Mierasaurus
| Mierasaurus Temporal range:Early Cretaceous(Valanginian?),
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|---|---|
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| Reconstructed skull ofMierasaurus | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Turiasauria |
| Genus: | †Mierasaurus Royo-Torreset al.,2017 |
| Species: | †M. bobyoungi
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| Binomial name | |
| †Mierasaurus bobyoungi Royo-Torreset al.,2017
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Mierasaurusis anextinctgenusofsauropoddinosaurfrom theEarly CretaceousofUtah,United States.The taxon was first described and named in2017byRafael Royo-Torresand colleagues, from a mostly complete skeleton including a disarticulated partialskullandmandible,teeth,multiplevertebraefrom along the length of the body, bothscapulae,radiusandulnabones, a leftmanus,a completepelvis,bothfemoraand the entire left hindlimb. Additionally, they referred a lower jaw and femur from juvenile individuals, which were found nearby, to the genus. Collectively,Mierasaurusis among the most completely knownNorth Americansauropods. The genus name honoursBernardo de Miera y Pacheco,the first European scientist to enter what is now Utah. Thetype speciesforMierasaurusisMierasaurus bobyoungi,named after Robert Glen Young, apaleontologistwho researched the Early Cretaceous of Utah.[1]
Along with its closest relativeMoabosaurus,also from the Early Cretaceous of Utah,Mierasaurusis among the last-surviving members of theTuriasauria,an otherwiseJurassicandEuropeangroup which can be distinguished by heart-shaped teeth, slenderhumeri,and the presence of an extra depression on the surface of theulnae,among other characteristics.Mierasaurusdiffers fromMoabosaurusin characteristics such as lacking vertical ridges on its teeth, having relatively smooth bottom surfaces on itscervical vertebrae,havingcervical ribsthat do not prominently split into two at their tips, and lacking a bulge on the side of thefemur.It is probable that the ancestors ofMierasaurusandMoabosaurusmigrated, shortly before the emergence of the former, to western North America from surviving populations of turiasaurs in either Europe or eastern North America.[1]
Discovery and naming
[edit]
Since 2005, theUtah Geological Surveyhas 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 Managementin 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 broadswalesover 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 ofsilificiedplant roots, andchertpebbles. It belongs to theCretaceous-agedYellow Cat Memberof theCedar Mountain Formation.[1]
In 2010, a skeleton of a subadultsauropoddinosaur was discovered in anarroyowithin 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; theatlasand eight othercervical vertebrae,along with elevencervical ribs;elevendorsal vertebrae,six dorsalribs,and sixsacral ribs;five other caudal vertebrae with twohaemal arches;a rightfemur;andpelvic girdleelements, including anilium,ischium,and twopubes.Although the remaining elements have been displaced anderodedby 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 Alcaláas part of aresearch paperpublished inScientific Reports.They named a newgenusfor the specimens,Mierasaurus;the name honorsBernardo de Miera y Pacheco,a Spanishcartographerwho was "the first European scientist to enter what is now Utah" in theDomínguez–Escalante expeditionof 1776. They also named thetype and only speciesM. bobyoungiafter Robert ( "Bob" ) Young, in order to acknowledge "the importance of [his] underappreciated research" the geology of the Early Cretaceous of Utah.[1]
Description
[edit]Skull
[edit]
The skull ofMierasaurusis overall similar to those of otherturiasaurs,which have rounded snouts with retractednostrilopenings.[1][2]The lower jaw ofMierasaurusbecomes increasingly taller towards the front, which is likeCamarasaurusbut unlike thebasalsauropodJobaria.A sharp ridge extends across the bottom edge of the front of the lower jaw, seen in bothdicraeosauridsanddiplodocids,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 roofis overall flattened, lacking any convexities. Unlike morederivedsauropods but also seen inTuriasaurus,[2]thefrontal boneofMierasaurusparticipates in the margin of thesupratemporal fenestra.Unique characteristics that differentiateMierasaurusfrom other turiasaurs can be found in itsbraincase:a ridge known as theotosphenoidalridge extends from the front of theparoccipitalprocess—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 (whichMoabosauruslacks). LikeTuriasaurus,[2]Mierasaurushas a pair offoraminaat the top end of the transversenuchal creston thesupraoccipital bone.Like inMoabosaurus,[6]the downward projections known as the basal tubera on thebasioccipital boneare L-shaped when viewed from the bottom.[1]
Vertebrae
[edit]
Mierasauruscan be excluded from theTitanosauriformesdue 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 centrumof the atlas) inMierasaurus,there is a pair of depressions that articulates with the odontoid process of theaxis(second cervical).Moabosauruslacks these depressions, and the cervical rib articulation (parapophysis) is more robust. As seen inKaatedocus,[9]thelaminaeextending from theneural spinesto theprezygapophysisof 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 ribsare visibly bifurcated either, unlike bothTuriasaurusandMoabosaurus,[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 thetubercleof each rib, which is a unique diagnostic feature ofMierasaurus.[1]
The rear dorsal vertebrae inMierasaurusare concave on both ends (amphicoelous), whereas the opposite (opisthicoelous) is true forMierasaurus,Camarasaurus,and titanosauriforms.[3][6][10]Mierasaurusdoes 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 thehypospheneis triangular inMierasauruswhile it is rectangular inMoabosaurus.However,Mierasaurusshares withMoabosaurusdorsal 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]
Limbs
[edit]
The forelimbs ofMierasaurusexhibit many shared (synapomorphic) turiasaurian traits. In theshoulder girdleofMierasaurus,the articulation with the scapula on thecoracoidis about half the bone's length. Thehumerusis 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,Moabosaurusalso 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 differentiatesMierasaurusfrom both titanosauriforms andMoabosaurus(which has a slight bulge[6]). Thefourth trochanterof the femur is only located 40% of the way down from the top of the bone, compared to halfway for most other sauropods, and thecondylesat the bottom end are, unusually, roughly the same size. Synapomorphic of turiasaurs, thecnemial crestof thetibiapoints forwards. The inner surface of theastragalusin the ankle narrows to form a triangular process, likeTuriasaurus.On the five-digited foot, the number ofphalangesis 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]
Classification
[edit]
Based on insertingMierasaurusinto 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 allowMierasaurusto 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 processesknown 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 unitesMierasauruswith other turiasaurs.[1]
Within the Turiasauria, Royo-Torres and colleagues considered the likewise-CretaceousMoabosaurusto 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.Moabosauruslacks 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 differentiatesMierasaurusfromTuriasaurusand potentially other turiasaurs.[1]
Thephylogenetic treesrecovered 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,andZbyoutside of theNeosauropoda(thus excluding them from both theDiplodocoideaand 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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Paleoecology
[edit]Locality and contemporaries
[edit]
The Doelling's Bowl locality is part of the lower segment of theYellow Cat Memberof theCedar Mountain Formation,as indicated by the presence of amarker horizonofcalcrete.Traditionally, based onbiostratigraphic correlationwithostracodsandcharophytes,the lower Yellow Cat Member has been considered to belong to theAptianepoch of the Cretaceous period, at 124.2 ± 2.6 Ma (million years) old.[16][17][18][19][20]However, based onuranium-lead datingof detritalzircon,this age has been more recently revised to less than ~139.7 ± 2.2 Ma, which is part of theValanginianepoch. 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 specimenof 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 theiguanodontianornithopodIguanacolossuswas 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 ofMierasaurusas well. A largeallosauroidtheropod is represented by teeth, and a new species ofpolacanthineankylosaurianhas 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 deformationof 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]
Paleobiogeography
[edit]Asides fromMierasaurusandMoabosaurus,turiasaurs are exclusivelyJurassicin 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 theTithonianepoch) 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 ofMierasaurusin 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 asharamiyidanmammals - to spread from Europe to North America.[1]
See also
[edit]References
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