| Mierasaurus | |
|---|---|
 | |
| Reconstructed skull ofMierasaurus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Turiasauria |
| Genus: | †Mierasaurus Royo-Torreset al.,2017 |
| Species: | †M. bobyoungi |
| Binomial name | |
| †Mierasaurus bobyoungi Royo-Torreset al., 2017 | |
Mierasaurus is anextinctgenus ofsauropoddinosaur from theEarly Cretaceous ofUtah,United States. The taxon was first described and named in2017 byRafael Royo-Torres and colleagues, from a mostly complete skeleton including a disarticulated partialskull andmandible,teeth, multiplevertebrae from along the length of the body, bothscapulae,radius andulna bones, a leftmanus, a completepelvis, bothfemora and the entire left hindlimb. Additionally, they referred a lower jaw and femur from juvenile individuals, which were found nearby, to the genus. Collectively,Mierasaurus is among the most completely knownNorth American sauropods. The genus name honoursBernardo de Miera y Pacheco, the first European scientist to enter what is now Utah. Thetype species forMierasaurus isMierasaurus bobyoungi, named after Robert Glen Young, apaleontologist who researched the Early Cretaceous of Utah.[1]
Along with its closest relativeMoabosaurus, also from the Early Cretaceous of Utah,Mierasaurus is among the last-surviving members of theTuriasauria, an otherwiseJurassic andEuropean group which can be distinguished by heart-shaped teeth, slenderhumeri, and the presence of an extra depression on the surface of theulnae, among other characteristics.Mierasaurus differs fromMoabosaurus in characteristics such as lacking vertical ridges on its teeth, having relatively smooth bottom surfaces on itscervical vertebrae, havingcervical ribs that do not prominently split into two at their tips, and lacking a bulge on the side of thefemur. It is probable that the ancestors ofMierasaurus andMoabosaurus migrated, shortly before the emergence of the former, to western North America from surviving populations of turiasaurs in either Europe or eastern North America.[1]
Since 2005, theUtah Geological Survey has conducted annual excavations for several weeks at a time at a site known as Doelling's Bowl, located on lands managed by the USBureau of Land Management in east-centralUtah, United States. The site is catalogued as UMNH VP.LOC.1208 at theNatural History Museum of Utah (UMNH), and as Utah Loc. 42Gr0300v at the Utah Geological Survey. Its exact location is not publicly available. Doelling's Bowl is known for a largebone bed, covering some 5,000 m2 (54,000 sq ft) and measuring approximately 1 metre (3 ft 3 in) thick. The rocks appear to be locally flat, but form broadswales over the whole site. Out of this area, 140 m2 (1,500 sq ft) has been excavated, producing 1500 vertebrate bones. Rocks in the Doelling's Bowl bonebed mainly consist of green-grey, sandymudstone, but also containsilcrete, casts ofsilificied plant roots, andchert pebbles. It belongs to theCretaceous-agedYellow Cat Member of theCedar Mountain Formation.[1]
In 2010, a skeleton of a subadultsauropod dinosaur was discovered in anarroyo within Gary's Island, a region at the western end of the bonebed named after its discoverer Gary Hunt. Only part of the specimen - a partial left forelimb (scapula,sternal plates,ulna,radius, and hand), a complete left hindlimb, and ten caudal (tail) vertebrae - was articulated. The feet of these limbs are buried in the sediment at a level deeper than the other remains, which suggests that the specimen died after being mired in soft mud. Other remains are scattered over an area of 10 m2 (110 sq ft), including a partial skull and lower jaw; three teeth; theatlas and eight othercervical vertebrae, along with elevencervical ribs; elevendorsal vertebrae, six dorsalribs, and sixsacral ribs; five other caudal vertebrae with twohaemal arches; a rightfemur; andpelvic girdle elements, including anilium,ischium, and twopubes. Although the remaining elements have been displaced anderoded by the arroyo, the specimen is still the most complete Cretaceous sauropod from North America. The specimen is catalogued as UMNH.VP.26004, but individual bones are catalogued with the prefix DBGI (Doelling's Bowl site).[1]
Additional specimens - a lower jaw from a juvenile specimen, UMNH.VP.26010, and a juvenile femur, UMNH.VP.26011 - were also referred to the sametaxon. In 2017, all of these specimens were described byRafael Royo-Torres, Paul Upchurch,James Kirkland, Donald DeBlieux,John Foster,Alberto Cobos, andLuis Alcalá as part of aresearch paper published inScientific Reports. They named a newgenus for the specimens,Mierasaurus; the name honorsBernardo de Miera y Pacheco, a Spanishcartographer who was "the first European scientist to enter what is now Utah" in theDomínguez–Escalante expedition of 1776. They also named thetype and only speciesM. bobyoungi after Robert ("Bob") Young, in order to acknowledge "the importance of [his] underappreciated research" the geology of the Early Cretaceous of Utah.[1]
The skull ofMierasaurus is overall similar to those of otherturiasaurs, which have rounded snouts with retractednostril openings.[1][2] The lower jaw ofMierasaurus becomes increasingly taller towards the front, which is likeCamarasaurus but unlike thebasal sauropodJobaria. A sharp ridge extends across the bottom edge of the front of the lower jaw, seen in bothdicraeosaurids anddiplodocids, and to some extentCamarasaurus.[3] The lower jaw bears thirteen teeth; the ones at the front are spatula-shaped while the ones at the rear are heart-shaped, which is a distinguishing characteristic of turiasaurs.[2][4][5] Compared toCamarasaurus, the teeth at the front of the jaw are more pointed and have taller crowns. There are no well-developed vertical ridges on the outer surface of the teeth, in contrast to its closest relativeMoabosaurus.[1]
InMierasaurus, theskull roof is overall flattened, lacking any convexities. Unlike morederived sauropods but also seen inTuriasaurus,[2] thefrontal bone ofMierasaurus participates in the margin of thesupratemporal fenestra. Unique characteristics that differentiateMierasaurus from other turiasaurs can be found in itsbraincase: a ridge known as theotosphenoidal ridge extends from the front of theparoccipital process—a bony spur to which neck muscles attach—and runs along its inner edge; and theoccipital condyle, which articulates with the atlas, has a pair of rounded ridges on the sides of its articular surfaces (whichMoabosaurus lacks). LikeTuriasaurus,[2]Mierasaurus has a pair offoramina at the top end of the transversenuchal crest on thesupraoccipital bone. Like inMoabosaurus,[6] the downward projections known as the basal tubera on thebasioccipital bone are L-shaped when viewed from the bottom.[1]
Mierasaurus can be excluded from theTitanosauriformes due to the solid internal structure of its vertebrae and ribs, which indicates they lackair-filled cavities.[3][7][8] Uniquely, on the bottom of the internal cavity of the atlantal intercentrum (term used for the unfused lower half ofvertebral centrum of the atlas) inMierasaurus, there is a pair of depressions that articulates with the odontoid process of theaxis (second cervical).Moabosaurus lacks these depressions, and the cervical rib articulation (parapophysis) is more robust. As seen inKaatedocus,[9] thelaminae extending from theneural spines to theprezygapophysis of the cervical vertebrae are well-developed and cap depressions underneath. The cervical neural spines are not strongly split (bifurcated) inMierasaurus, less so than those ofTuriasaurus. Also unlikeMoabosaurus, there are no keels or hollows on the bottom of the cervical centra. None of thecervical ribs are visibly bifurcated either, unlike bothTuriasaurus andMoabosaurus,[4][6] although there are small upward projections at the tips of the first few cervical ribs which may represent incipient or rudimentary bifurcation. There is a ridge on the side of thetubercle of each rib, which is a unique diagnostic feature ofMierasaurus.[1]
The rear dorsal vertebrae inMierasaurus are concave on both ends (amphicoelous), whereas the opposite (opisthicoelous) is true forMierasaurus,Camarasaurus, and titanosauriforms.[3][6][10]Mierasaurus does not have the prespinal laminae (ridges on the front of the spine) present in the rear dorsal vertebrae ofMoabosaurus, and the articular process known as thehyposphene is triangular inMierasaurus while it is rectangular inMoabosaurus. However,Mierasaurus shares withMoabosaurus dorsal neural spines with spinodiapophyseal laminae that bifurcate towards the top of each spine, with one branch extending forwards and one backwards. The caudal (tail) vertebrae are concave in front and convex behind (procoelous), as inTuriasaurus,[4]Moabosaurus,[6]Losillasaurus, and theTitanosauria.[3] Also like titanosauriforms, the neural arches are located on the front halves of each centrum, and the haemal canals are long compared to the haemal arches that form them, at 41% of the bones' length. There are characteristic depressions at the lower outer sides of the haemal arches.[1]
The forelimbs ofMierasaurus exhibit many shared (synapomorphic) turiasaurian traits. In theshoulder girdle ofMierasaurus, the articulation with the scapula on thecoracoid is about half the bone's length. Thehumerus is slender, with an HRI (humerus robustness index) of only 0.27; the maximum width of the bottom end, similarly, measures only 40% of the bone's total length. A T-shaped/Y-shaped profile is present at the top end of the ulna due to the strong development of the rear-projecting process. Besides the depression for articulating with the radius, there is a second deep depression likeMoabosaurus,Turiasaurus,Losillasaurus,Zby, andDystrophaeus. The top end of the radius is large, being at least half the length, of the bottom end.[1]
Unlike the rounded margins seen titanosauriforms,[11] the front margin of the ilium is triangular inMierasaurus. Uniquely,Moabosaurus also has a very short ischium compared to its pubis; the former measures only 75% of the length of the latter, otherwise seen only amongtitanosaurs.[11] However, the bulge on the side of the femur is missing, which differentiatesMierasaurus from both titanosauriforms andMoabosaurus (which has a slight bulge[6]). Thefourth trochanter of the femur is only located 40% of the way down from the top of the bone, compared to halfway for most other sauropods, and thecondyles at the bottom end are, unusually, roughly the same size. Synapomorphic of turiasaurs, thecnemial crest of thetibia points forwards. The inner surface of theastragalus in the ankle narrows to form a triangular process, likeTuriasaurus. On the five-digited foot, the number ofphalanges is 2, 3, 3, 2, and possibly 0. Unlike most other sauropods, with the exception ofVulcanodon[12] andSanpasaurus,[13] the claws on the second and third digits are compressed vertically, not horizontally.[1]
Based on insertingMierasaurus into two prior analyses, one by José Carbadillo and Martin Sander in 2013[14] and another by Philip Mannion and colleagues in 2017,[15] Royo-Torres and colleagues identified a number of synapomorphic traits that allowMierasaurus to be placed among the Turiasauria. Asides from limb characteristics noted above, other synapomorphies include the slightly concave profile of the rear surface of the basal tubera; the heart-shaped teeth; the height of the dorsal neural arches below the level of thearticular processes known as postzygapophyses being at least equal to that of their corresponding centra; the opisthocoelous condition of the rear dorsals; the slightly procoelous front caudals; the presence of depressions known as pleurocoels in the front caudals; the neural spines of the front caudals being less than 1.2 times the height of their corresponding centra; and the centra of the middle caudals being at least as wide as they are long. Although not an explicit synapomorphy, the secondary depression on the ulna also unitesMierasaurus with other turiasaurs.[1]
Within the Turiasauria, Royo-Torres and colleagues considered the likewise-CretaceousMoabosaurus to be the closest relative ofMierasaurus. They share the L-shaped profile of the basal tubera, and the bifurcating spinodiapophyseal laminae of the dorsal neural spines. However, they differ from each other by twelve characteristics.Moabosaurus lacks rounded ridges on its occipital condyle; has vertical ridges on its teeth; has a robust parapophysis; lacks depressions on its atlas intercentrum; has hollows and keels on the bottom of its cervical centra; has convex as opposed to straight centroprezygapophyseal laminae in its front cervicals; has bifurcating cervical ribs; lacks ridges or bulges accompanying the tubercles of its cervical ribs; has prespinal laminae in its middle and rear dorsals; has rectangular as opposed to triangular hyposphenes in its rear dorsals; has uniformly opisthocoelous dorsals; and has a bulge on its femur.[6] The lack of prominent bifurcation in the cervical ribs also differentiatesMierasaurus fromTuriasaurus and potentially other turiasaurs.[1]
Thephylogenetic trees recovered by Royo-Torres and colleagues from the two different analyses agreed upon the position of turiasaurs, placing them as a unified group containingMierasaurus,Moabosaurus,Turiasaurus,Losillasasurus, andZby outside of theNeosauropoda (thus excluding them from both theDiplodocoidea and theMacronaria). Below, the tree based on the analysis of Mannion and colleagues[15] is reproduced; it yielded well-resolved relationships within the Turiasauria, as opposed to the tree based on the analysis of Carbadillo and Sander. The continent-level geographic ranges of various taxa are included in the tree.[1]
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The Doelling's Bowl locality is part of the lower segment of theYellow Cat Member of theCedar Mountain Formation, as indicated by the presence of amarker horizon ofcalcrete. Traditionally, based onbiostratigraphic correlation withostracods andcharophytes, the lower Yellow Cat Member has been considered to belong to theAptian epoch of the Cretaceous period, at 124.2 ± 2.6 Ma (million years) old.[16][17][18][19][20] However, based onuranium-lead dating of detritalzircon, this age has been more recently revised to less than ~139.7 ± 2.2 Ma, which is part of theValanginian epoch. Detrital zircon dating provides only a maximum age, since the time between crystallization and deposition is unknown. The same dating measures produced two maximum age estimates of ~136.4 ± 1.1 Ma and ~132 Ma for the age of the upper segment.[21][22] An older age of up to 142 Ma remains possible, as does a younger age around 124 Ma, which would be congruent with the ostracod and charophyte data.[1]
Doelling's Bowl is the origin of thetype specimen of thedromaeosauridtheropodYurgovuchia.[23] Additionally, in a layer about 25 centimetres (9.8 in) below the preserved feet of the type specimen ofMierasaurus, a large specimen of theiguanodontianornithopodIguanacolossus was uncovered along with some smaller individuals.[24] Indeterminate bones from small ornithopods, as well as part of the tail of a large ornithopod, were mixed in with the remains ofMierasaurus as well. A largeallosauroid theropod is represented by teeth, and a new species ofpolacanthineankylosaurian has also been found. Non-dinosaurs are represented by skull fragments and teeth from possiblygoniopholididaecrocodyliforms, as well as shell fragments of a turtle similar toNaomichelys.[21][25] The area was a waterloggedbog-like environment, judging by theplastic deformation of the bones, the presence of horizontal root systems, and the better-preserved condition of the top surfaces of the bones (which suggests that invertebrates grazed on the bottom surfaces).[1]
Asides fromMierasaurus andMoabosaurus, turiasaurs are exclusivelyJurassic in age; turiasaurs had previously been considered to have gone extinct at the Jurassic-Cretaceous boundary.[26][27] However, turiasaurs are not known from the Jurassic-aged (more specifically, dating to theTithonian epoch) deposits of theMorrison Formation, or from any other equivalent deposits in western (Laramidian) North America. This means that they may have reached North America at some point after the Tithonian, although the possibility that they were present but have not been found cannot be excluded. It is also possible that they were present in eastern (Appalachian) North America in the Late Jurassic, and spread to Laramidian North America subsequently. However, the presence ofMierasaurus in Valanginian deposits correlates with a substantial drop in sea level that occurred during the epoch, which may have formed aland bridge[28][29][30] and allowed turiasaurs - and potentially other groups, such asharamiyidan mammals - to spread from Europe to North America.[1]
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