New therizinosaurid dinosaur from the marine Osoushinai Formation (Upper Cretaceous, Japan) provides insight for function and evolution of…

In 2008, Murakami et al.1 described a partial manus of a theropod dinosaur, consisting of metacarpal I and manual unguals I-2, II-3, and III-4 from the right side in a concretion, from the upper Campanian Osoushinai Formation of the Yezo Group in Nakagawa Town of Hokkaido Island of Japan (Fig.1ac). The Yezo Group is composed of mainly Upper Cretaceous marine sediments and rich in invertebrate and vertebrate fossils, including sharks, plesiosaurs, mosasaurs, turtles, pterosaurs, non-avian dinosaurs, and birds. So far, dinosaur materials of hadrosaurids, tyrannosauroid, and nodosaurid, in addition to the Nakagawa theropod, have been reported from the group2,3,4. The Nakagawa specimen was identified as a maniraptoran dinosaur and Murakami et al.1 argued further that it may belong to a derived therizinosaur because of the lack of a groove between the flexor tubercle and the proximal articular surface of the unguals. It was extremely difficult to resolve the identification of Nakagawa specimen to the family level or lower at the time of publication because of the limited comparative information in the literature. However, more recent studies have provided a great deal of information on therizinosaur unguals, permitting comparisons of manual morphologies among therizinosaurs and for testing the phylogenetic status of the Nakagawa specimen5,6,7,8,9,10.

Map of Japan, showing the location of Nakagawa Town and other dinosaur localities (a). Two other names in red are locations of other therizinosaur materials. This figure was created by using Adobe Photoshop 21.2.0 and Adobe Illustrator 24.2.1 (https://www.adobe.com/). A photo of the concretion containing materials of Paralitherizinosaurus japonicusgen. et sp. nov.before preparation (b). Dark parts in the concretion are exposed bones of Paralitherizinosaurus, showing that all elements were preserved in this block. See Murakami et al.1 for stratigraphic column of the Oshoushinai Formation and the horizon of the specimen. Manual unguals and silhouette of Paralitherizinosaurus japonicus, showing recovered skeletal elements in white (c) (Courtesy of Genya Masukawa). Life reconstruction of Paralitherizinosaurus japonicus (d) (Courtesy of Masato Hattori).

Asian therizinosaurs radiated in the Early Cretaceous, and their diversification continued into the Late Cretaceous. The Late Cretaceous taxa are larger than the Early Cretaceous forms in body size, exemplified by Therizinosaurus cheloniformis from Mongolia with elongate, large and nearly straight manual unguals. Manual unguals of therizinosaurs have a large diversity in shapes and functions. Based on the shape analysis with extinct and extant mammals by Lautenschlager10, basal therizinosaurs, such as Alxasaurus and Erliansaurus, have short and compact unguals for a proposed generalist functionality because these are placed near the boundaries of the morphospace of scansorial, fossorial, and terrestrial mammals, whereas other therizinosaurs with elongate unguals occupy outside of these mammals for potentially different functions. Only Nothronychus has unguals like those found in fossorial mammals. Because some therizinosaurs such as Beipiaosaurus and Therizinosaurus, have similar unguals to those of ornithomimosaurs, elongate unguals are presumed to have a similar function, possibly to pull the base of branches to bring leaves to their heads10.

Therizinosaurs have been found mainly from the Cretaceous deposits in Mongolia and China. All of the Early Cretaceous taxa, except Falcarius, have been named from China so far, and two taxa are recovered from northern (Alxasaurus from the Inner Mongolia)11 and northwestern (Suzhousaurus from Gansu Province) regions12. Other Chinese taxa (Jianchangosaurus, Beipiaosaurus, and Lingyuanosaurus)6,13,14 were recovered from the Jehol Group in the Liaoning Province of China, located in the eastern part of the country. The Late Cretaceous therizinosaurs have been discovered from the Gobi Desert (Inner Mongolia of China and southern Mongolia) except for Nanshiungosaurus (Guangdong Province in the southeastern China along the Pacific). Although it has not been named yet, therizinosaur materials have been recovered from the Bissekty Formation in Uzbekistan, which may represent more than two taxa9. However, we follow Sues and Averianov9 in their treatment of the Bissekty therizinosauroid material as a single taxon. Japan, which was located at the eastern edge of the Asian continent during the Cretaceous before the opening of the Japan Sea during the Miocene, has produced two therizinosaur specimens from the Lower and Upper Cretaceous deposits15,16 (Fig.1a) but neither is named because of the fragmentary nature of the specimens. The Nakagawa specimen is the third therizinosaur from Japan. It is important because it is the youngest occurrence of therizinosaurs from Japan and preserves important elements that enlighten our understanding of its finer-scale taxonomic identification as well as providing insights into and morphological function.

This study will describe the Nakagawa material in detail, compare with other therizinosaurs, demonstrate its phylogenetic placement within Maniraptora/Therizinosauria, quantify the ungual shapes through geometric morphometric analysis to elucidate the evolution and function of therizinosaur claws, and discuss its paleogeographic and paleoecological implications.

Detailed information is provided by Murakami et al.1 and summarized here. This specimen was contained in an isolated concretion as float, sitting near the confluence of the Rubeshibe River and 36 Ten-zawa Creek in Nakagawa Town in Hokkaido Prefecture, Japan (Fig.1a). The concretion was discovered in an area where the Nishichirashinai Formation (Coniacian to lower Santonian) is exposed. Murakami et al.1 concluded that it was originally from an upstream area, where the Oshoushinai Formation (early Campanian) crops out because of the presence of the inoceramid bivalve Sphenoceramus in the concretion and the features of lithofaces (heavily bioturbated matrix and fine grain size), which matches the Osoushinai Formation. The depositional environment of this formation is considered as deeper than outer shelf because of the lack of storm deposits. The concretion is roughly 25cm15cm15cm (Fig.1b). It is plausible to consider that all materials belong to a single individual based on its depositional environment and close proximity of all preserved elements as mentioned by Murakami et al.1.

Theropoda Marsh17.

Coelurosauria von Huene18.

Therizinosauria Russell19.

Therizinosauridae Maleev20.

Paralitherizinosaurus japonicus gen. et sp. nov.

Zoobank ID: urn:lsid:zoobank.org:pub:8397F6AE-4791-4EE6-B6CF-B25950FB617C (for this publication), urn:lsid:zoobank.org:act:7934F3DA-B1CA-4CAA-B5E4-86D41B93E7CE (for the new genus) and urn:lsid:zoobank.org:act:BC19DCA2-8F4A-4FDC-83C4-B9EB8D3C30EF (for the new species).

Etymology Paralos means by or near the sea in Greek, therizo means reap in Greek, and sauros means reptile in Latin. Specific name, japon refers to Japan.

Holotype NMV-52 (NMV, Nakagawa Museum of Natural History, Japan), a partial vertebra and a partial right hand, including metacarpal I, proximal ends of unguals I and II, and nearly complete ungual III (Fig.1c).

Horizon and locality The Osoushinai Formation (early Campanian) of the Yezo Group in Nakagawa Town in Hokkaido Prefecture, Japan.

Diagnosis A therizinosaurid with the following unique characters: dorsoventrally flattened metacarpal I (dorsoventral height less than half of transverse width) with no rotation of the axis of the distal end; presence of proximodorsally projecting dorsal lip in digits I and III; a shallow depression at the proximal portion of ungual III-4, connecting to the collateral groove; the presence of proximally extending ventral process; a weak flexor tubercle expressed as a small ridge.

Description and comparisons The original study described four manual elements from the right side (metacarpal I and manual unguals I-2, II-3, and III-4), but this study identified another bone as a partial cervical centrum (Fig.2) although the fragmentary nature of this bone limits the certainty of its identification. We interpret the specimen as the anteroventral portion of a cervical centrum. Therizinosaur presacral vertebrae are highly pneumatic8,14,21, a condition that is present in this specimen. The parapophysis is ventrally placed on the lateral surface. Two laminae, posterodorsal and ventral laminae, extend posterior to the parapophysis. The arrangement of the posterodorsal lamina is similar to the centrodiapophyseal lamina in Northonychus8. Ventral surface is flat and featureless.

A partial vertebra of Paralitherizinosauruss japonicus gen. et sp. nov. in anterior (a) and right lateral (b) views. This figure was created by using Adobe Photoshop 21.2.0 and Adobe Illustrator 24.2.1 (https://www.adobe.com/).

Metacarpal I is nearly complete. It is 57.87mm long, which is approximately twice the length of the transverse width (29.41mm) at the proximal end. In anterior view, the distal half of metacarpal I diverges medially by 17 degrees from the contact surface with metacarpal II (Fig.3a,a,c,c). A similar condition is present in two Chinese therizinosaurids (Alxasaurus and Erliansaurus), while the shaft of metacarpal I is nearly straight in basal therizinosauroids (Falcarius and Jianchangosaurus) and the therizinosaurid Therizinosaurus5,14,22. The shaft width (21.72mm) is roughly two-thirds of the proximal width. The medial border of this element is strongly concave in anterior view (Fig.3a,a), whereas it is nearly straight in Nothronychus and Therizinosaurus and weakly concave in Falcarius, Alxasaurus, and Erliansaurus5,8,11,22,23. The width of the proximal end is greater than the dorsoventral height (24.47mm). In proximal view, the proximal end is triangular with three processes: medial, anterolateral, and posterolateral processes. In their preliminary description of this specimen, Murakami et al.1 noted that the presence of a rectangular buttress was not clear because of its preservation. Our examination shows that the base of the ventrolateral process is preserved, and a ridge extends ventrally from the process, indicating the presence of a rectangular buttress (Fig.3b,b,c,c). The buttress would underlie the ventral surface of the metacarpal II if entirely preserved, and this is a synapomorphic feature for the clade of Therizinosauroidea11. In anterior view, the proximal edge of the proximal end between the dorsal and mediolateral processes is straight as in therizinosaurids (e.g., Alxasaurus and Therizinosaurus). Falcarius has a concave proximal edge of the proximal end for a dorsomedial notch22. The proximal surface has a depression with a size of 21mm14mm (Fig.3e,e). The medial edge of the element bears a medial ridge (Fig.3d,d), connecting the medial process of the proximal end and medial condyle of the distal end. This feature is present only in Therizinosaurus and diagnostic for this taxon5. The cross-section of the shaft is triangular as in Therizinosaurus, whereas it is oval in Falcarius22. It is anteroposteriorly compressed, and its dorsoventral height is 10.12mm, less than half of lateromedial width of the shaft. A rotation of the axis of distal end from the long axis of the element is noted in Falcarius22 and Erliansaurus, but this rotation is absent in Paralitherizinosaurus. The distal end has lateral and medial condyles, separated by a shallow sulcus (Fig.3f,f). The lateral condyle has a circular collateral ligament fossa, whereas the medial condyle lacks a fossa and has a flat medial surface. A collateral ligament fossa is absent in Falcarius22 and Therizinosaurus. The medial condyle is proximally positioned with respect to the lateral condyle in therizinosaurs, but this condition is more subtle in Paralitherizinosaurus.

Right metacarpal I of Paralitherizinosauruss japonicus gen. et sp. nov. in anterior (a), posterior (b), lateral (c), medial (d), proximal (e), and distal (f) views. (a) to (f) are corresponding images in black and white with labels. This figure was created by using Adobe Photoshop 21.2.0 and Adobe Illustrator 24.2.1 (https://www.adobe.com/).

Manual ungual I-2 preserves only lateral side of the proximal end (Fig.4a,b,a,b). It has a dorsal lip, which is a proximodorsally projecting process above the phalangeal articular surface of manual unguals. This process is commonly seen in manual unguals of therizinosaurs, oviraptorosaurs, and dromaeosaurids24. In derived therizinosaurs, Therizinosauridae, no taxa have a dorsal lip in manual ungual I-2. The only therizinosaur with a dorsal lip is the basal therizinosaur Beipiaosaurus13. In lateral view, the outline of the surface is like the other manual unguals of Paralitherizinosaurus (Fig.4a,a).

Right manual unguals of Paralitherizinosauruss japonicus gen. et sp. nov. Ungual of digit I in lateral (a) and medial (b) view. Ungual of digit II in proximal (c), lateral (d), medial (e), dorsal (f), and ventral (g) views. Ungual of digit III in proximal (h), lateral (i), medial (j), dorsal (k), and ventral (l) views. (a) to (l) are corresponding images in black and white with labels. This figure was created by using Adobe Photoshop 21.2.0 and Adobe Illustrator 24.2.1 (https://www.adobe.com/).

Manual ungual II-3 preserves the proximal portion of the element and exhibits a pronounced dorsal lip as seen in Falcarius, Lingyuanosaurus, Alxasaurus, and Therizinosaurus (Figs. 4df,df and Fig.5)6,11,22,25. The ventral surface of this lip forms a dorsal portion of the phalangeal articular surface. The medial side of the base of the lip has a shallow depression, extending ventrally (Fig.4e,e). The ventral half of the medial surface has a wide depression. A depression on lateral and medial surfaces of proximal portions in Jianchangosaurus, Lingyuanosaurus, and Therizinosaurus is continuous from the collateral groove6,14,25. Ventral to the phalangeal articular surface bears a ventral process, which extends slightly more proximally than the articular surface, similar to the condition observed in Erliansaurus (Fig.4e,e,g,g). It is square in lateral view as in Therizinosaurus. The phalangeal articular surface is divided asymmetrically by a vertical ridge, and the medial side is larger than the lateral side (Fig.4c,c).

Comparisons of manual unguals in digits I-III of therizinosaurs. An image at the lower left corner is a photo of the proximoventral end of Therizinosaurus ungual in oblique view. Not to scale. This figure was created by using Adobe Photoshop 21.2.0 and Adobe Illustrator 24.2.1 (https://www.adobe.com/).

Manual ungual III-4 is transversely narrow as in other unguals (Fig.4k,k,l,l) and strongly curved as in other therizinosaurs, other than Therizinosaurus5,25 (Fig.4i,i,j,j). The collateral groove on the lateral surface of this element approaches the dorsal margin of the ungual distally. Although the distal end is missing, the collateral groove may be extended to the dorsal edge of the distalmost part of the ungual in Paralitherizinosaurus (Fig.4i,i), which is a potential diagnostic feature for Therizinosauroidea6. The groove at the proximal end is close to the ventral edge of the ungual, like the ungual of digit II of Therizinosaurus and isolated unguals from the Bissekty taxon (Fig.5), and is continuous with a shallow depression. This depression is much smaller than the ones observed in Jianchangosaurus, Lingyuanosaurus, Nothronychus, and Therizinosaurus6,8,14, which have a large triangular depression. Extension of the collateral groove to the proximal end is only seen in isolated unguals of the Bissetky taxon (Fig.5). The medial surface of the ungual is flat and featureless (Fig.4j,j). Distally, the collateral groove is faintly present and migrates dorsally towards its tip. In most therizinosaurs, both the lateral and the medial surfaces have distinct collateral grooves. The asymmetry of features on the lateral and the medial surfaces is present to some extent, but this strong asymmetry may be a unique feature for Paralitherizinosaurus. The ungual has a dorsal lip at the proximal end (Fig.4i,i,j,j). In Therizinosauria, a complete set of manual unguals is rarely preserved but has been reported in five taxa (Falcarius, Beipiaosaurus, Martharaptor, Erliansaurus, and Nothronychus). Among these taxa, only two taxa have a dorsal lip, which is present in digits II and III in Falcarius and in digits I in Beipiaosaurus. Three therizinosaurids (Lingyuanosaurus, Alxasaurus, and Nothronychus) preserve the manual ungual of digit III, but none of these taxa preserves a dorsal lip because of damage, suggesting the presence of a dorsal lip in manual ungual III-4 may be unique to Paralitherizinosaurus. At the base of the dorsal lip is a shallow depression as in the ungual of digit II. The phalangeal articular surface is divided by a vertical ridge, but it is nearly symmetrical (Fig.4h,h). Ventral to the articular surface has a ventral process, which extends more proximally than the articular surface. This large ventral process is present in the ungual of the digit III of Erliansaurus. The ventral surface of the ventral process bears a weak ridge, which is a flexor tubercle. The original description of this ungual interpreted that the flexor tubercle was missing because of bioerosion1. Subsequent preparation of this ungual shows that this portion was not damaged and there is an extremely weak flexor tubercle on the ventral surface (Fig.4l,l). A similar condition is present in Therizinosaurus (Fig.5), where the other therizinosaurs have a strong flexor tubercle.

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New therizinosaurid dinosaur from the marine Osoushinai Formation (Upper Cretaceous, Japan) provides insight for function and evolution of...