally extended depressions on the proxi-
modorsal surface of manual phalanx I-1; long dorsal and ventral proximal lips on manual phalanx III-2 for tight
articulation with phalanx III-1; a proximodistally longitudinal medial ridge on the anterior surface of the femur
proximal to the medial condyle; and distorted distal condyles with a widely convex distoventral margin on pedal
phalanx III-3. Additionally, it is characterized by the following combination of two features: the thickest portion
near the middle portion of the distal end of the ulna, and an angle of less than 11 degrees between the anterior
edge of the cnemial crest and the anterior edge of the tibial sha.
Description
eropod bones found in 2010 (Fig.2A) and 2011 (Fig.2B) were assigned to a single individual skeleton (Sup-
plementary Text S2). e le articulated forelimb bones are folded at angles of 26 degrees at the elbow and 53
degrees at the wrist. Medial to the forelimb, the disarticulated gastralia are arranged in subparallel and form
angles of 100 to 130 degrees with the long axis of the humerus. Ventral to the posterior half of the gastralia area,
right pedal digits extend without any fold. Medial to the right pedal digits, the le articulated femur and tibia
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are folded at an angle of 42 degrees. Both ankles are tightly folded. If the posterior surface of the le tibia is
considered as a horizontal plane, the right ankle is positioned 40mm anterior and 50mm dorsal to the le one.
e tail bones are just medial to the le ankle and nearly vertical to the long axis of the le tibia.
e preserved length of the caudal centrum is three times as long as the height of the anterior articular surface
(Fig.3A; Supplementary Fig.S1). e lateral surfaces have a shallow concavity along the central length, showing
that the midpoint of the centrum is hourglass-shaped in cross-section, as in Daliansaurus, Gobivenator, Sinorni-
thoides, and MPC-D 100/1405,7,21,36, but unlike Troodon, Urbacodon, and Zanabazar, which have smooth lateral
surfaces37–39. Based on the remarkedly long centrum without transverse processes, the caudal vertebra may be
assigned to one posterior to 9th4,6,7,36. e distal chevron is dorsoventrally compressed and anteroposteriorly
longer than high (Fig.3A).
e preserved shas of the dorsal rib are lateromedially wide and have at or slightly convex anterior and
posterior surfaces (Supplementary Fig.S2), resembling the mid-sha of the posterior dorsal rib in Liaoningve-
nator and Troodon6,40 rather than those of the anterior to middle ones in the taxa with lateromedially narrow
rib shas. In at least three of thirty-eight rod-like gastralia, a half of the sha is straight with an expanded end,
Figure2. Hypnovenator matsubaraetoheorum gen. et sp. nov. Blocks including the forelimb, knee (A), and
heel (B). (C) Reconstructed skeleton. Red and yellow colors show the conrmed and probable positions of
the recovered elements, respectively (Courtesy of Genya Masukawa). (D) Le manus in medial view. (E) Le
manual phalanx I-1 in dorsal view. (F) Removed fragmentary le manual phalanx II-3 (manual ungual phalanx
II) for preparing the le manus. (G) Cross-section of the bent right ankle. (H) Le metatarsus in posterior
view. Abbreviations:as, astragalus; dp, depression; fe, femur; , bula; hu, humerus; mc I, metacarpal I; mc II,
metacarpal II; mc III, metacarpal III; mp I-1, manual phalanx I-1; mp I-2, manual phalanx I-2 (manual ungual
phalanx I); mp II-1, manual phalanx II-1; mp II-3, manual phalanx II-3 (manual ungual phalanx II); mp III-3,
manual phalanx III-3; mp III-4, manual phalanx III-4 (manual ungual phalanx III); mr, medial ridge; mt II,
metatarsal II; mt III, metatarsal III; mt IV, metatarsal IV; mt V, metatarsal V; ra, radius; ti, tibia; ul, ulna. Almost
all elements are from the le side. Abbreviations for elements from the right side added ‘(r)’ at the end. is
gure was created using Adobe Photoshop 25.5.1 and Adobe Illustrator 28.3 (https:// www. adobe. com/).
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whereas the rest is curved with a thinner end. is morphology is consistent with the medial segments17,21,40–42.
ree other segments are longer and slenderer than the medial ones and are assigned as the lateral segments19,21.
e humeral sha is posteriorly bowed as in Gobivenator and Liaoningvenator6,36, but unlike Daliansaurus,
Jianianhualong, Linhevenator, and Mei, which have a straight humeral sha4,7,11,20 (Figs.2A, 3B; Supplementary
Fig.S3). e expanded distal end is 231% as wide as the mid-sha and close to 257% in Gobivenator (MPC-D
100/86). e ulna is almost straight with a slightly bowed distal extent unlike Daliansaurus, Sinornithoides, and
Talos, which have a slightly bowed sha3,7,21, and Jianianhualong and Mei, which bear a strongly bowed sha4,20
(Figs.2A, 3C; Supplementary Fig.S4). e mid-sha of the ulna is 71% as dorsoventrally high as that of the
radius, resembling Sinornithoides (69%)21 and Daliansaurus (74%)7. e subtriangular proximal end has a sin-
gle concave articular facet as in Gobivenator (MPC-D 100/86). e distal end is 148% wider than its maximum
height, which is positioned in the middle portion. e radius is slightly curved dorsally as in Jianianhualong and
Xiaotingia20,43 and unlike Mei and Sinornithoides, which have a straight sha4,21 (Figs.2A, 3D; Supplementary
Fig.S5). e semilunate carpal has a transverse trochlear groove on the proximal surface, a mediodorsal process
for articulation with metacarpal I, and a ventrolateral process for covering the ventral side of the proximal end
of metacarpal III (Fig.3F; Supplementary Fig.S6). A small distal carpal 3 is found in some basal troodontids44
but not identied in Hypnovenator because the carpal fuses with the semilunate carpal to form the ventrolateral
process. Metacarpal I has a concave medial edge, forming a ventromedial ange (Figs.2A,D–E, 3E,F; Supple-
mentary Figs.S6, S7). e distal articular facet is deeply ginglymoid by an intercondylar groove with a higher
lateral condyle than the medial one. e orientation of the groove is nearly parallel to that of metacarpal II.
Metacarpal II is straight and nearly parallel to metacarpal III. e distal articular facet is ginglymoid with a
Figure3. Selected elements of Hypnovenator matsubaraetoheorum gen. et sp. nov. (A) Distal caudal vertebrae
and a chevron in le lateral view. (B) Le humerus in anterior view. (C) Le ulna in medial view. (D) Le radius
in medial view. (E) Le manus, missing the proximal ends of metacarpals I-III and manual phalanges II-2 to
3, in lateral view. (F) Le distal carpal and proximal ends of metacarpals I-III in ventral view. Distal part of le
femur in posterior (G) and distal (H) views. (I) Proximal part of le bula in lateral view. (J) Proximal part
of le tibia in lateral view. (K) Distal parts of tibia and bula, astragalus, and calcaneum from thele side in
anterior view. (L) Proximal half of le metatarsals II-IV in anterior view. (M) Right pedal phalanges II-3, III-1
to 4, and IV-1 to 5 in dorsal view. (N) Right pedal phalanx III-3 in lateral view. Abbreviations:as, astragalus;
cav, caudal vertebra; ch, chevron; cn, cnemial crest; ecte, ectepicondyle; ectt, ectocondylar tubercle; ente,
entepicondyle; entt, entocondylar tubercle; fc, bular crest; , bula; , bular trochlea (= trochlea bularis);
lco, lateral condyle; mc I, metacarpal I; mc II, metacarpal II; mc III, metacarpal III; mco, medial condyle; mp I-1,
manual phalanx I-1; mp I-2, manual phalanx I-2 (manual ungual phalanx I); mp II-1, manual phalanx II-1; mp
III-1, manual phalanx III-1; mp III-2, manual phalanx III-2; mp III-3, manual phalanx III-3; mp III-4, manual
phalanx III-4 (manual ungual phalanx III); mr, medial ridge; mt II, metatarsal II; mt III, metatarsal III; mt IV,
metatarsal IV; op, olecranon process; pdl, proximodorsal lip; pf, popliteal fossa; pp II-3, pedal phalanx II-3 (pedal
ungual phalanx II); pp III-1, pedal phalanx III-1; pp III-2, pedal phalanx III-2; pp III-3, pedal phalanx III-3; pp
III-4, pedal phalanx III-4 (pedal ungual phalanx III); pp IV-1, pedal phalanx IV-1; pp IV-2, pedal phalanx IV-2;
pp IV-3, pedal phalanx IV-3; pp IV-4, pedal phalanx IV-4; pp IV-5, pedal phalanx IV-5 (pedal ungual phalanx
IV); sc, semilunate carpal; suc, supracondylar crest (= lateral posterior ridge, tibiobular crest); ti, tibia. is
gure was created using Adobe Photoshop 25.5.1 and Adobe Illustrator 28.3 (https:// www. adobe. com/).
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higher medial condyle than the lateral one. Metacarpal III is straight as in Sinornithoides and MPC-D 100/4421,23
and unlike Daliansaurus and Mei, which have a moderately curved sha4,7. e distal articular facet is rounded.
All non-ungual manual phalanges are narrow and elongate with a distal ginglymoid condyle with the exception
of phalanges III-1 and III-2 (Figs.2A,D,F, 3E; Supplementary Fig.S7). e dorsal surface of the proximal pha-
lanx I-1 possesses a pair of subtriangular and shallow fossae, which are dorsally divided by a faint longitudinal
ridge. e fossae are 8.2mm long and 2.1mm high on the lateral side and 9.1mm long and 2.2mm high on the
medial side. Phalanx I-1 bears a prominent proximoventral heel, twice as wide as the mid-sha, as in phalanx
II-1. Ungual phalanx I is highly curved and bears a proximally placed and prominent exor tubercle, which
is slightly lower than the articular facet. Dorsal to the facet, ungual phalanx I lacks a proximodorsal lip as in
Sinornithoides and Xixiasaurus21,45. Phalanx II-1 is similar to phalanx I-1 in morphology except for lacking a
pair of fossae in the proximodorsal surface as seen in phalanx I-1. Ungual phalanx II has a highly curved ventral
edge and a proximally placed exor tubercle. However, it is dicult to further compare other ungual phalanges
due to its fragmentary condition. e proximal articular facet of phalanx III-1 is not transversely expanded,
which diers from phalanges I-1 and II-1, but has a proximoventral heel with 185% as high as the minimum
height of the sha just proximal to the distal condyles. e proximal articular facet of phalanx III-2 bears long
dorsal and ventral lips for tight articulation with phalanx III-1. is immovable articulation between phalanges
III-1 and III-2 is also known in SDUST-V1042 lacking the proximodorsal lip of phalanx III-246. Phalanx III-3 is
barely exed because its proximoventral lip contacts the ventrodistal surface of phalanx III-2 with slight exion.
Ungual phalanx III is shorter than ungual phalanx I, and the dorsal arch of ungual phalanx III is higher than
the level of the dorsal extremity of the proximal articular facet with the proximal articular surface of ungual
phalanx orientated vertically. e highest point of the dorsal arch is positioned at a half of the ungual phalanx
length, which is more distal than those of other troodontids5,20,21,43. A part of the keratinous sheath is preserved
in contact with the distal tip.
e distal femur bears a shallow notch to separate the supracondylar crest from the lateral condyle as in Gobi-
venator36 but unlike Almas and Daliansaurus7,41, which have a smooth distal edge of the crest (Figs.2A, 3G–H;
Supplementary Fig.S8). A thick and longitudinal ridge with 15.6mm in length and 3.6mm in height extends
proximomedially in the anteromedial edge of the distal femur. e position of this ridge resembles a prominent
process in Linhevenator and Philovenator, but which is low mound-like11,47. If the ridge is homologous to the
process in the two taxa, the ridge is remarkedly developed from the process. In lateral view, the straight anterior
edge of the tibial cnemial crest is nearly parallel to the tibial sha unlike Almas41, Gobivenator (MPC-D 100/86),
Liaoningvenator6, and Sinusonasus18, which have an inclined anterior edge of the crest (Figs.2A,B,G, 3J–K; Sup-
plementary Figs.S9, S11). e medial surface of the proximal bula bears a shallow concavity unlike Troodon
with a pronounced fossa3 and Mei, Tal os, and Xiaotingia with a at medial surface3,4,43 (Figs.2A,B, 3I,K; Sup-
plementary Figs.S10, S11). e astragalus is fused with the calcaneum unlike Talos having no fusion3 (Figs.2G,
3K; Supplementary Fig.S11). e distal condyles are anteroventrally divided by a shallow intercondylar groove
resembling Gobivenator and Mei but unlike Borogovia, Talos, and Troodon, which possess a deeper intercondylar
groove3,4,10,36,40. e ascending process is triangular with its center along the midline, resembling the general
shape in Gobivenator and Philovenator36,47, unlike Talos and Troodon with a medially displaced process3. e
base of the process bears a shallow depression as in Talos, Troodon, and Zanabazar3,38, but unlike Liaoningvena-
tor and Philovenator, which have a at anterior surface6,47. e ratio of metatarsals IV to II at the mid-sha in
posterior view is 250%, which is higher than those of most troodontids except for Gobivenator (272%)36 and
Talos (276%)3 (Figs.2B,G–H, 3L; Supplementary Fig.S12). Metatarsal III is strongly pinched in a trough formed
between metatarsals II and IV along its sha. Anteriorly, the proximal end of metatarsal III is obscured by the
anteroproximal contact between metatarsals II and IV, forming an arctometatarsalian condition48. e anterior
exposure of the proximal metatarsal III is wider until one h of the preserved length from distal to the proxi-
mal articular surface and narrower distally unlike Linhevenator with a wider anterior exposure distally11 and
Philovenator with no anterior exposure in the proximal extent47. All non-ungual pedal phalanges are slender
with a width of less than 30% of their length and have a ginglymoid distal articular surface (Fig.3M,N; Supple-
mentary Fig.S13). Ungual phalanx II has a mediolaterally compressed elliptical cross-section with a rounded
ventral edge. Phalanx III-3 is the most dorsoventrally compressed phalanx of digit III, measuring 64% as high as
its width at mid-sha, compared with 90% in phalanges III-1 and III-2. Unlike other non-ungual phalanges, the
distal condyles of phalanx III-3 bear a widely convex distoventral margin. e lateral distal condyle of phalanx
III-3 extends distally. Ungual phalanx III is triangular in cross-section and has a small proximodorsal lip as in
Talos3. Ungual phalanx IV is less curved than ungual phalanx III.
Discussions
Phylogenetic analysis
e phylogenetic analysis produced 10 most parsimonious trees with 12,235 steps with a consistency index of
0.072 and a retention index of 0.618. A strict consensus tree revealed that Hypnovenator belongs to Troodontidae,
having two of seven synapomorphies of the clade (minimum transverse width of metatarsus distally compared
to the proximal width < 60% [character 388] and the posterior projection of the posterior surface just proximal
to the lateral condyle of femur distinctly more posteriorly projected than the medial surface [character 657])
(Fig.4; Supplementary Data S1 and Figs.S14, S15). Furthermore, the taxon is placed within Troodontinae, sup-
ported by two of eleven synapomorphies of the clade (transverse width at midsha of metatarsal IV compared to
metatarsal II > 166% [character 226] and expanded ventrally, triangular cross-section of pedal ungual phalanges
III and IV [character 274]).
In addition to Hypnovenator, this study includes eight taxa (Borogovia, Gobivenator, Linhevenator, Saurorni-
thoides, Tal o s , Troodon, Urbacodon, and Zanabazar) from the Upper Cretaceous as members of Troodontinae,
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Figure4. Phylogenetic relationships of Hypnovenator matsubaraetoheorum gen. et sp. nov. and states of four characters. (A) States
of character 219 (proximal transverse constriction of metatarsal III: anterior exposure wider or subequal to metatarsals II and IV
[0], subarctometatarsal [1], arctometatarsal [2], hyperarctometatarsal [3]). (B) States of character 274 (cross section of pedal ungual
phalanges III and IV: expanded ventrally, triangular [0], subequal in transverse width dorsally and ventrally [1]). (C) Ratios for states of
character 226 (ratio of metatarsals IV to II in transverse widths at midsha in posterior view: < 66% [0], 66–165% [1], > 166% [2]). (D)
Ratios for states of character 674 (ratio of metatarsals II to IV in transverse width of trochlea: < 80% [0], 80–130% [1], > 130% [2]). (E)
Comparison of troodontid metatarsus in anterior view. Red, yellow, and blue colors represent metatarsals II, III, and IV, respectively.
Metatarsus with an asterisk is reserved from the original image. (F) Phylogenetic relationships of Hypnovenator matsubaraetoheorum
gen. et sp. nov. Strict consensus phylogenetic tree (CI: 0.072, RI: 0.618) with character distribution of 10 most parsimonious trees of
12,235 steps. Green numbers on the right side of each branch show synapomorphies on the pes. For character descriptions, see Sellés
etal.16. is gure was created using Adobe Illustrator 28.3 (https:// www. adobe. com/).
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which diers from previous works. Van der Reest and Currie37 placed Latenivenatrix in Troodontinae and
excluded Talos from the clade, considering the former a junior synonym of Stenonychosaurus49, assigned to
Troodon in this study50. Cau and Madzia10 included Albertavenator, Almas, Philovenator, and Xixiasaurus in
Troodontinae and positioned Borogovia as a sister taxon to the clade. However, Albertavenator was not included
in our analysis, and Almas, Philovenator, and Xixiasaurus were placed outside of Troodontinae. Two Early Cre-
taceous troodontids, Geminiraptor and Sinornithoides, were previously assigned within Troodontinae by Hart-
man etal.8 and Sellés etal.16, respectively, but our analysis placed them outside of the clade. With the exclusion
of the two taxa from Troodontinae, Hypnovenator stands as the only Early Cretaceous troodontine troodontid,
representing the oldest record of the clade.
Hypnovenator forms a clade with Gobivenator, positioned at the basal position of Troodontinae and united by
four synapomorphies: a single proximal cotyla of the ulna [character 152], mediolateral width of the ascending
process of astragalus < 58% width of astragalocalcaneum, when measured halfway up [character 209], at anterior
surface at base of the ascending process of astragalus compared to rest of its process [character 212], and 51–71%
with a ratio of anteroposterior diameters at midsha of radius to ulna [character 256].
Posture
e partial skeleton of Hypnovenator was buried in muddy sandstone of uvial deposits under low-energy water
ow, containing plant fragments. It shows slight displacement but remain in proximity to their original positions,
such as subparallel arranged gastralia segments, a tail positioned vertically to the long axis of the le tibia, and
a right hindlimb shied anterodorsally compared to the le one. Hypnovenator exhibits an intriguing posture,
characterized by a loosely folded forelimb lateral to the gastralia, tightly folded ankles, and unbent pedal digits
positioned under the gastralia, resembling the sleeping style of two Chinese non-troodontine troodontids (Mei
and Sinornithoides)17,51. It has been suggested that the posture of the Chinese troodontids from the Lower Cre-
taceous may have indicated sheltering within a burrow or protective responses to volcanic and eolian events4. In
contrast, two Mongolian alvarezsaurids (Jaculinykus and Shuvuuia) with a similar sleeping style52,53 have been
reported from the Upper Cretaceous alluvial deposits54,55. is new report on a sleeping posture suggests that
this posture was common within the clade of Troodontidae. Furthermore, it indicates that the sleeping posture
of small-bodied maniraptorans is prevalent not only in environments with volcanic and eolian events or alluvial
systems but also in uvial systems.
Evolution of manual ungual phalanges
Geometric morphometric analysis of troodontid manual ungual phalanges revealed that more than 71% of the
total shape variation is described by the rst two principal components (PC1 and PC2) (Fig.5C; Supplementary
Data S6). PC1 is primarily associated with the curvature of ungual phalanges. is indicates that strongly curved
ungual phalanges, with the highest point of the dorsal edge placed in the proximodorsal corner, are positioned
on the positive side of PC1. While non-troodontine troodontids have PC1 values of higher than -0.07, Hyp-
novenator has the lowest PC1 value in ungual phalanx III (-0.12) and the third highest PC1 value in ungual
phalanx I (0.05). is results in a remarkably wider PC1 range of 0.17 for the two ungual phalanges compared to
non-troodontine troodontids, which generally have values of less than 0.08. e wide range in PC1 values may
indicate a characteristic in Troodontinae. On the other hand, PC2 is related to the length and height of ungual
phalanges and the size of the exor tubercles. Short and high ungual phalanges with enlarged exor tubercles
are positioned on the positive side of PC2. Hypnovenator has nearly equal PC2 values in ungual phalanges I and
III (0.02), similar to those in ungual phalanges I and III of Sinornithoides (0.03). With the exception of Sinorni-
thoides and Xiaotingia, ungual phalanx II shows a positive shi in PC1 and a negative shi in PC2 from ungual
phalanx III, whereas ungual phalanx III demonstrates negative shis in both PC1 and PC2 from ungual phalanx
I. e mechanical advantage (MA) of ungual phalanges shows a stronger correlation with PC2 than with PC1
(R2 = 0.7154, p < 0.0001, n = 20) (Fig.5D). e MA value of ungual phalanx III of Hypnovenator is the highest
(0.48), resulting in the ungual phalanx being exceptionally high from the regression line and plotted well outside
of 95% condence intervals. Boxplots of MAs in each ungual phalanx show that both ungual phalanges I and
III are higher than ungual phalanx II, except for ungual phalanx III of Xiaotingia, which has a notably low MA
value (0.30), and that ungual phalanx I of Hypnovenator is plotted near the median value of MA (Fig.5E). Among
troodontids, ungual phalanx I tends to have a larger exor tubercle than other ungual phalanges (Fig.5F). Both
preserved ungual phalanges of Hypnovenator show nearly median values of the development of exor tubercle
(DFT) (sensu Kobayashi etal.56). e hypothesized outputs (HO) (sensu Kobayashi etal.56) of ungual phalanx
I tend to be higher than those of other ungual phalanges (Fig.5G). In Hypnovenator, ungual phalanx I is plot-
ted near the median value of HO, while ungual phalanx III is higher than the median value of HO, resulting in
both values of HO being subequal. Consequently, digit III functioned as eectively as digit I in Hypnovenator,
whereas digit I shows greater functionality than other ungual phalanges in non-troodontine troodontids. is
transition in function involves unique motions of digit III, such as immobile phalanx III-2 (also in non-named
troodontid SDUST-V 1042)46 and the minimally exed phalanx III-3 (also in Deinonychus)46, which may be also
recognized features in other troodontines.
Evolution of pedal structure
ree of the eleven synapomorphies for Troodontinae [characters 226, 274, 674] and one of the ve synapo-
morphies for derived troodontines (Urbacodon, Saurornithoides, Borogovia, and higher taxa) [character 218]
specically pertain to the pes (Fig.4). e asymmetrical metatarsus is characterized by a transverse width ratio
of metatarsals IV to II exceeding 166% at midsha [character 226]. Most Liaoning troodontids andSinorni-
thoidesexhibit a plesiomorphic condition, with ratios of less than 120%6,17,20,21,57, but this increases to 158% in
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MPC-D 100/140 of the late Early Cretaceous troodontid5. More derived taxa, such asHypnovenator(250%)
andGobivenator (272%), show a pronounced increase in metatarsal asymmetry, inuencing the arctometa-
tarsalian condition [character 219] (Supplementary Text S3). e arctometatarsalian condition is linked to the
cursoriality of troodontines, supported by the weight-bearing function and the relative length of hind limbs48,58,59.
Figure5. Manual ungual phalanges I (A) and III (B) of Hypnovenator matsubaraetoheorum gen. et sp.
nov. Fixed and sliding landmarks are displayed on each ungual phalanx. Points are colored as follows: xed
landmarks (red) and sliding landmarks (blue). (C) Scatter plot showing PC1 and PC2 values obtained from
geometric morphometric analysis. Symbols of manual ungual phalanges I-2, II-3, III-4, and unnumbered
manual ungual phalanges on the graph are shown by circles, triangles, squares, and pluses, respectively.
Dark green, light green, red, and black symbols are shown by members of Sinovenatorinae and the earlier
branching clade than sinovenatorines, members of a sister clade to the Sinovenatorinae excluding Hypnovenator
matsubaraetoheorum, Hypnovenator matsubaraetoheorum (= Troodontinae), and members with undecided
position, respectively. e gray dots on the thin plate spline represent the “average” positions of the landmarks
and the black dots represent the positions aer deformation. (D) Regression plot of PC2 values obtained from
geometric morphometric analysis and the mechanical advantage of troodontid ungual phalanges. e light
blue area shows 95% condence intervals. Boxplots represent the (E) mechanical advantages, (F) development
of exor tubercle, and (G) hypothesized output force in each ungual phalanx of troodontids. e red asterisk
shows Hypnovenator matsubaraetoheorum in boxplots. Abbreviations: Dl, Daliansaurus liaoningensis; Hm,
Hypnovenator matsubaraetoheorum ; Jt, Jianianhualong tengi; MA, MPC-D 100/44; MB, MPC-D 100/140;
SV, SDUST V1042 (not included in our phylogenetic analysis); Sy, Sinornithoides youngi; Xh, Xixiasaurus
henanensis; Xz, Xiaotingia zhengi; cs, claw sheath; , exor tubercle; DFT, development of exor tubercle; MA,
mechanical advantage; HO, hypothesized output force. is gure was created using Adobe Photoshop 25.5.1
and Adobe Illustrator 28.3 (https:// www. adobe. com/).
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e metatarsal arrangement in troodontines resembles those in ostriches, which possess only two pedal digits,
with the midline shi between metatarsals III and IV60. Although troodontids have four digits, only two (digits
III and IV) bear body weight due to the proximal position of digit I and the hyperextensible digit II1, suggest-
ing that this similarity likely arises from convergence due to cursorial habits with only two weight-bearing
digits.Non-cursorial eudromaeosaurs also have only two weight-bearing digits but the midline of metatarsus
positioned on metatarsal III as in most other theropods. Hypnovenatoris noteworthy as the oldest troodontine
with an asymmetrical arctometatarsus, extending the record by 35 million years compared with previousworks.
In addition to the metatarsal asymmetry, there is a trend towards increased cursoriality seen in the shorter digit
IV of some derived troodontines compared to non-troodontine troodontids61. For instance, the ratios of digits
IV to III are 75% inTroodon, 80% inTalos, and 86% inBorogovia(assuming the non-preserved phalanx III-1 is
approximately 33mm in length based onSaurornithoides). ese ratios are notably lower than those of theEarly
Cretaceous troodontids (98% inSinornithoidesand 96% inSinovenator).
e pedal features indicative of enhanced cursoriality are expressed through the morphological character-
istics of metatarsal and phalangeal articulations: ginglymoid for grasping and the absence or weakness of the
intercondylar groove for cursoriality61. Hypnovenator preserves the interphalangeal articulations of digits but
is missing the distal end of metatarsal III. Most non-troodontine troodontids and Gobivenator, a sister taxon to
Hypnovenator, retain ginglymoid articulations to some extent for grasping, indicated by the presence of a deep
distal articular groove on metatarsal III21,23,36,47,57,62, whereas there is no or shallow distal articular groove of
metatarsal III in derived troodontines [character 218]. Derived troodontines such as Troodon, Borogovia, Talos,
and Saurornithoides exhibit non-ungual interphalangeal articulation of phalanges of digit III with roller joints
and phalanges of digit IV with weakly ginglymoid articulation3,10,38,61. Hypnovenator, on the other hand, displays
ginglymoid articulations in the digits. e mosaic features in the pes for grasping (ginglymoid articulations in
digits) and cursoriality (asymmetrical arctometatarsus) in basal troodontines suggest that Hypnovenator, along
with Gobivenator, serves as a pivotal taxon, supporting a transition towards more ecient cursorial locomotion
in derived troodontines.
RegardingHypnovenator, although preservation issues hinder a complete assessment, members of the
Troodontinae exhibit a relatively narrow trochlea of metatarsal II, dened by a transverse width ratio of meta-
tarsals II to III trochlea less than 80% [character 674]. is narrow trochlea corresponds to the asymmetry of the
metatarsus, yet its presence suggests some level of grasping ability in the hyperextensible digit II61. Triangular
cross-sections of pedal ungual phalanges III and IV [character 274] are present inHypnovenator,Borogovia,Lin-
hevenator, andTroodon10,11. is feature is also found in non-avialan theropods except for therizinosaurs63,
microraptorine dromaeosaurs64, and non-troodontine troodontids43, indicating a reversal in the morphology
of ungual phalanges III and VI within Troodontinae, likely associated with adaptations for ground-dwelling
habitats and cursoriality.
Materials and methods
CT-scan
To construct the holotype materials of Hypnovenator within host rock in three-dimension, the materials under-
went microcomputed tomography (micro-CT) using a TESCO Microfocus CT TXS320-ACTIS at the National
Science Museum (Tokyo, Japan), and digital images were processed and measured using Amira 2019.3 (ermo
Fisher Scientic). e complete preparation was disturbed by extremely fragile bones (gastralia and dorsal ribs)
and unremovable host rock on both ends of long bones.
Denition of taxonomic names
is study follows Sereno65 for the denition of Troodontidae, a stem-based monophyletic group containing
Troodon and all coelurosaurs closer to it than Velociraptor or Passer. Sinovenatorinae was dened as the most
inclusive clade including Sinovenator but not Troodon, Saurornithoides, Anchiornis, Archaeopteryx, Gallus, Unen-
lagia, or Dromaeosaurus7 and redened as a stem-based monophyletic group containing Sinovenator closer to it
than to Jinfengopteryx, Troodon, and Passer8. is study follows the denition of Hartman etal.8. Troodontinae is
redened as the least inclusive clade containing Troodon (when included), Gobivenator, and Zanabazar but not
Sinovenator and Jinfengopteryx for the more stable clade than those of Martnuiuk66, van der Reest and Currie37,
Hartman etal.8, Cau and Madzia10, and Sellés etal.16. is study also follows the denition of Halszkaraptorinae
by Cau etal.67, the most inclusive clade that contains Halszkaraptor, but not Dromaeosaurus, Unenlagia, Sau-
rornithoides or Vultur.
Phylogenetic analysis
To test the phylogenetic position of Hypnovenator among troodontids, the data matrix of Sellés etal.16 was used.
e broad-scale data matrix was used to resolve the phylogenetic relationship among coelurosaurian theropods
including at least twenty-seven troodontid taxa. An analysis was run using the data matrix of Sellés etal.16,
which Hypnovenator adds. All cordings for troodontids were checked, and some scorings were modied based
on the papers and original materials (Supplementary Data S2 and TableS2). e data matrix comprises 503 taxa
and 700 characters. e analysis was conducted with equally weighted parsimony using TNT v. 1.568. We set the
maximum number of trees saved in memory at 10,000 and used a traditional search, performing 10,000 replica-
tions of Wagner trees (using random addition sequences) followed by tree bisection reconnection (TBR) as the
swapping algorithm, saving 10 trees per replicate. However, a strict consensus tree forms a polytomy within BYU
2023, ISMD-VP09, and other taxa because of the high ratio of missing data in BYU 2023 (99%). Second analysis
run with the same setting as the rst one excluding BYU 2023.
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Geometric morphometric analysis
Geometric morphometric analysis was performed to quantify the two-dimensional ungual phalanx morphologi-
cal variations using the R package geomorph version 4.0.769. Ungual outlines in lateral view were obtained from
the original materials of Hypnovenator, MPC-D 100/140, and MPC-D 100/44, the cast of Sinornithoides (FPDM-
V-7218), and the literatures on other troodontids7,20,43,45,46. Hypnovenator is the sole troodontine. e outlines
were digitalized into four xed landmarks and 12 sliding semi-landmarks (Supplementary Data S3–6), following
Chinzorig etal.70. e landmarks underwent generalized Procrustes analysis71,72 to align the specimens. Firstly,
this involved scaling all shapes (in this case, ungual phalanx landmarks) to a uniform size, followed by rotating
the shape coordinates around the origin to minimize shape dierences. Subsequently, principal component
analysis (PCA) was applied to the covariance matrix of the Procrustes coordinates. PCA identies maximum
variance in multidimensional datasets, summarizing the original data as PC1, PC2, and so on. Consequently, PCA
facilitates the graphical representation of multivariate data in a two-dimensional graph, as shown in Fig.573,74.
To evaluate the functionality of troodontid ungual phalanges, mechanical advantage (MA) was computed. As
MA corresponds to a class 3 lever75, resultant MA values indicate the proportion of output force exerted on the
ungual phalanx tip relative to the input force at the exor tubercle.
e mechanical advantage of the ungual phalanx can be determined using above equation, where ’a’ repre-
sents the output lever length from the fulcrum to the resistance, ’d’ denotes the length from the fulcrum to the
exor tubercle, ’θ’ signies the angle of the input force vector to the line of output lever, and ’δ’ represents the
angle between the line from the fulcrum to the exor tubercle and the line of output lever69. e size of the exor
tubercle is closely associated with the cross-sectional area of the attached muscle, correlating with the maximum
input force. us, the exor tubercle size was quantied as a ratio of the perpendicular length from the apex of
the exor tubercle to the segment between the base of the exor tubercle, serving as a proxy of the input force.
Multiplying the exor tubercle size by the mechanical advantage yields the inferred output force at the tip. ese
inferred output forces were compared across digits I to III. Standardized major axis (SMA) regression analyses,
utilizing R package smatr version 3.4.8, were employed to examine the relationship between the obtained PC
scores and the inferred output force, thereby assessing the shape-function relationships of troodontid ungual
phalanges. All statistical analyses were conducted on soware R version 4.3.276 using the R script, with modifying
le names, provided by Kobayashi etal.56.
Data availability
All data generated or analyzed during this study are included in this published article and its Supplementary
Information les.
Received: 22 May 2024; Accepted: 4 July 2024
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Acknowledgements
e authors deeply appreciate Kaoru Matsubara and Takaharu Ohe (Research Group on the Sasayama Group
[Sasayama-sougun wo shiraberu kai]) for their initial discovery of theHypnovenator matsubaraetoheorum speci-
men, and the late Haruo Saegusa (University of Hyogo, Japan) for managing subsequent excavation eorts.We
wish to express our gratitude to Kazumi Wada (Museum of Nature and Human Activities, Hyogo, Japan) for
his outstanding preparation of the specimen and to Genya Masukawa for drawing the reconstructed skeleton of
Hypnovenator matsubaraetoheorum. We thank all museum sta, related organizations, and volunteers for assist-
ing in excavation of the specimen in Nishikosa Area, Takanobu Tsuihiji and Makoto Manabe (National Museum
of Nature and Science, Tokyo, Japan) for providing CT-scan data of the specimen, Ryuji Takasaki (University
of Toronto) for giving K.K. useful advice on geometric morphometric analysis, and Kohei Tanaka (Tsukuba
University) for discussions on Canadian troodontids. We also would like to thank Rinchen Barsbold, Khishigiav
Tsogtbaatar, and Damdinsuren Idersaikhan (Institute of Paleontology, Mongolian Academy of Sciences), Mark
Norell, Carl Mehling, and Mick Ellison (American Museum of Natural History), James Gardner and Donald
Henderson (Royal Tyrrell Museum), Kieran Shepherd, Xiao-Chun Wu, Alan McDonald (Canadian Museum of
Nature), Soki Hattori and Masateru Shibata (Fukui Prefectural University) for providing access to comparative
specimens. is work was supported by a part of the Foundation of Kinoshita Memorial Enterprise.
Author contributions
K.K. designed and directed the project. K.K. wrote the description of Hypnovenator and performed phylogenetic
and geometric morphometric analyses. K.K. and Y.K. contributed to discussions. All authors shared the role of
editing the manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https:// doi. org/
10. 1038/ s41598- 024- 66815-2.
Correspondence and requests for materials should be addressed to K.K.
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