Charonosaurus

Credit: Connor Ashbridge on the Wikimedia Commons.

*Charonosaurus* is an extinct genus of parasaurolophinin lambeosaur that lived in Asia and Russia during the Late Cretaceous.

## History

*Charonosaurus* is a very large lambeosaurine, known from partial skull CUST J-V1251-57 from the Late Maastrichtian-aged Yuliangze Formation, Jiayin in fluvial sandstone/mudstone. Adults and juveniles have been found in the same area, probably representing the same taxon.

Chinese institutions have been excavating near Jiayin since 1975, along the right bank of the Amur River. This borders Russia and China. This limited area has produced hadrosaur bonebeds. Among these were remains reported by Pascal Godefroit, Shuqin Zan and Liyong Ji (2000), placing them in *Charonosaurus*. This is from Charon, the ferryman of the River Styx in Greek mythology. "*Jiayinensis*" derives from Jiayin, the type locality. *Charonosaurus* were found on the southern bank of the Amur River; 8 kilometers west of Jiayin, Heilonjiang Province, northeastern China. A. Kristofovich, M. Manakin, NP Stepanov and V. Arseniev (1902, 1914-1917 and 1925) made the first excavations, but Bolotsky et al. (2014) mention the site may have been discovered as far back as 1866 by F. Schmidt.

The Yuliangze sediment of-which *Charonosaurus* occurs was mainly determined with palynology; it is in part from the *Wodehouseia spinata*-*Aquilapollenites subtilis* palynozone (Early-Middle Maastrichtian; 70.6-66.043 million years ago), according to Markevich and Bugdaeva. This is also based on comparison with palynological communities in adjacent basins with sediment. The Late Cretaceous North America and eastern Asia were part of the *Aquilapollenites* Province, from this reason is which it may be beneficial to compare palynozones from eastern Russia and from the Western Interior Basin. This may find a strong comparison between the (North American) *Wodehouseia spinata* palynozone and the (Asian) *Wodehouseia spinata*-*Aquilapollenites subtilis* palynozone. From this, it was dated to the Late Maastrichtian, but the specific age for the sediment bearing dinosaurs can only be found with exact calibrations. Radiometric dating of the K/PG boundary in some parts of this region is stratigraphically lower than first thought, which argues in favour of the Late Maastrichtian date.

## Description

 File:Charonosaurus head.png
The femur is up to 1.35 meters (4.5 feet) long, and what is known of the skull resembles *Parasaurolophus*. Because of this, it is often depicted with a long, backwards-projecting, hollow crest formed by the dorsal surface of the frontals. However, updated classification brought to light in the *Tlatolophus* naming paper suggests otherwise; this crest may resemble *Tlatolophus* and *Parasaurolophus cyrtocristasus*, projecting back, but curving down, forming a broader end (similar to the "inverse comma" shape observed in Tlatolophus) and an elongate beak. This is mentioned only in diagrams, where the authors choose to restore the crest this way because *Charonosaurus* resolves close to *Tlatolophus*, so the trait can be inferred. *Charonosaurus* is one of the largest Asian hadrosaurs, and shows that lambeosaurines persisted until the Latest Cretaceous.

The supratemporal windows are relatively short and wide, with the structure on the frontal modified to fit a hollow crest. It forms a wide, hollow platform that bends strongly forwards, with long, deep, longitudinal grooves. The caudolateral of the frontal posteriorly projects, overhanging the anterior of the parietal and supratemporal windows. The rostrodorsal plane of the postorbital makes a thick promontorium. The branch near the posterior of the postorbital leg is long and broad, and almost completely overlaps the squamosal leg. The parietal is somewhat short, not reaching the occipital, having a slightly convex top with no trace of a sagittal crest. The anterior protuberance of the zygomatic is dorsoventrally broadened and symmetrically rounded in the lateral. As in all hadrosaurs, the ectopterygoid ridge is very well-developed. However, unlike Hadrosaurinae, the superior projection lies posterior to the maxillary midpoint. The square leg is very similar to North American hadrosaurids. The elongate articular surface runs atop the quadratojugal notch, suggesting a lack of the semiquadratic foramen. The ventral head of the quadratojugal leg is dominated by a large and hemispheric lateral condyle, which hinges with the surangular. The occipital neck is shortened, like all hadrosaurids, with the articular surface of the condyle being vertical. The basal palate projections extend below the condyle level. Above each protuberance of the basal palate has a large symmetrical, detached, wing-shaped protuberance at the lateral side of the sphenoid base. This protuberance has a caudal and rostral wing. The supreoccipital has 2 craniodorsal cusps who anchor to the caudal recesses in the parietal. The dental is robust, strongly-bending rostrally downwards, which forms an angle of ~30° with the longitudinal axis. ~40 rows of teeth comprise the battery observed in adult individuals. Since they grow high, each row reaches about 2 or 3 teeth tall, with only 1 or 2 being functional. The diastema is shorter in juveniles and extended in adults. Each tooth grows up to ~5 centimeters high, having a height/length ratio of >4 and have one sinuous, vertical ridge.

The sacrum is comprised of 9 ossified vertebrae. The forearm extending led to shoulder girdle and forelimb adjustments, which also causes the biceps and triceps to develop. The proximal head of the scapula bears a larger strut on the caudolateral, which marks the anchor points of the triceps scapulare. The coracoid is generic for a hadrosaurid, having a large cranioventrally-oriented, hooked projection; also present in young individuals. The raven bone has an extremely large cusp that anchors the biceps. The humerus bears a large deltopectoral crest, extending down below the center of the leg. At the cranial side lays the broad intertubercular sulcus, where the caput longum and biceps branchii run. The bottom, lateral and medial walls of this groove have anchor points for 3 other muscles. The acrista tricipitalis is present in all specimens on the end of the humerus; this is especially developed in older specimens. The radius and ulna are slender and very elongate, with the ulna having a total length/maximum width ratio of >6.3 on the proximal head. It is >6.6. for the radius. The preacetabular protuberance of the ilium is more elongate than other hadrosaurs. The ilium/preacetabular length has a ratio of >2.1, rotating slightly to the ventral. The antitrochanter is large. The distal ischium forms a wide, foot-shaped knob. The blade of the pre-pubisi is somewhat short, broadening abruptly cranially. The distal fibular head is strongly broadened and shaped like a club. The cranial ascending protuberance of the talus is shaped like an equilateral triangle.

## Classification

Pascal Godefroit, Shuqin Zan and Liyong Jin (2000) performed a cladistic analysis based on 33 cranial, tooth and postcranial features; they found it may be closer to *Parasaurolophus* than to any other lameosaur. Characters that are unavailable were not included, with *Eolamia* and *Tsintaosaurus* excluded. It has the following synapomorphies with lambeosaurinae: the squamosals prevent contact between the (shortened) parietal (having a length/minimum width ratio of <2) and the occipital, the frontal forms a broad, hollowed platform that founds the crest and becomes postorbital through the connection, the prefrontalis not part of the orbital rim, the vertical-posterior section of the frontal, the raised end of the squamosal, the rounded and symmetrical front of the protrusion (in the zygomatic), the sinuous middle carina of the teeth dentin and deltopectoral crest are strongly developed in length and width of the humerus, extending below the middle of the leg. In lambeosaurine, *Charonosaurus* and *Parasaurolophus* are very close, since both have the frontal platform extending posteriorly above the anterior of the parietal and supratemporal fenestra, the club-like fibular distal head being enlarged and the upward cranial projection of the ankle bone being equilateral. In 2021, it was found:

## Paleoecology

4 thanatocoenosis, Jiayin, Wulaga, Blagoveschensk and Kundr of the Amur region, have varies paleoenvironments that often oppose. This may suggest that Maastrichtian East Asian hadrosaurids had a strict habitat preference. Similar phenomena are seen in North American taxa. However, these patterns, may be due to lesser knowledge, and not due to an irregular occurrence. Unlike most of the world at this time, titanosaurs are excluded from all of Asia. Lambeosaur-dominated environments shows that differing dinosaur communities were present outside North America before the K/PG event. These sites may give information about dinosaur diversity.

Bonebeds span several tens of meters², consisting mostly of *Charonosaurus*. In these layers house juveniles and adults, unarticulated, and scattered amongst one-another. Taphonomic features, such as long bones oriented in one direction and the vertebral arches, spinous processes and the apophyses are broken off, suggesting a thanatocoenosis formed in a river or stream, with strong currents. This results in their corpses accumulating and piling in a low point in the environment. ~90% of the recovered material are from lambeosaurines. Ankylosaurs, theropods (including tyrannosaurids), turtles, crocodiles make the remaining ~10%. There is no evidence it coexisted with other lambeosaurus, but *Saurolophus* have been recovered. Many isolate theropod teeth show that carcasses were scavenged upon by many dinosaurs, or that they were hunted and killed by the riverside.

## References

1. ↑ The Princeton Field Guide to Dinosaurs, Gregory S. Paul
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