''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), fu ...
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
Pr ...
of
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")
, ...
. A
gobiconodontid 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. ...
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,
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 animal coverings, such as the arthropod exoskeleton, have different d ...
, 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 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 ...
s.
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. 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 ...
, 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 ...
.
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. 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 muscle in humans and other mammals that extends across the bottom of the thoracic cavity. The diaphragm is the ...
.
The ear is very well preserved. The external ear (
pinnae) 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 firs ...
, 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 ...
after
Thomas Martin 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 preced ...
, 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. T ...
habitat, based on its vegetation:
Bennettitales, ''
Brachyphyllum'', ''
Pagiophyllum'', ''
Sphenolepis'' 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 exta ...
, ''
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 liv ...
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)