Astrodon is agenus of large herbivoroussauropoddinosaur, measuring 20 m (66 ft) in length, 9 m (30 ft) in height and 20 metric tons (22 short tons) in body mass.^[2][3][4] It lived in what is now the easternUnited States during theEarly Cretaceous period, and fossils have been found in theArundel Formation, which has been dated throughpalynomorphs to theAlbian about 112 to 110 million years ago.^[5]

Astrodon Temporal range:Early Cretaceous,112 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Life restoration
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	†Sauropodomorpha
Clade:	†Sauropoda
Clade:	†Macronaria
Clade:	†Titanosauriformes
Genus:	†Astrodon Leidy, 1865[1]
Species:	†A. johnstoni
Binomial name
†Astrodon johnstoni Leidy, 1865
Synonyms
Pleurocoelus nanusMarsh, 1888 Pleurocoelus altusMarsh, 1888

Two dinosaur teeth were received in late November 1858 by chemistPhilip Thomas Tyson from John D. Latchford. They had been found in Latchford's open iron ore pit in the Arundel Formation atSwampoodle nearMuirkirk inPrince George's County,Maryland. Tyson let them be studied by thedentistChristopher Johnston, professor at theBaltimore Dental College, who cut one tooth in half and thereby discovered a characteristic star-formed cross-section. Johnston namedAstrodon in 1859. However, he did not attach aspecific epithet, soJoseph Leidy is credited with namingAstrodon johnstoni (thetype species) in 1865, with asholotype specimenYPM 798. If Johnston had attached a specific epithet, it would have been the second dinosaur species identified in the United States. Johnston incorrectly stated that the site of the discovery had been an iron mine near the town ofBladensburg.

In 1888,O. C. Marsh named some bones from the Arundel found near Muirkirk, MarylandPleurocoelus nanus andP. altus. However, in 1903John Bell Hatcher, taking into account the similarity of the teeth ofAstrodon johnstoni and the teeth from the Arundel Formation referred toPleurocoelus nanus, argued that the latter represents the same species as the former and that the nameAstrodon therefore had priority.^[6] In 1921Charles W. Gilmore agreed that the genusPleurocoelus is a junior synonym ofAstrodon, but at the same time keptP. nanus andP. altus as separate species ofAstrodon. Other species at one time assigned to the genus includeAstrodon valdensis^[7] andAstrodon pussilus.^[8] In 1962 R. F. Kingham assignedBrachiosaurus, including all its species, toAstrodon as asubgenus.^[9] Carpenter and Tidwell (2005) accepted Hatcher's argument that there is only one species of sauropod dinosaur known from the Arundel Formation and thatAstrodon johnstoni is the senior synonym ofPleurocoelus nanus (as well asP. altus) in the first in-depth description of this dinosaur. The majority of the bones ofAstrodon are of juveniles, and Carpenter and Tidwell considered the two species named by Marsh,P. nanus andP. altus, as different growth stages ofAstrodon johnstoni.^[5]

However, other authors did not find the argument in favor of the synonymization ofAstrodon andPleurocoelus so convincing. According to Peter Rose (2007) it has not been demonstrated that either the teeth ofAstrodon johnstoni or those attributed toPleurocoelus are "morphologically diagnostic among titanosauriforms", which limits their utility when it comes to distinguishing them from the teeth of other taxa. The type series ofPleurocoelus nanus andP. altus (four vertebrae and two hindlimbs bones, respectively) cannot be directly compared to the teeth from the type series ofAstrodon, so any comparison has to be conducted based on the referred specimens ofPleurocoelus. These, however, are all isolated bones from the Arundel Formation, which themselves were referred toPleurocoelus only based "on proximity of the localities and the size of the bones". Rose concludes that, asAstrodon is not based on the diagnostic material, "new discoveries should not be aligned with that genus" and that "the argument to synonymize the two taxa,Astrodon andPleurocoelus, seems unfounded". The type material ofPleurocoelus may not be diagnostic as well, according to the author.^[10]

A similar argument was made by Michael D. d'Emic (2013). The author did not find any diagnostic features of the type material ofAstrodon johnstoni,Pleurocoelus nanus andP. altus and considered the three taxa to benomina dubia; according to the author there is no direct evidence that any sauropod bones from the Arundel Formation other than their type series can be referred to these taxa. D'Emic also stated that the exact provenance of the bones from the type series ofPleurocoelus nanus is uncertain and thus "these bones could represent achimera of individuals or taxa". The author also commented on the diagnosis ofAstrodon johnstoni proposed by Carpenter and Tidwell (which was based on all of the sauropod material from the Arundel Formation, not only on the teeth from the type series); he claimed that most of the supposedautapomorphies of this taxon "are indistinguishable compared to other sauropods such asCamarasaurus (...) and/or are related to the juvenile nature of the material".^[11]

TheArundel Formation of Maryland has been dated throughpalynomorphs to theAlbian stage of the Early Cretaceous period, about 112 million years ago.^[5] This formation is part of the Potomac Group that includes formations not only in Maryland but also in parts of Washington D. C., Delaware and Virginia. The Arundel Formation was deposited on the edge of the expanding Atlantic Ocean basin. The region preserved in this formation was a broad, and generally flat plain with several streams running across it, probably similar to the modern day coastal regions of Alabama, Georgia, Louisiana and Mississippi.

Fossil material assigned toAstrodon has also been found in two Oklahoma localities of theAntlers Formation, which stretches from southwest Arkansas through southeastern Oklahoma and into northeastern Texas.^[12][13] This geological formation has not been dated radiometrically. Scientists have used biostratigraphic data and the fact that it shares several of the same genera as the Trinity Group of Texas, to surmise that this formation was laid down during the Albian stage of the Early Cretaceous Period, approximately 110 mya.^[14] ThePapo Seco Formation of theLusitanian Basin in Portugal also provided fossils referred toAstrodon.

In what is now Maryland,Astrodon shared itspaleoenvironment with dinosaurs such ascoelurosaurians, theankylosaurianPriconodon crassus, thenodosauridPropanoplosaurus marylandicus,^[15] a possiblebasalceratopsian, and potentially theornithopodTenontosaurus. The fossil evidence points to the presence of the poorly known theropods "Dryptosaurus"medius,Capitalsaurus potens, "Coelurus"gracilis, and the well known largetheropodAcrocanthosaurus atokensis, which likely was theapex predator in this region.^[16][17] Othervertebrates are not as well known from the formation, but include freshwatersharks,lungfish, at least three genera ofturtles includingGlyptops caelatus, and thecrocodyliformGoniopholis affinis. Evidence has shown that themultituberculate early mammalArgillomys marylandensis was also present.^[18] Trace fossils included theropod tracks known asEubrontes and others assigned to the ichnogenusPteraichinus belonging to a pterosaur, which demonstrate that these animals were present in abundance.^[19] The plant life known from this area included trees preserved assilicified wood, cycads likeDioonites,Ginkgo, the ground plantSelaginella and the giant redwood coniferSequoia.

In prehistoric Oklahoma,Astrodon lived alongside other dinosaurs, such as thesauropodSauroposeidon proteles, thedromaeosauridDeinonychus antirrhopus and thecarnosaurAcrocanthosaurus atokensis.^[20][21] The most common dinosaur in the paleoenvironment preserved in theAntlers Formation is theornithopodTenontosaurus. Other vertebrates present at the time ofAstrodon included theamphibianAlbanerpeton arthridion, the reptilesAtokasaurus metarsiodon andPtilotodon wilsoni, thecrurotarsan reptileBernissartia, thecartilaginous fishHybodus buderi andLissodus anitae, theray-finned fishGyronchus dumblei, the crocodilianGoniopholis, and the turtlesGlyptops andNaomichelys.^[22][23] Possible indeterminatebird remains are also known from this formation. The fossil evidence suggests that thegarLepisosteus was the most common vertebrate in this region. The early mammals known from this region includeAtokatherium boreni andParacimexomys crossi.^[24]

In 1998,Astrodon johnstoni was named thestate dinosaur ofMaryland.^[25]Astrodon was also previously the state dinosaur of Texas, but has since been replaced withSauroposeidon.^[26]

A life-sizedAstrodon model (featuring a wound on its left rear leg) is displayed in theTerror of the South exhibit on the third floor of theNorth Carolina Museum of Natural Sciences.^[27][28] AnotherAstrodon model is also on display at theMaryland Science Center in Baltimore, Maryland.^[29]

• List of Maryland state symbols

• Johnston, C., 1859, "Note on odontography," Amer. Journal Dental Sci. 9:337–343.
• Kranz, P. M. 1996, "Notes on the Sedimentary Iron Ores of Maryland and their Dinosaurian Fauna", in Maryland Geological Survey Special Publication No. 3, pp. 87–115.
• Leidy, J 1865, Memoir on the extinct reptiles of the Cretaceous formations of the United States. Smithson. Contrib. Knowl. XIV: atr. VI: 1–135.
• Lucas, F. A. 1904, "Paleontological notes," Science (n.s.) XIX (480): 436–437.
• Lull, R. S. 1911, "The Reptillian Fauna of the Arundel Formation" and "Systematic Paleontology of the Lower Cretaceous Deposits of Maryland – Dinosauria", Lower Cretaceous: Maryland Geological Survey Systematic Reports, pp. 173–178, 183–211.
• Marsh, O. C. 1888, "Notice of a New Genus of Sauropoda and Other New Dinosaurs from the Potomac Group," American Journal of Science, 3rd Series, Vol. XXXV, pp. 89–94.

• Maryland State Archives