Jerry Harris

In November 2018, a team of researchers from the National Park Service and the St. George Dinosaur Discovery Site documented fossil vertebrate tracks at the Andre’s Alcove Tracksite (AAT) in the Lower Jurassic (?190–185 Ma) Navajo Sandstone in Glen Canyon National Recreation Area, Utah. The tracks at the AAT were photo-documented for photogrammetric processing using Agisoft Photoscan Professional to enable the track surface to be studied remotely. Ichnotaxa already identified at the AAT include those of small ornithischian dinosaurs (Anomoepus), theropod dinosaurs (Grallator, Eubrontes), and protosuchian crocodylomorphs (Batrachopus). Vertebrate body fossils are rarer in the Navajo Sandstone than in underlying, earlier Jurassic strata; the AAT site therefore provides important insights into late Early Jurassic vertebrate diversity and behaviors at the onset of the Navajo Erg in the American Southwest.

Fossil footprints provide important evidence regarding the morphology, behavior, distribution, and ecology of ancient animals. In recent years, the ichnological record (pertaining to fossils other than skeletal or body parts, most familiarly and commonly tracks) of major tetrapod clades has been studied intensively. The body fossil record amply demonstrates that the origin of birds lies within the theropod dinosaur lineage (making birds extant dinosaurs, in an evolutionary sense), but the ichnological record contributes much valuable information concerning behavioral shifts during both this evolutionary transition and the early diversification of birds. Here, for the first time, we review the entire avian track record, including its specialized ichnotaxonomy, from the Mesozoic (the “Age of Reptiles,” 250-65 million years ago) and Cenozoic (the “Age of Mammals and Birds,” 65 million years ago through the present, including the Holocene) and consider how the evidence impacts the understanding of avian evolution and ecology.
Growing evidence from both the skeletal and track records indicates that the initial avian taxonomic, morphological, and ecological radiations took place around the Jurassic-Cretaceous boundary (about 145 million years ago). Tracks similar to, and in some cases indistinguishable from, those made by modern shorebirds (Charadriiformes), small ducks (Anseriformes), small herons (Ciconiidae), and even roadrunners (Cuculiformes) appeared, and were even regionally abundant only 15-20 million years thereafter. In contrast, the oldest body fossil records of anseriforms and possibly charadriiforms occur very close to the end of the Cretaceous (roughly 70 million years ago), and later still for ciconiiforms and cuculiforms. This strongly implies that the early track makers were members of extinct, early avian lineages with which later taxa converged in foot morphology. Feeding traces associated with some of these tracks demonstrate that behaviors reminiscent of extant herons and spoonbills had also evolved by this time. However, despite significant skeletal and footprint finds, there is little correspondence between the records — few footprints match the feet of birds represented by skeletal remains. In short, the familiar morphologies and behaviors of many modern birds actually evolved convergently with many of their extinct, Mesozoic relatives. Footprints thus have the dual benefits of providing an important, and unexpected, complementary record of early avian morphological and ecological diversity while highlighting the importance of morphological and behavioral convergence.
Although the skeletal record suggests an avian taxonomic shift at the “dinosaur-killing” Cretaceous-Paleogene (K-Pg) boundary extinction event, the track record provides insufficient evidence to support or refute such a shift because the dominance of shorebird-like tracks continues uninterrupted from Mesozoic to Cenozoic. Early Paleogene tracks provide evidence of large, Diatryma- or Gastornis-like, ground dwelling birds in addition to typical shorebirds and waterbirds like the Eocene anseriform Presbyornis. Neogene tracks include those of a few large ratites and a turkey-like species; Holocene tracks include those of several species of moa. Unlike its Mesozoic counterpart, the Cenozoic avian body fossil and ichnological records correspond much more closely.
Tracks of perching birds, raptors, and other groups that do not habitually frequent wet shorelines — the most suitable environment for track preservation — are rare. Indeed, the avian track record is dominated by the footprints of shorebirds, with a minor component attributable to large flightless and cursorial forms. Nevertheless, the body of literature on fossil bird tracks is still relatively small (~200 reports), describing about 6 ichnofamilies, comprising about 38 named ichnogenera and 65 ichnospecies.

Herein we describe a single trackway that pertain to Menglongipus sinensis ichnogen. et ichnosp. nov. from the Nijiagou track site in the Chicheng county, Hebei Province. China. The tracks occur in the Tuchengzi Formation, which spans the Jurassic-Cretaceous boundary. The discovery of M. sinensis indicates that deinonychosaurians occupied this area prior to deposition of the Yixian Formation. from which the oldest deinonychosaur body fossils in the region have been found. The body length (about 65cm) of the M. sinensis track maker is very similar to that estimated for basal paravians. Additional details are provided about the type Velociraptorichnus sichuanensis and the association between dromaeopodid and other theropod tracks is discussed.

Herein we describe three trackways that pertain to Hadrosauropodus nanxiongensis ichnosp. nov. from the Yangmeikeng track site in the Nanxiong Basin, Guangdong Province, China. The tracks occur in the Upper Cretaceous Zhutian Formation. The nearby Gushi track site preserves trackways attributable to Hadrosauropodus isp. These represent the first occurrences of hadrosaur footprints in China. These discoveries expand the known distribution of hadrosaur tracks from North America to China, and provides evidence for the existence of large hadrosaurs in the Cretaceous of the Nanxiong Basin. Other previously-described, large, ostensible ornithopod track occurrences in China are discussed: Sinoichnites is represented only by an informal “plastotype,” Yangtzepus is attributable to a theropod, and Iguanodonopus is considered a nomen nudum and its specimens attributable to Iguanodontipus. New specimens of Jiayinosauropus allow some redescription of the ichnotaxon, and the unnamed Neixiang footprint is redescribed.

Herein we describe three and one half footprints that pertain to Grallator isp. from the Lower Cretaceous Yixian Formation, Sihetun, Liaoning Province, China. This is the first description of dinosaur footprints from the Yixian Formation. The tracks were left by at least three individual track makers. It is estimated from the tracks that the body lengths of the track makers were 1.51 m, which is the average length of known theropods from the Yixian Formation. The feet of Caudipteryx and Sinosauropteryx were reconstructed. The former was more similar than the latter to the Grallator isp. track outlines. Feet capable of registering Grallator morphotype tracks may therefore have been widely distributed in small-medium sized theropods (other than dromaeosaurids and troodontids) from the Yixian Formation.

Background
Fossil tracks made by non-avian theropod dinosaurs commonly reflect the habitual bipedal stance retained in living birds. Only rarely-captured behaviors, such as crouching, might create impressions made by the hands. Such tracks provide valuable information concerning the often poorly understood functional morphology of the early theropod forelimb.

Methodology/Principal Findings
Here we describe a well-preserved theropod trackway in a Lower Jurassic (~198 million-year-old) lacustrine beach sandstone in the Whitmore Point Member of the Moenave Formation in southwestern Utah. The trackway consists of prints of typical morphology, intermittent tail drags and, unusually, traces made by the animal resting on the substrate in a posture very similar to modern birds. The resting trace includes symmetrical pes impressions and well-defined impressions made by both hands, the tail, and the ischial callosity.

Conclusions/Significance
The manus impressions corroborate that early theropods, like later birds, held their palms facing medially, in contrast to manus prints previously attributed to theropods that have forward-pointing digits. Both the symmetrical resting posture and the medially-facing palms therefore evolved by the Early Jurassic, much earlier in the theropod lineage than previously recognized, and may characterize all theropods.

Herein described are two footprints that belong to the new ichnospecies Changpeipus pareschequier ichnosp. nov.from the Lower Jurassic Lufeng Formation near Yaozhan Village, Lufeng County, Yunnan Province, China. This is the first discovery of dinosaur footprints in the Lufeng Formation. The ichnogenus Changpeipus is revised after the re-study of its type specimens .Specimen IVPP V2472 2a, a supposed manus print that is part of the ichnogenoholotype of Changpeipus carbonicus, is re-studied herein and assessed as a pes print of a juvenile individual of the Changpeipus carbonicus track maker. Changpeipus luanpingeris is a junior synonym of Changpeipus carbonicus. Changpeipus pareschequier ichnosp.nov. resembles the ichnotaxon Kayentapus. Based on its characteristics,the footprints are presumably made by a member of the Coelophysoidea from the Lufeng Formation.

At least 50 pterosaur track sites have been reported from Late Jurassic through Late Cretaceous localities in North America, Europe, East Asia and South America, plus one possible site from north Africa. Tracks from these sites have been assigned to 11 ichnospecies in four ichnogenera. Of these, Pteraichnus is by far the most prevalent, well-preserved, and represented by multiple (presently eight) ichnospecies. The majority of Pteraichnus tracksites are Late Jurassic or earliest Cretaceous (Berriasian) in age. In contrast, the other three ichnogenera – Purbeckopus, Haenamichnus and possibly Agadirichnus – are all represented by single ichnospecies from single localities and are based on relatively poorly-preserved, earliest through latest Cretaceous material. At least 16 Late Jurassic, well-preserved Pteraichnus samples from marginal marine deposits in western North America are dominated by small tracks (pes length ~2–10 cm), often in trackways. The two ichnospecies from this region (P. saltwashensis and P. stokesi) are both based on trackway segments and differ in style of preservation. Other Late Jurassic trackways from marginal marine deposits in France and Poland are also small (pes length typically ~3–5 cm); much larger tracks (pes length 18 cm) are known from a single locality in Asturias, Spain. It is unclear whether most Pteraichnus represent pterodactyloid or “rhamphorhynchoid” pterosaurs because the diagnostic impressions of pedal digit V are rarely clearly and unambiguously impressed. Six basal Cretaceous (Berriasian) Pteraichnus ichnospecies from a least a dozen sites in Soria, Spain are also mostly small (footprint length ~1–5 cm) and based on as yet insufficiently described, isolated footprints, not trackways. As a result, several of these ichnospecies are probably nomina dubia. The contemporaneous Purbeckopus from England is much larger (foot length ~19–22 cm). Although small, Pteraichnus-like tracks have been reported sporadically from the post-Berriasian Cretaceous, most are much larger (foot length 10–20 cm) and, in the case of Haenamichnus from Korea, reach 30–33 cm. It is unclear whether Agadirichnus from the Late Cretaceous of Morocco (foot length 10–12 cm) is pterosaurian. Most Cretaceous sites represent lacustrine, not marginal marine, habitats. Both Jurassic and Cretaceous assemblages often contain very high track densities and a range of track sizes associated with invertebrate traces. This suggests that diverse pterosaurian flocks may have congregated in large numbers to feed. Some assemblages reveal swim tracks that suggest pterosaurs floated in shallow water, touching the submerged substrate with only their hind feet. These swim track assemblages also contain possible beak traces that may indicate feeding.

Deinonychosaurian theropods, the dinosaurian sister group of birds, are characterized by a large raptorial claw borne on a highly modified second digit that was thought to be held in a retracted position during locomotion. In this study, we present new trackway evidence for two coeval deinonychosaurian taxa from the Early Cretaceous of Shandong, China that indicate a hitherto unrecognized body size diversity for this period and continent. These fossil tracks confirm diversity and locomotory patterns implied by phylogeny and biogeography, but not yet manifested in the body fossil record. Multiple parallel and closely spaced trackways generated by the larger track maker provide the best evidence yet discovered for gregarious behavior in deinonychosaurian theropods.

Appendicular elements of the sauropod dinosaur Suuwassea emilieae, from the Upper Jurassic Morrison Formation of Montana, USA, display a peculiar mix of autapomorphic and plesiomorphic features. While more similar in overall morphology to Apatosaurus than other flagellicaudatans, the coracoid of Suuwassea lacks the quadrangular shape of Apatosaurus. The humerus of Suuwassea bears a pronounced proximal tuberculum, a feature seen elsewhere only in saltasaurine titanosaurian sauropods. The rectangular proximal articular surface of the tibia is proportioned neither like Diplodocus nor Apatosaurus type specimens, although this region may be intraspecifically variable. The pes of Suuwassea possesses plesiomorphically elongate phalanges and a small, uncompressed ungual, unlike other flagellicaudatans except Dyslocosaurus. The localization of tooth marks on the pedal elements suggests that sauropod feet may have been singled out by scavengers, as has been noted for elephants.

Fossil footprints are important in understanding Cretaceous avian diversity because they constitute evidence of paleodiversity and paleoecology that is not always apparent from skeletal remains. Early Cretaceous bird tracks have demonstrated the existence of wading birds in East Asia, but some pedal morphotypes, such as zygodactyly, common in modern and earlier Cenozoic birds (Neornithes) were unknown in the Cretaceous. We, herein, discuss the implications of a recently reported, Early Cretaceous (120–110 million years old) trackway of a large, zygodactyl bird from China that predates skeletal evidence of this foot morphology by at least 50 million years and includes the only known fossil zygodactyl footprints. The tracks demonstrate the existence of a Cretaceous bird not currently represented in the body fossil record that occupied a roadrunner (Geococcyx)-like niche, indicating a previously unknown degree of Cretaceous avian morphological and behavioral diversity that presaged later Cenozoic patterns.

Although recent discoveries from Lower Cretaceous sediments in northeastern China have greatly improved our understanding of the initial stages of avian diversification in eastern Asia, the early evolution of Aves elsewhere on the continent remains poorly understood. In 2004, a collaborative field effort directed by personnel from the Chinese Academy of Geological Sciences and Carnegie Museum of Natural History recovered multiple partial to nearly complete avian skeletons from outcrops of the Lower Cretaceous Xiagou Formation exposed in the Changma Basin of northwestern Gansu Province, China. Here we describe a thrush-sized partial skeleton comprised of a fragmentary pelvic girdle and largely complete hind limbs. A phylogenetic analysis of 20 avian ingroup taxa and 169 anatomical characters places the specimen in Enantiornithes, and within that clade, in Euenantiornithes. When coupled with additional recent discoveries from the Changma Basin, the new skeleton improves our understanding of early avian evolution and diversification in central Asia.

Vertebrae of Suuwassea demonstrate an interesting combination of plesiomorphies and autapomorphies among known members of the Flagellicaudata. The cranial cervical vertebrae have proportions close to Diplodocus but resemble those of Apatosaurus except by having greatly reduced cranial and caudal spinozygapophyseal laminae. As a result, they have craniocaudally compressed, caudally positioned spinous processes excavated on all sides by fossae. The cranial thoracic vertebrae are again similarly proportioned as those of Diplodocus but are morphologically similar to those of Apatosaurus. The most distinguishing feature of Suuwassea caudal vertebrae are the short, amphiplatyan, distalmost ‘whiplash’ caudal vertebrae. These may be either a retention of or a reversal to the plesiomorphic sauropod condition because classic flagellicaudatan, biconvex distalmost caudals occur in the Middle Jurassic of England.

Three-dimensional specimens of the volant fossil bird Gansus yumenensis from the Early Cretaceous Xiagou Formation of northwestern China demonstrate that this taxon possesses advanced anatomical features previously known only in Late Cretaceous and Cenozoic ornithuran birds. Phylogenetic analysis recovers Gansus within the Ornithurae, making it the oldest known member of the clade. The Xiagou Formation preserves the oldest known ornithuromorph-dominated avian assemblage. The anatomy of Gansus, like that of other non-neornithean (nonmodern) ornithuran birds, indicates specialization for an amphibious life-style, supporting the hypothesis that modern birds originated in aq

A new specimen of an enantiornithean bird from the Lower Cretaceous Xiagou Formation of Gansu Province, northwestern China, consists of an articulated distal left humerus, ulna, radius, carpus, and manus. The specimen may represent a primitive enantiornithean because it lacks a longitudinal sulcus on the radius, has incompletely fused alular and major metacarpals, and possibly retains a remnant of a second phalanx on the minor digit. It differs from all other known enantiornitheans, and exhibits possible autapomorphies, including peculiar, flat humeral epicondyles, a pair of eminences on the distal minor metacarpal, and an enormous flexor tuberculum on the alular ungual. The specimen probably pertains to the same taxon as a previously described enantiornithean arm from Changma; the incompleteness of the taxon precludes erecting a new name, but it provides new information concerning enantiornithean diversity in the Early Cretaceous of central Asia.

Suuwassea emilieae is a recently described dinosaur taxon discovered in the Upper Jurassic Morrison Formation of the western United States and the only non-diplodocid flagellicaudatan (Dinosauria: Sauropoda) known from North America. It retains sauropod symplesiomorphies that are unexpected in a Late Jurassic taxon and thus sheds light on the evolutionary origins of the Flagellicaudata. Despite being comparatively small, the holotype of Suuwassea demonstrates hallmarks of relatively advanced age. A phylogenetic analysis of 30 taxa and 331 characters retains Suuwassea in a trichotomy with the Diplodocidae (Apatosaurus + (Diplodocus + Barosaurus)) and the Dicraeosauridae (Dicraeosaurus + Amargasaurus). This lack of resolution is probably due to a combination of missing data, character conflict and poor incorporation of specimens referred to diplodocid taxa that differ from their holotype specimens and species holotypes. Middle Jurassic palaeobiogeographical reconstructions conflict with the hypothetical distribution of flagellicaudatans in the Middle and Late Jurassic based on their phylogeny, implying that physical barriers, such as epeiric seas, were not responsible for limiting their initial radiation. The postparietal foramen shared by Suuwassea, Dicraeosaurus, Tornieria and Amargasaurus may correlate to preferred existence in near-shore, terrestrial environments.

Cranial elements of Suuwassea emilieae (Sauropoda: Diplodocoidea) from the Upper Jurassic Morrison Formation of Montana, U.S.A., represent one of only a few flagellicaudatan skulls known. Preserved elements include a left premaxilla, a fragment of right maxilla, a right squamosal, a right quadrate, a basicranium and skull roof lacking only the rostral end of the frontals, basipterygoid processes, and parasphenoid rostrum. Autapomorphic features of the skull include: premaxillary teeth projecting parallel to long axis of premaxilla; single optic nerve foramen; postparietal foramen present and larger than parietal foramen; supraoccipital with elongate ventral process contributing little to dorsal margin of foramen magnum; basioccipital not contributing to floor of median condylar incisure; and antotic processes with no dorsal contact with frontals. The basicranium more closely resembles that of Apatosaurus rather than Diplodocus and is also unlike the skull of Dicraeosaurus, despite its possession of a similar postparietal foramen, a feature unique among Morrison Formation sauropods. Pending reanalysis of Tornieria africana, which also possesses it, the postparietal foramen must be viewed as a symplesiomorphic retention in the Dicraeosauridae, with its loss a synapomorphy of the Diplodocidae, or at least of the North American members of the latter clade.

The Sundance Formation (Middle-Upper Jurassic) of Wyoming is well known for pterosaur footprints. Two new partial trackways from the upper Sundance Formation of the Bighorn Canyon National Recreation Area (BICA) of north-central Wyoming are enigmatic. The trackways are preserved in rippled, flaser bedded, glauconitic sand and mud. The deposits were laid down in tidal flats, behind barrier islands, along the mesotidal Sundance Sea.
    The best-preserved print of the primary trackway possesses four impressions: three shorter digits with negative rotation and an elongate, caudally-oriented mark. The primary trackway has low pace angulation. The combination of morphology and pace angulation matches neither tracks nor body fossils of horseshoe crabs, theropod dinosaurs, pterosaurs, crocodylomorphs, "lacertoids," or mammaliforms. The secondary trackway, possibly consisting of undertracks, similarly possesses elongate caudal impressions but differs from the former by possessing four subparallel, cranially-oriented digits. These prints also do not closely resemble any of the aforementioned taxa. While the secondary trackway does not lend itself to conclusion, the primary track maker could have been either an injured, pathologic pterosaur or a pterosaurian taxon otherwise unknown from the ichnological record.

A partial skeleton of a new sauropod dinosaur from the Upper Jurassic Morrison Formation (?Tithonian) of Montana is described. Suuwassea emilieae gen. et sp. nov. is diagnosed by numerous cranial, axial, and appendicular autapomorphies. The holotype consists of a premaxilla, partial maxilla, quadrate, braincase with partial skull roof, several partial and complete cranial and middle cervical, cranial dorsal, and caudal vertebrae, ribs, complete scapulocoracoid, humerus, partial tibia, complete fibula, calcaneus, and partial pes. It displays numerous synapomorphies of the Diplodocoidea, including characters of both the Diplodocidae (Apatosaurus + (Diplodocus + Barosaurus)) and Dicraeosauridae (Dicraeosaurus + Amargasaurus). Preliminary phylogenetic analysis indicates that Suuwassea is a diplodocoid more derived than rebbachisaurids but in a trichotomy with both the Diplodocidae and Dicraeosauridae. Suuwassea represents the first well-supported, North American, non-diplodocid representative of the Diplodocoidea and provides new insight into the origins of both the Diplodocidae and Dicraeosauridae.