''Spinolestes'' is an extinct

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

genus from the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = ''Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , i ...

. A

gobiconodontid Gobiconodontidae is a family of extinct mammals that ranged from the mid-Jurassic to the early Late Cretaceous, though most common during the Early Cretaceous. The Gobiconodontids form a diverse lineage of carnivorous non-therian mammals, and in ...

eutriconodont Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the ...

, it is notable for the remarkable degree of preservation, offering profound insights to the biology of non-

theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Ch ...

n mammals.Thomas Martin, Jesús Marugán-Lobón, Romain Vullo, Hugo Martín-Abad, Zhe-Xi Luo & Angela D. Buscalioni (2015). A Cretaceous eutriconodont and integument evolution in early mammals. Nature 526, 380–384. doi:10.1038/nature14905

Description

''Spinolestes

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

, MCCMLH30000A', hails from Las Hoyas, Spain. The living animal was about 24 centimeters long and weighted somewhere between 50 and 70 grams. As a Konservat-Lagerstätten specimen, it is famous for being remarkably well preserved, including not only the skeleton but also multiple soft tissues like

fur Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket t ...

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

, internal organs and ears (both external and internal), a rarity among Mesozoic mammals. Besides soft-tissues, ''Spinolestes'' is also remarkable for its xenarthrous vertebrae, convergent with those of

xenarthrans Xenarthra (; from Ancient Greek wikt:ξένος, ξένος, xénos, "foreign, alien" + wikt:ἄρθρον, ἄρθρον, árthron, "joint") is a major clade of Placentalia, placental mammals native to the Americas. There are 31 living species: ...

and to a lesser extent

hero shrew The hero shrew (''Scutisorex somereni''), also known as the armored shrew, is a large shrew native to the Congo Basin of Africa. Its features are typical of a white-toothed shrew − short legs, slender snout, dense fur − except for a highly u ...

Soft tissue

Hair is very well preserved, down to the cellular level; among fossil mammals it is among the best preserved. ''Spinolestes fur was similar to that of modern mammals, possessing compound

hair follicles The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

with primary and secondary hair. The guard hairs are proto-spines similar to those of modern

spiny mice The term spiny mouse refers to any species of rodent within the genus ''Acomys''. Similar in appearance to mice of the genus '' Mus'', spiny mice are small mammals with bare, scaled tails. However, their coats are endowed with unusually stiff gu ...

. In some places on the body, hairs appear to have broken off close to the skin and were discolored near the broken tip – possible signs of

dermatophytosis Dermatophytosis, also known as ringworm, is a fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Multiple ar ...

, making it the first Mesozoic mammal with a clear record of a skin infection. Besides hair, ''Spinolestes'' also had keratinous

scutes A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...

. Uniquely among Mesozoic mammal fossils the internal organs are also preserved, deposits of iron marking the position of the liver and microscopic structures being interpreted as the

bronchioles The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start o ...

. These are separated by a curved line, assumed to be the

thoracic diaphragm The thoracic diaphragm, or simply the diaphragm ( grc, διάφραγμα, diáphragma, partition), is a sheet of internal Skeletal striated muscle, skeletal muscle in humans and other mammals that extends across the bottom of the thoracic cavit ...

. The ear is very well preserved. The external ear (

pinnae The auricle or auricula is the visible part of the ear that is outside the head. It is also called the pinna (Latin for "wing" or "fin", plural pinnae), a term that is used more in zoology. Structure The diagram shows the shape and location of ...

) is fairly large and broad in a

mouse A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

-like fashion. The inner ear is also exceptionally preserved in the form of the

Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left) also known as Meckelian cartilages; above this the incus and malleus are developed. Meckel's cartilage arises from the first ...

, which may be of extreme relevance in understanding the evolution of the mammalian ear.

Phylogeny

Cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

after

Thomas Martin Thomas or Tom Martin may refer to: Born 16th century *Thomas Martin (politician, died 1583) (1530–1583), MP for Dorchester *Thomas Martin (lawyer) (1521–1593), MP for Ludgershall, Saltash and Hindon Born 17th century *Thomas Martin of Pal ...

et al. 2015:

Ecology

The environment of Las Hoyas dates to the

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...

, and it was probably a tropical or subtropical

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

habitat, based on its vegetation:

Bennettitales Bennettitales (also known as cycadeoids) is an extinct order of seed plants that first appeared in the Permian period and became extinct in most areas toward the end of the Cretaceous. Bennettitales are among the most common Mesozoic seed plants ...

, ''

Brachyphyllum ''Brachyphyllum'' (meaning "short leaf") is a form genus of fossil coniferous plant foliage. Plants of the genus have been variously assigned to several different conifer groups including Araucariaceae and Cheirolepidiaceae. They are known from ...

'', ''

Pagiophyllum ''Pagiophyllum'' is a form genus of fossil coniferous plant foliage. Plants of the genus have been variously assigned to several different conifer groups including Araucariaceae and Cheirolepidiaceae. They were found around the globe during the ...

'', ''

Sphenolepis ''Sphenolepis'' is a genus of hymenopteran insects of the family Eulophidae. ReferencesKey to Nearctic eulophid genera

'' and '' Cupressinocladus''

conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extan ...

, '' Weitchselia reticulata'', '' Montsechia vidali'' and several others. Various species of

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

and aquatic invertebrates are also known.Fossilworks: Las Hoyas
/ref>

References

{{taxonbar, from=Q21110978 Barremian life Cretaceous mammals of Europe Cretaceous Spain Early Cretaceous animals of Europe Early Cretaceous mammals Eutriconodonts Fossils of Spain Fossil taxa described in 2015 La Huérguina Formation Prehistoric mammal genera Taxa named by Thomas Martin (paleontologist)