The taxon was first described and named in 2017 by Rafael Royo-Torres and colleagues, from a mostly complete skeleton including a disarticulated partial skull and mandible, teeth, multiple vertebrae from along the length of the body, both scapulae, radius and ulna bones, a left manus, a complete pelvis, both femora and the entire left hindlimb.

The genus name honours Bernardo de Miera y Pacheco, the first European scientist to enter what is now Utah.

[1] Along with its closest relative Moabosaurus, also from the Early Cretaceous of Utah, Mierasaurus is among the last-surviving members of the Turiasauria, an otherwise Jurassic and European group which can be distinguished by heart-shaped teeth, slender humeri, and the presence of an extra depression on the surface of the ulnae, among other characteristics.

Rocks in the Doelling's Bowl bonebed mainly consist of green-grey, sandy mudstone, but also contain silcrete, casts of silificied plant roots, and chert pebbles.

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.

Although the remaining elements have been displaced and eroded by the arroyo, the specimen is still the most complete Cretaceous sauropod from North America.

In 2017, all of these specimens were described by Rafael Royo-Torres, Paul Upchurch, James Kirkland, Donald DeBlieux, John Foster, Alberto Cobos, and Luis Alcalá as part of a research paper published in Scientific Reports.

They also named the type and only species M. bobyoungi after Robert ("Bob") Young, in order to acknowledge "the importance of [his] underappreciated research" the geology of the Early Cretaceous of Utah.

A sharp ridge extends across the bottom edge of the front of the lower jaw, seen in both dicraeosaurids and diplodocids, and to some extent Camarasaurus.

[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.

Unlike more derived sauropods but also seen in Turiasaurus,[2] the frontal bone of Mierasaurus participates in the margin of the supratemporal fenestra.

[1] Mierasaurus can be excluded from the Titanosauriformes due to the solid internal structure of its vertebrae and ribs, which indicates they lack air-filled cavities.

[3][7][8] Uniquely, on the bottom of the internal cavity of the atlantal intercentrum (term used for the unfused lower half of vertebral centrum of the atlas) in Mierasaurus, there is a pair of depressions that articulates with the odontoid process of the axis (second cervical).

As seen in Kaatedocus,[9] the laminae extending from the neural spines to the prezygapophysis of the cervical vertebrae are well-developed and cap depressions underneath.

The fourth 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 the condyles at the bottom end are, unusually, roughly the same size.

[1] The phylogenetic trees recovered by Royo-Torres and colleagues from the two different analyses agreed upon the position of turiasaurs, placing them as a unified group containing Mierasaurus, Moabosaurus, Turiasaurus, Losillasasurus, and Zby outside of the Neosauropoda (thus excluding them from both the Diplodocoidea and the Macronaria).

Traditionally, based on biostratigraphic correlation with ostracods and charophytes, the lower Yellow Cat Member has been considered to belong to the Aptian epoch of the Cretaceous period, at 124.2 ± 2.6 Ma (million years) old.

[16][17][18][19][20] However, based on uranium-lead dating of detrital zircon, this age has been more recently revised to less than ~139.7 ± 2.2 Ma, which is part of the Valanginian epoch.

However, the presence of Mierasaurus in Valanginian deposits correlates with a substantial drop in sea level that occurred during the epoch, which may have formed a land bridge[28][29][30] and allowed turiasaurs - and potentially other groups, such as haramiyidan mammals - to spread from Europe to North America.