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Sonorasaurus
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Sonorasaurus
Temporal range: Early-Late Cretaceous, Albian–Cenomanian
Speculative life restoration
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Sauropoda
Clade: †Macronaria
Family: †Brachiosauridae
Genus: †Sonorasaurus
Ratkevich, 1998
Species:
†S. thompsoni
Binomial name
†Sonorasaurus thompsoni
Ratkevich, 1998

Sonorasaurus is a genus of brachiosaurid sauropod dinosaur from the Early to Late Cretaceous (Albian to Cenomanian stages, around 112 to 93 million years ago). Its fossils have been found in southern Arizona, United States. Its name, which means "Sonora lizard", comes from the Sonora River that flows in the Sonoran Desert where its fossils were first found. The type species is S. thompsoni, described by Ratkevich in 1998.

Discovery

[edit]

Fossilized remains were discovered in November 1994 by geology student Richard Thompson in the Turney Ranch Formation in the Whetstone Mountains, located in the Chihuahua Desert region of the Sonoran Desert in southern Arizona. Thompson had investigated a previously almost unexplored region, where fossils proved to be plentiful and directly accessible on the surface. A relatively complete sauropod skeleton was weathering out on a rock wall. He informed paleontologist Ronald Paul Ratkevich of the Arizona-Sonora Desert Museum, Tucson of the find. Ratkevich assembled a team of volunteers and began securing the bones in the spring of 1995; excavations would only end in 1999. He assumed the fossils represented a sauropod but was no expert on that taxon. The curator of geology of the museum, David W. Thayer, thought it might be a therizinosaur, mistaking a tail chevron bone for the long hand claw typical of that group. In 1995, Ratkevich and Tayer first reported the find, already using the name "Sonorasaurus" but informally, so that it remained a nomen nudum.[1]

Both men now asked dinosaur expert Edwin Harris Colbert to identify the animal. Colbert, having seen only pictures, suggested it might be a member of the Hadrosauridae. Ratkevich and Thayer then visited the displays in the American Museum of Natural History, concluding their find was rather dissimilar to the hadrosaurid skeletons shown there, so that it must represent a species new to science. Ratkevich considered naming it "Chihuahuasaurus" but ultimately shied away from the comical contrast between the gigantic sauropod and the minute dog breed. In 1996, a subsequent article tried to fit the bones found, in a diagram of the hadrosaurid Kritosaurus. This attempt largely failed, with an ilium being mistaken for a shoulder blade. Again, the name "Sonorasaurus" was used but still invalidly.[2]

Sonorasaurus was finally formally described in 1998 by Ratkevich, who identified it as a brachiosaurid sauropod. Dating of the specimen found it to be the earliest known brachiosaurid to have lived in the 'middle' Cretaceous Period of North America.[3] On April 10, 2018, Sonorasaurus was declared the state dinosaur of Arizona.[4]

Description

[edit]

The holotype of Sonorasaurus, ASDM 500, is an incomplete skeleton consisting of various postcranial elements, many of which are fragmentary. A complete dorsal rib from the same horizon, ASDM 807, may also be referrable to S. thompsoni.[3] Sonorasaurus is estimated to have been about 15 meters (49 feet) long, 8 meters (26 feet) tall, and weighed 10 metric tons (11 short tons), which is about one third of the size of Brachiosaurus.[3][5]

Classification

[edit]

Ratkevich initially identified Sonorasaurus as a brachiosaurid.[3] However phylogenetic studies in the following years failed to find a consensus, with some finding it to lie within Brachiosauridae[6] and others outside of it.[7] In no analysis was the recovered phylogenetic position of Sonorasaurus strongly supported until D'Emic et al. (2016), which found Sonorasaurus to fall confidently within the Brachiosauridae. However the authors noted that additional data was still required to firmly establish its lower-level affinities.[8]

Sonorasaurus in a cladogram of Brachiosauridae after Mannion et al. (2017).[9]

Brachiosauridae

In Mannion et al. (2019) it is still described as a brachiosaurid.[10]

References

[edit]
[edit]
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Sonorasaurus

View on Grokipedia
from Grokipedia
Sonorasaurus thompsoni is a species of brachiosaurid sauropod that lived during the mid- period, approximately 98–110 million years ago, in what is now , . Known from a partial including vertebrae, limb bones, and other elements, it represents one of the geologically youngest members of the family and the first such taxon identified in during the . As a adapted to a semiarid, evergreen-dominated environment with variable , S. thompsoni likely fed on trees and other vegetation, reaching an estimated adult length of about 49–50 feet (15 meters), a height of up to 27 feet (8.2 meters) at the shoulder, and a weight of around 38,000–84,000 pounds (17,000–38,000 kilograms). The specimen (ASDM 500) was discovered in November 1994 by amateur paleontologist Richard C. Thompson in the Turney Ranch Formation of the Bisbee Group, within the Whetstone Mountains of County, marking the first relatively complete skeleton found in . Excavation efforts by the from 1995 to 1999 recovered approximately one-third of the skeleton, which includes distinctive features such as an extreme posterior ulnar condyle on the and non-pneumatic regions in the ilium, supporting its classification as a valid brachiosaurid closely related to . Bone histology reveals slow, sporadic growth with at least eight lines of arrested growth, indicating the preserved individual was a subadult at death. Named in 1998 by Ronald P. Ratkevich after the region and its discoverer (though sometimes misspelled as thompsonii in later accounts), Sonorasaurus highlights a transitional fauna in the region, bridging Jurassic brachiosaurids with later titanosauriforms before a notable 30-million-year gap in North American sauropod records. In 2018, it was officially designated as the state dinosaur of through Senate Bill 1517, proposed by 11-year-old student Jax Weldon, underscoring its significance to the state's paleontological heritage. The species' remains are housed at the , contributing to ongoing research on sauropod evolution and mid- ecosystems in the American Southwest.

Discovery and Naming

Discovery History

In 1994, student Richard Thompson discovered the first fossilized remains of Sonorasaurus thompsoni while exploring Mattie Canyon in the Turney Ranch Formation of the Whetstone Mountains, . Thompson, a geosciences student, identified exposed limb bone fragments protruding from the sandstone using a geologic map and promptly contacted paleontologist Ronald Paul Ratkevich at the . Ratkevich assembled a team of volunteers and secured the necessary permits, initiating formal excavation in January 1995 at the site on . The fieldwork, led by Ratkevich and conducted under federal regulations, continued intermittently until 1999, during which the team employed hand tools such as rock hammers, sledges, and chisels to carefully extract the fossils. Over the course of the effort, more than 50 tons of overlying rock were removed to access the specimens, yielding approximately one-third of the . The excavation faced significant logistical challenges due to the remote desert location, including extreme summer temperatures exceeding 110°F (43°C), lack of shade or nearby water sources, and the need to haul supplies over rugged terrain. The fossils' partial exposure beneath heavy rock and the inherently fragmentary condition of the further complicated the process, requiring meticulous on-site documentation and protection. Following extraction, the specimens underwent initial and reassembly in the museum's and have since been housed in the Arizona-Sonora Museum's collection in Tucson.

Etymology and Taxonomy

The genus and the species S. thompsoni were formally named and described in 1998 by paleontologist Ronald Paul Ratkevich. The description appeared in the Journal of the Arizona-Nevada Academy of Science. The generic name combines a reference to the region of , with the Greek saurus (lizard). The specific epithet thompsoni honors Richard Thompson, the discoverer of the fossils. Ratkevich initially classified Sonorasaurus thompsoni as a member of the , a family of long-necked sauropod dinosaurs known from the and periods. The specimen, cataloged as ASDM 500 and housed at the , comprises a partial representing a subadult individual, including dorsal and caudal vertebrae, ribs, a partial , and elements of the fore- and hind limbs.

Description

Known Fossil Material

The holotype specimen of Sonorasaurus thompsoni, designated ASDM 500 and housed at the , comprises a partially articulated subadult . This material includes five posterior dorsal vertebrae, dorsal rib fragments, caudal vertebrae, fragmentary left ilia, partial scapulae, a partial right , partial ulnae and radii, metacarpals, manual phalanges, tibiae, fibulae, metatarsals, and pedal phalanges. The fossils are fragmentary and incomplete, with no preserved or . An additional element, a possible dorsal cataloged as ASDM 807, has been associated with the . Post-excavation, the specimens were cleaned and stabilized by staff at the .

Size and Anatomy

Sonorasaurus thompsoni was a large sauropod , with the known subadult specimen estimated to have been approximately 15 meters long, 8 meters tall at the shoulder, and weighing nearly 17 metric tons based on limb bone scaling using the equation of Campione and Evans (2012). The overall body plan followed that of a typical long-necked, long-tailed herbivorous sauropod, featuring elevated shoulders indicative of its brachiosaurid affinities. Key anatomical features include pneumatic dorsal vertebrae with a camerate to semicamellate internal bone structure and prominent lateral fossae and foramina. Posterior dorsal vertebrae exhibit the hyposphene-hypantrum articulation system, a trait common in non-titanosaurian sauropods. The neural arches are positioned anteriorly on the elongate , situated about one-third the distance posterior to the centropostzygapophyseal laminae, differing from the more posterior placement observed in titanosaurs. Limb proportions reflect long forelimbs typical of brachiosaurids and somphospondylans, emphasizing a high-shouldered posture. The humerus measures approximately 150 cm in length, with a midshaft transverse width of 16.8 cm and anteroposterior width of 12.2 cm; its distal end features a triangular process on the ulnar condyle. Although the femur is not preserved, its midshaft circumference is estimated at 598 mm, derived from the humeral midshaft circumference of 475 mm via allometric scaling.

Classification

Phylogenetic Analysis

Sonorasaurus thompsoni was originally classified within upon its description by Ratkevich in 1998. A cladistic analysis by D'Emic et al. in 2016 confirmed and strengthened this placement, positioning Sonorasaurus firmly within and resolving prior uncertainties from earlier studies. In this analysis, based on modifications to the character matrix from D'Emic (2012), Sonorasaurus forms a with other brachiosaurids, reflecting limited resolution among certain postcranial characters. Supporting synapomorphies for its brachiosaurid affinity include a gracile relative to body size and vertebral pneumaticity that lacks the sub-centimeter-scale, pervasive camerate structure typical of somphospondylans, instead exhibiting more robust, less invasive foramina consistent with basal titanosauriforms. These features distinguish it from more derived titanosaurs while aligning it with taxa like and . As a late taxon (approximately 113–100 Ma), Sonorasaurus represents one of the geologically youngest brachiosaurids and sauropods known from , occurring just prior to the ~30-million-year "" that followed around 100 Ma and marked the decline of non-titanosaurian sauropods on the continent. Sonorasaurus thompsoni exhibits notable similarities to the brachiosaurid altithorax, particularly in its elevated shoulder region supported by elongated forelimbs and a tall neural arch positioned anteriorly on the dorsal vertebrae, traits that contribute to a giraffe-like posture characteristic of brachiosaurids. However, Sonorasaurus displays more derived features within Titanosauriformes, such as a strongly beveled medial margin on metatarsal IV and a D-shaped proximal end on metacarpal I, which distinguish it from the more plesiomorphic morphology seen in . In contrast to derived titanosaurs, Sonorasaurus lacks ventrolateral ridges on the caudal vertebral centra and shows less extensive pneumaticity in the neural arches, reflecting its basal position within the brachiosaurid rather than the highly pneumatized skeletons typical of later titanosaurs. It shares some convergent traits with titanosaurs, including a plank-like cross-section in the dorsal ribs and a posteriorly projecting ulnar condyle on the , but its overall pelvic morphology, with a transversely broad and anteroposteriorly narrow iliac peduncle, aligns more closely with primitive titanosauriform conditions. Compared to the North American somphospondylan sanjuanensis, Sonorasaurus represents an earlier, more basal form, with differences in pubis morphology—featuring a notably tall pubic peduncle similar to brancai—versus the broader, less elevated pubis in . This distinction underscores Sonorasaurus's placement as a late-surviving brachiosaurid, predating the dominance of titanosaurian forms like . Overall, Sonorasaurus fills a critical gap in the North American sauropod fossil record during the – interval, bridging the diversity of brachiosaurids and the reappearance of giant sauropods like following a roughly 30-million-year hiatus.

Paleobiology

Growth and Age

The only known specimen of Sonorasaurus thompsoni represents a subadult individual estimated to have been 11–12 years old at death, determined through the counting of lines of arrested growth (LAGs) in the . These growth marks, visible in histological thin sections, indicate annual pauses in deposition, providing a minimum age estimate after accounting for potential resorption of inner lines. Histological analysis of the long bones, following methods outlined by D’Emic et al. (2016), reveals fibrolamellar tissue consistent with rapid juvenile growth phases typical of sauropod dinosaurs, where vascular canals are densely packed and oriented longitudinally during early . However, the presence of multiple LAGs and transitions to slower-depositing lamellar in outer cortical regions suggest a deceleration in growth rate later in life, likely influenced by physiological maturity. This pattern implies that the specimen had not yet reached full adulthood and could have grown considerably larger—potentially exceeding its estimated 15-meter length and 17 metric ton mass—had it survived longer. The ontogenetic stage of immaturity is further supported by partial fusion in the vertebrae, including neurocentral sutures, and in limb bones such as the and , where epiphyseal elements remain unfused or incompletely ossified. Such features distinguish the specimen from mature sauropods, where complete fusion typically occurs, and align with histological ontogenetic stage (HOS) 11 on the scale of Klein and Sander (2008), representing an advanced subadult phase at 75–80% of maximum body size based on comparisons to related brachiosaurids like Giraffatitan.

Diet and Locomotion

Sonorasaurus thompsoni, as a member of the , was herbivorous, consuming plant matter typical of sauropods. Its elongate neck and robust cranial structure suggest adaptations for high browsing, allowing access to elevated foliage such as in its semiarid environment. Although no teeth are preserved in the , brachiosaurid dental morphology generally features spoon-shaped crowns suited for stripping leaves from branches, with low-wear patterns indicating non-abrasive processing. Locomotion in S. thompsoni was quadrupedal, supported by pillar-like limbs that distributed its body weight efficiently for a graviportal sauropod. The forelimbs were notably longer than the hindlimbs, with measurements showing a of approximately 150 cm, ulna of 99.8 cm, and of 95.2 cm, facilitating a brachiosaurid-like elevated for reaching high . These proportions imply a stable, columnar posture during movement, with no evidence of specialized defensive structures such as osteoderms. Paleobiological inferences suggest S. thompsoni may have browsed solitarily or in small herds within forested areas, consistent with the social patterns observed in other sauropods lacking herd-defending features.

Geological Setting

The fossils of Sonorasaurus thompsoni were discovered in the Turney Ranch Formation, a stratigraphic unit within the broader Bisbee Group of southeastern . The Bisbee Group represents a thick sequence of to sedimentary rocks deposited across a large extensional basin spanning what is now southern and northern . The Turney Ranch Formation, where the holotype and paratype specimens of Sonorasaurus were found, consists primarily of repetitive successions of fluvial channel sandstones and overbank mudstones and siltstones, with thicknesses ranging from 900 to 1,420 meters in the type area. These sediments indicate a dominated by river systems and associated floodplains, with occasional lacustrine influences, within a rift basin setting characterized by active faulting and during the . Stratigraphic and geochronologic data place the Turney Ranch Formation in the late () to early () stages, near the Albian-Cenomanian boundary, approximately 100.5 million years ago. K-Ar dating of authigenic from bracketing strata yields ages of 96.7 ± 2.4 Ma and 97.7 ± 2.2 Ma, supporting this temporal assignment. The Sonorasaurus locality is situated in the Whetstone Mountains of southeastern , spanning Pima and counties, an area that during deposition formed part of the northern margin of the expansive Bisbee rift basin within the proto-Chihuahua region. This tectonic context reflects the ongoing extension associated with the opening of the and early stages of North American breakup.

Associated Fauna and Environment

The paleoenvironment of Sonorasaurus thompsoni in the Turney Ranch Formation of was a low-relief inland setting characterized by semiarid conditions with highly variable, possibly seasonal precipitation. This environment featured woodlands dominated by , including Cupressinoxylon, with trees reaching heights of up to 32 m, interspersed with fluvial channels, floodplains, and ephemeral ponds that supported a mix of riverine and lacustrine features. Associated fauna from the formation includes crocodyliforms, , ray-finned fishes, and gastropods, reflecting a diverse aquatic and semi-aquatic community in the ponds and rivers. Among dinosaurs, theropods such as a carcharodontosaurine (comparable to ) and possible dromaeosaurids (cf. ) are represented by fragmentary remains, alongside ornithischians including cf. , nodosaurids, and iguanodontians. No other sauropods are known from these deposits, making Sonorasaurus the sole representative of its group in this assemblage. As a brachiosaurid sauropod, Sonorasaurus occupied the role of a primary , browsing on the foliage of tall in this woodland ecosystem and serving as a high-level in a community increasingly dominated by ornithischian herbivores. This positions it within a pre-decline sauropod , where potential with ornithischians may have contributed to ecological pressures. Sonorasaurus represents one of the geologically youngest brachiosaurid sauropods in , occurring just prior to the regional extinction of sauropods around 100 Ma and the onset of a roughly 30-million-year hiatus from the to stages.

Cultural Significance

State Dinosaur Designation

Sonorasaurus was officially designated as the state of on April 10, 2018, through Senate Bill 1517, which was signed into law by Governor . This legislation marked the first time recognized an official state , highlighting the significance of the ' fossils discovered in Cochise County in 1994. The designation was advocated by 11-year-old Jax Weldon, a student from Phoenix, who wrote a letter to Ducey proposing Sonorasaurus due to its discovery within the state and its representation of Arizona's prehistoric legacy. Weldon's initiative drew attention to the dinosaur's unique status as a brachiosaurid endemic to the region, emphasizing local paleontological heritage over more widely known species from other states. This recognition has boosted public awareness of Arizona's fossil resources, integrating Sonorasaurus into state educational curricula to foster interest in among students. Additionally, institutions like the have featured exhibits on the dinosaur, promoting its role in the state's and encouraging community engagement with local scientific discoveries.

Representation in Media

Sonorasaurus has been represented primarily through scientific anatomical illustrations and educational exhibits rather than fictional portrayals. In peer-reviewed publications, the dinosaur is depicted via skeletal reconstructions and detailed drawings of its elements, emphasizing its brachiosaurid features such as elongated forelimbs and a robust . For instance, the original includes line drawings of the bones, including vertebrae and limb elements, to illustrate its morphology relative to other sauropods. Similarly, a revision provides updated illustrations of key bones like the and metatarsals, highlighting growth patterns and phylogenetic placement within . In educational media, Sonorasaurus features prominently in museum displays at the in Tucson, where the fossils are housed and conserved. Visitors can view replicas of the embedded in and participate in interactive digs simulating its discovery, as part of exhibits like "Dinos in the Desert," which brings life to the Sonoran landscape through life-sized installations and educational programs. The museum also hosts events such as Dino Night, allowing amateur to examine real Sonorasaurus bones and learn about its role as Arizona's state . Online, a 2020 YouTube episode from the "Dinosaur of the Day" series by the I Know Dino podcast provides an accessible overview of its discovery, , and significance, reaching audiences interested in . Popular media coverage underscores Sonorasaurus's uniqueness as the only known dinosaur skeleton from southern Arizona, often portraying it as a symbol of regional prehistoric heritage. A Discover Magazine article details the 1994 discovery by amateur fossil hunter Richard Thompson, describing the partial skeleton's brachiosaur-like traits and its estimated 35-ton size in a narrative that highlights the rarity of Cretaceous sauropods in the American Southwest. Following its 2018 designation as Arizona's state dinosaur, a Tucson.com feature emphasized its forested habitat during the Middle Cretaceous and its distinction as the sole complete specimen from the area, using it to engage readers with local paleontological pride. Despite these representations, Sonorasaurus has no major appearances in films, novels, or other fiction, reflecting its niche status compared to more iconic dinosaurs like . Instead, it serves primarily in outreach, such as programming and online educational content, to promote awareness of Arizona's record.

References

  1. https://azlibrary.gov/state-dinosaur/7079
  2. http://www.desertmuseum.org/members/newsletters/news202501.pdf
  3. https://www.azleg.gov/legtext/53leg/2R/summary/S.1517GOV.DOCX.htm
  4. https://www.researchgate.net/publication/304004908_Anatomy_systematics_paleoenvironment_growth_and_age_of_the_sauropod_dinosaur_Sonorasaurus_thompsoni_from_the_Cretaceous_of_Arizona_USA
  5. https://doi.org/10.1017/jpa.2015.65
  6. https://doi.org/10.1186/1741-7007-10-60
  7. https://doi.org/10.1017/jpa.2015.67
  8. https://ucmp.berkeley.edu/diapsids/saurischia/sauropoddiet.html
  9. https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.24198
  10. https://pmc.ncbi.nlm.nih.gov/articles/PMC3045712/
  11. https://pubs.geoscienceworld.org/jpaleontol/article/90/1/102/588422/Anatomy-systematics-paleoenvironment-growth-and
  12. https://cinty-lee.squarespace.com/s/Dickinson-Tectonic-Setting-AZ-Through-Geologic-Time.pdf
  13. https://libres.uncg.edu/ir/asu/f/Heckert_A_2005_29_Distribution.pdf
  14. https://ngmdb.usgs.gov/Geolex/UnitRefs/TurneyRanchRefs_10989.html
  15. https://www.sciencedirect.com/science/article/abs/pii/S0895981101000463
  16. https://www.tandfonline.com/doi/abs/10.1080/02724634.2012.671204
  17. https://www.azcentral.com/story/news/politics/legislature/2018/02/19/arizona-has-no-state-dinosaur-so-11-year-old-proposed-one/316691002/
  18. https://www.abc15.com/news/state/arizona-dinosaur-sonorasaurus-named-official-state-dinosaur
  19. https://tucson.com/thisistucson/todo/article_3581d100-78ab-11e8-a570-3b593e5a41e9.html
  20. https://www.cambridge.org/core/journals/journal-of-paleontology/article/anatomy-systematics-paleoenvironment-growth-and-age-of-the-sauropod-dinosaur-sonorasaurus-thompsoni-from-the-cretaceous-of-arizona-usa/D9DC3AF687CAC37535D207F018E2522F
  21. https://www.desertmuseum.org/dinosaurs/
  22. https://infosci.arizona.edu/events/2024/dino-night
  23. https://www.youtube.com/watch?v=sXh7pMhFeP4
  24. https://www.discovermagazine.com/sonorasaurus-39840
  25. https://www.kold.com/story/38810043/fossils-of-new-state-dinosaur-reside-at-arizona-sonora-desert-museum/
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