Eodromaeus

Eodromaeus; Temporal range:Late Triassic(Carnian); ~231.4–229Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
	Restored skeletal mount in Japan
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Genus:	†Eodromaeus; Martinezet al.,2011
Species:	†E. murphi
Binomial name
	†Eodromaeus murphi; Martinezet al.,2011

Eodromaeus(meaning "dawn runner" ) is anextinct genusof probablebasal theropod dinosaursfrom theLate TriassicofArgentina.Like many other of the earliest-known dinosaurs, it hails from theCarnian-age (~230 Ma)Ischigualasto Formation,within theIschigualasto-Villa Unión Basinof northwesternArgentina.Upon its discovery, it was argued to be one of the oldest true theropods, supplanting its contemporaryEoraptor,which was reinterpreted as a basal sauropodomorph.^[1]

Discovery

Skeletal diagram ofEodromaeus murphi,known remains depicted in white and light grey, unknown in dark grey.

Fossils fromEodromaeuswere first discovered in 1996 by Argentinean paleontologist Ricardo N. Martinez andEarthwatchvolunteer Jim Murphy, and it was first believed that the fossils were a new species ofEoraptor.However, as the researchers started to take a closer look at the fossils, they found that it had many skeletal features which were absent inEoraptor,and they understood that it came from a new genus.^[1]^[2]

Eodromaeusis known from six specimens found at various levels of theIschigualasto FormationofSan Juan Province,Argentina.By far the most complete specimen is theholotype,PVSJ 560, a crushed but nearly complete articulatedskeletonrecovered from the base of the Valle de la Luna Member. The Valle de la Luna Member is the thick third-oldest member of the formation. It overlies the thinner Cancha de Bochas and La Peña Members, the second-oldest and oldest members of the formation, respectively. Other referred specimens include PVSJ 561, 562, and 563 from the Valle de la Luna Member, PVSJ 534 from the La Peña Member, and PVSJ 877 from the Cancha de Bochas Member. EveryEodromaeusspecimen lived within the humidScaphonyx-Exaeretodon-Herrerasaurusbiozone, which occupies the first half of the Ischigualasto Formation. Abentonitedeposit in the La Peña Member has been dated to 231 ± 0.4Ma,indicating that the Ischigualasto Formation corresponds to the later part of theCarnianstage, the first stage in theLate Triassic.^[1]^[3]

Eodromaeuswas named byRicardo N. Martínez,Paul C. Sereno,Oscar A. Alcober, Carina E. Colombi, Paul R. Renne, Isabel P. Montañez and Brian S. Currie in2011and thetype speciesisEodromaeus murphi.Thegeneric nameis derived from theGreekwordsEos( "Dawn", "Early" ) andDromaeus( "Runner" ). Thespecific namehonors Jim Murphy, who used to work the area nearby where the fossils were found.^[1]

Description

Size comparison betweenEodromaeusand a human.

Eodromaeuswas a relatively small dinosaur, like mostCarniandinosaurs apart fromherrerasaurids.^[1]A press release estimated that its total length reach about 1.2 meters (3.9 feet) from nose to tail, with a weight of about 4.5 to 6.8 kg (9.9 to 15.0 lb).^[2]Bensonet al.(2018) estimated thatEodromaeushad a mass of 7.1 kg (16 lb).^[4]The animal was lightly built and had long hindlimbs, suggesting that it was well-built for running even by the standards of most early dinosaurs. Kubo & Kubo (2012) found that, among 23 sampled Triassicarchosaurs,onlyMarasuchusexceededEodromaeusin adaptations forcursoriality.^[5]Paul Sereno has estimated that it could run about 32 km per hour (20 miles per hour).^[2]Eodromaeushas been cited by Sereno as resembling a predictedcommon ancestorto all dinosaurs, the "Eve" of dinosaurs.^[6]

Life restoration

The skull was low and rectangular, about 12 cm (4.7 in) in length. It had an expansiveantorbital fenestraedged from below by a sharp ridge. The antorbital fenestra was preceded a promaxillary fenestra, an additional hole in the skull characteristic oftheropodsandHerrerasaurus.Thebraincasehad deep depressions on its side like many theropods, and connected to thepalatevia thin and blade-likebasipterygoid processes.The cranium had a relatively low number of knife-shaped teeth (fifteen in total) which were longest in the front half of themaxilla.There was also a row of tiny teeth on thepterygoid boneof the palate. Palatal teeth are very rare in dinosaurs (and true archosaurs in general), though they have also been found inEoraptor.The lower jaw was slender, with widely spaced teeth extending to the tip of the snout as in early theropods.^[1]

Thecervicals(neck vertebrae) were significantly more elongated than those ofEoraptor,and those near the shoulders had large pits which would have housed air sacs. The shorter but more numerousdorsals(trunk vertebrae) were reinforced byhyposphene-hypantrum articulations,while the caudals (tail vertebrae) were connected by elongatedprezygapophyses,as seen in other theropods. The shoulder girdle had a deepcoracoidconnecting to a long and rod-shapedscapula.The large forelimbs had closely-appressed forearm bones and strongly-developed wrist and elbow joints. Themanus(hand) had five fingers, with the second and third elongating towards their tips. In contrast, the fourth and fifth fingers were very thin and short. Thepelvis(hip) had a talliliumwhich connected to threesacrals(hip vertebrae). It also included an elongatedischiumandpubis,the latter of which tapers before expanding into a small pubic boot similar to that of other theropods. The hindlimb possessed several muscle scars characteristic of theropods, such as a depression on thefemurfor extensor muscles, and a flange fortibialligaments on thefibula.^[1]

Classification

Reconstructed skeletal mount

When originally described in 2011,Eodromaeuswas regarded as the earliest member ofTheropoda.Theropods include the vast majority of carnivorous dinosaurs, such asTyrannosaurus,Allosaurus,andVelociraptor,as well as the only living dinosaurs,birds.Eodromaeuswas considered one of the mostbasaltheropod and placed outside of the groupNeotheropoda,which contains coelophysoids and post-Triassic theropods. The only putative theropods found to be more basal were theherrerasaurids,a group of relatively large early carnivorous dinosaurs with an unstable relationship to other dinosaurs.^[1]The discovery ofEodromaeusalso lead to scrutiny regarding the contemporary early dinosaurEoraptor.Eoraptorhad previously been regarded as a theropod, but Martinezet al.(2011) instead concluded that it likely represented one of the most basalsauropodomorphs,the group that includes animals likeApatosaurus.^[1]There is still a small amount of debate over the position ofEoraptor,with a few studies reclaiming it as a theropod alongsideEodromaeus.^[7]^[8]Most studies since 2011 have continued to placeEodromaeusas a basal theropod akin to the results of Martinezet al.There are some disagreements; many subsequent studies considerEodromaeusto liecrownward(i.e. more 'advanced') relative toTawaand/orChindesaurus.^[9]^[10]^[11]^[12]A few also reject the notion of herrerasaurids being theropods.^[8]

An alternative hypothesis, first presented by Cabreiraet al.(2016),argues thatEodromaeuswas not a theropod, but rather a more basalsaurischian.^[13]This means that its lineage would have evolved prior to the split between sauropodomorphs and true theropods. Other putative theropods such asTawa,Chindesaurus,Daemonosaurus,and herrerasaurids fall even furtherstemwardsthanEodromaeusaccording to this study and derived works.^[14]^[15]^[16]A few other unusual placements have been found in studies relating to the controversialOrnithoscelidahypothesis of Baronet al.(2017).^[8]Although Baronet al.'s original paper did placeEodromaeusas a theropod, a critique and recoding by Langeret al.(2017) recovered Saurischia and movedEodromaeusout of Theropoda.^[17]^[18]A subsequent response to that critique, by Baronet al.(2017), placedEodromaeusas a basal herrerasaurian and simultaneously shifted the entire family Herrerasauridae outside of Dinosauria.^[19]