Metrioxenini
Encyclopedia
Metrioxenini are a tribe
of belids
, primitive weevil
s of the family
Belidae
, containing about 30 species
. They are only found in two widely separated areas, Southeast Asia
extending to Indonesia
, and South Africa
. In the Paleogene
, they were found at least in North America
and Europe
also, occurring perhaps across the entire Northern Hemisphere
.
As in other belids, their antenna
e are straight, not elbowed as in the true weevils (Curculionidae
). They are far less characteristic than their presumed closest living relatives, the Aglycyderini
. They can be recognized by the sharp-sided rostrum
("snout"). Also, their eyes are bordered by a wide and ridged groove. Metrioxena shows two sharp ribs on each elytron
, which are absent in the other genera. These, by contrast, have their tarsal
claws fused together at the base (they are unfused in Metrioxena) and grooves on the pygidium
which are absent in Metrioxena.
Metrioxena contains twelve described species
apart from the fossil
ones mentioned below. They are today restricted to parts Southeast Asia
, extending into the Indonesia
n archipelago
. Roughly, they occur between Thailand
and the Philippines
and from Malaysia to the Tanimbar Islands
. They have been found on Sumatra
, Java
, Sulawesi
and offshore Ternate
but are hitherto unknown from Borneo
and the Lesser Sunda Islands
where they conceivably might also occur. Considering that in the Paleogene
the genus (or very close relatives) were found in North America
and Europe
, it may well be that the present distribution is a relict
of a formerly Holarctic
range. The Paleogene was warmer than our time, and the climate
in today's temperate
latitudes was by then indeed more like that in today's Southeast Asia.
Afrocorynus contains presently two described species, which are only known to occur in the mountains and hills that extend on the southern coast of South Africa
, between Mosselbay in the west, Grahamstown
in the east, and the Karoo
inland. Hispodes occurs in similar habitat
extending eastwards and inland, with the single described species (H. spicatus) found hitherto only in the Katberg region, and an undescribed species known from the Drakensberg
escarpment
south of Tzaneen
.
sugar palm
s (Arecaceae
). Larvae, though little is known about them, are probably eating the wood of dead or dying trees, while the adults eat pollen
. The Paleogene
fossil Metrioxena electrica was found in conifer amber
from the Baltic region
, where palms are known to have grown some 50 million years ago.
The South African species seem to be dependent on the Knysna-Amatole montane forests
biome
. Very little is known about their host plants. The first specimen of Afrocorynus asparagi was found on "wild asparagus
" but this seems a most unlikely host plant for Belidae
; in any case it has never again been found on an Asparagus species but only on Putterlickia pyracantha (Celastraceae
). Hispodes spicatus has not been recorded from any other plant than Rhoicissus tridentata (Vitaceae
). And though the Knysna-Amatole montane forests hold the conifers Podocarpus
and African cypress (Widdringtonia), which seem plausible host plants considering what other Belidae feed on, the Rosidae
P. pyracantha and R. tridentata actually grown in shrub
by habitat
interspersed with the actual montane forest. It may be that H. spicatus larvae feed - atypically for the family - on flower buds, as they have not been found in immature fruit
s which, apart from wood, are the other known food-type of Belidae larvae. More probably, they feed on the decaying wood like most of their relatives.
Presumably, the original host plants of belid larvae were conifers, namely Araucariaceae
. From there the Metrioxenini shifted to palms. The African lineages might have host-shifted to the Rosidae shrubs directly (from conifers) or indirectly (via palms, which used to occur in theior present range but eventually disappeared). This issue could be resolved by checking whether the Pondoland Palm (Jubaeopsis caffra), the only survivor from the original palmtree flora of Southern Africa, is host to a metrioxenine.
of Metrioxenini are occasionally assigned to subtribes, that seem almost superfluous, as it seems that the presently monotypic
Metrioxenina simply represent a more basal lineage and the Afrocorynina (Afrocorynus and Hispodes) a more advanced one. But the relationship among the genera is not resolved with complete certainty. Indeed, Metrioxena was sometimes placed in the Oxycoryninae
and the Afrocorynina elevated to full tribe status, but this seems to be in error. It is possible that Metrioxena iwill eventually be split up into several genera (e.g. Prometrioxena). Hispodes on the other hand is occasionally separated in a subtribe of Hispodini, which Marvaldi and co-writers 2006 find to be an unwarranted arrangement, since it would render all subtribes monotypic and thus redundant.
The fossil
record shows that the Metrioxenini were well distinct by the mid-Paleogene
, about 50 million years ago (mya). The belids as a whole are of Jurassic
origin, and the Metrioxenini must thus have evolve
d in the Late Cretaceous
or perhaps Paleocene
, roughly some 100-60 mya.
There are three fossil species
of Metrioxenini known to date: Paltorhynchus bisulcatus of the Early Eocene Green River Formation
and "Paltorhynchus" narwhal from the latest Eocene
or earliest Oligocene
of the Florissant Fossil Beds
are both from North America
. If the latter species actually belongs in the modern genus Metrioxena, since it is the type species
of Paltorhynchus, that name would become a junior synonym of Metrioxena and a new genus would have to be established for the more primitive "P." bisulcatus. That species might represent another genus of the Metrioxenina or a still more basal member of the tribe. Metrioxena electrica known from Baltic
amber
and of comparable age to the North America
n fossils, is sometimes separated in Archimetrioxena.
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...
of belids
Belidae
Belidae is a family of weevils, called belids or primitive weevils because they have straight antennae, unlike the "true weevils" or Curculionidae which have elbowed antennae...
, primitive weevil
Weevil
A weevil is any beetle from the Curculionoidea superfamily. They are usually small, less than , and herbivorous. There are over 60,000 species in several families, mostly in the family Curculionidae...
s of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Belidae
Belidae
Belidae is a family of weevils, called belids or primitive weevils because they have straight antennae, unlike the "true weevils" or Curculionidae which have elbowed antennae...
, containing about 30 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. They are only found in two widely separated areas, Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...
extending to Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
, and South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. In the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
, they were found at least in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
also, occurring perhaps across the entire Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
.
As in other belids, their antenna
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
e are straight, not elbowed as in the true weevils (Curculionidae
Curculionidae
Curculionidae is the family of the "true" weevils . It was formerly recognized in 1998 as the largest of any animal family, with over 40,000 species described worldwide at that time...
). They are far less characteristic than their presumed closest living relatives, the Aglycyderini
Aglycyderini
Aglycyderini are a tribe of belids, primitive weevils of the family Belidae. Like in other belids, their antennae are straight, not elbowed as in the true weevils...
. They can be recognized by the sharp-sided rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
("snout"). Also, their eyes are bordered by a wide and ridged groove. Metrioxena shows two sharp ribs on each elytron
Elytron
An elytron is a modified, hardened forewing of certain insect orders, notably beetles and a few of the true bugs ; in most true bugs, the forewings are instead called hemelytra, as only the basal half is thickened while the apex is membranous...
, which are absent in the other genera. These, by contrast, have their tarsal
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
claws fused together at the base (they are unfused in Metrioxena) and grooves on the pygidium
Pygidium
The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...
which are absent in Metrioxena.
Distribution and ecology
The type genusType genus
In biological classification, a type genus is a representative genus, as with regard to a biological family. The term and concept is used much more often and much more formally in zoology than it is in botany, and the definition is dependent on the nomenclatural Code that applies:* In zoological...
Metrioxena contains twelve described species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
apart from the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ones mentioned below. They are today restricted to parts Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...
, extending into the Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
n archipelago
Archipelago
An archipelago , sometimes called an island group, is a chain or cluster of islands. The word archipelago is derived from the Greek ἄρχι- – arkhi- and πέλαγος – pélagos through the Italian arcipelago...
. Roughly, they occur between Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
and from Malaysia to the Tanimbar Islands
Tanimbar Islands
The Tanimbar Islands, also called Timor Laut, are a group of about 65 islands in the Maluku province of Indonesia, including Fordata, Larat, Maru, Molu, Nuswotar, Selaru, Selu, Seira, Wotap, Wuliaru and Yamdena.-Geography:...
. They have been found on Sumatra
Sumatra
Sumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538...
, Java
Java
Java is an island of Indonesia. With a population of 135 million , it is the world's most populous island, and one of the most densely populated regions in the world. It is home to 60% of Indonesia's population. The Indonesian capital city, Jakarta, is in west Java...
, Sulawesi
Sulawesi
Sulawesi is one of the four larger Sunda Islands of Indonesia and is situated between Borneo and the Maluku Islands. In Indonesia, only Sumatra, Borneo, and Papua are larger in territory, and only Java and Sumatra have larger Indonesian populations.- Etymology :The Portuguese were the first to...
and offshore Ternate
Ternate
Ternate is an island in the Maluku Islands of eastern Indonesia. It is located off the west coast of the larger island of Halmahera, the center of the powerful former Sultanate of Ternate....
but are hitherto unknown from Borneo
Borneo
Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....
and the Lesser Sunda Islands
Lesser Sunda Islands
The Lesser Sunda Islands or Nusa Tenggara are a group of islands in the southern Maritime Southeast Asia, north of Australia. Together with the Greater Sunda Islands to the west they make up the Sunda Islands...
where they conceivably might also occur. Considering that in the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
the genus (or very close relatives) were found in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, it may well be that the present distribution is a relict
Relict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
of a formerly Holarctic
Holarctic
The Holarctic ecozone refers to the habitats found throughout the northern continents of the world as a whole. This region is divided into the Palearctic, consisting of Northern Africa and all of Eurasia, with the exception of Southeast Asia and the Indian subcontinent, and the Nearctic,...
range. The Paleogene was warmer than our time, and the climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
in today's temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
latitudes was by then indeed more like that in today's Southeast Asia.
Afrocorynus contains presently two described species, which are only known to occur in the mountains and hills that extend on the southern coast of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, between Mosselbay in the west, Grahamstown
Grahamstown
Grahamstown is a city in the Eastern Cape Province of the Republic of South Africa and is the seat of the Makana municipality. The population of greater Grahamstown, as of 2003, was 124,758. The population of the surrounding areas, including the actual city was 41,799 of which 77.4% were black,...
in the east, and the Karoo
Karoo
The Karoo is a semi-desert region of South Africa. It has two main sub-regions - the Great Karoo in the north and the Little Karoo in the south. The 'High' Karoo is one of the distinct physiographic provinces of the larger South African Platform division.-Great Karoo:The Great Karoo has an area of...
inland. Hispodes occurs in similar habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
extending eastwards and inland, with the single described species (H. spicatus) found hitherto only in the Katberg region, and an undescribed species known from the Drakensberg
Drakensberg
The Drakensberg is the highest mountain range in Southern Africa, rising to in height. In Zulu, it is referred to as uKhahlamba , and in Sesotho as Maluti...
escarpment
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...
south of Tzaneen
Tzaneen
Tzaneen is a small town situated in the Mopani district of the Limpopo province in South Africa. It is in a fertile region with tropical and subtropical agriculture taking place in a 20,000 km² region...
.
Larval host plants
Metrioxena apparently feed only on ArengaArenga
Arenga is a genus of 24 species of palms, native to tropical regions of southern and southeastern Asia. They are small to medium-sized palms, growing to 2-20 m tall, with pinnate leaves 2-12 m long.Species*Arenga australasica*Arenga brevipes...
sugar palm
Sugar palm
Sugar palm is a common name for several species of palms used to produce sugar.*Arenga pinnata *Borassus flabellifer*Caryota urens*Cocos nucifera...
s (Arecaceae
Arecaceae
Arecaceae or Palmae , are a family of flowering plants, the only family in the monocot order Arecales. There are roughly 202 currently known genera with around 2600 species, most of which are restricted to tropical, subtropical, and warm temperate climates...
). Larvae, though little is known about them, are probably eating the wood of dead or dying trees, while the adults eat pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
. The Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
fossil Metrioxena electrica was found in conifer amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...
from the Baltic region
Baltic region
The terms Baltic region, Baltic Rim countries, and Baltic Rim refer to slightly different combinations of countries in the general area surrounding the Baltic Sea.- Etymology :...
, where palms are known to have grown some 50 million years ago.
The South African species seem to be dependent on the Knysna-Amatole montane forests
Knysna-Amatole montane forests
The Knysna-Amatole montane forests ecoregion, of the Tropical and subtropical moist broadleaf forests Biome, is in South Africa. It covers an Afromontane area of in South Africa's Eastern Cape and Western Cape provinces.-Setting:...
biome
Biome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
. Very little is known about their host plants. The first specimen of Afrocorynus asparagi was found on "wild asparagus
Asparagus
Asparagus officinalis is a spring vegetable, a flowering perennialplant species in the genus Asparagus. It was once classified in the lily family, like its Allium cousins, onions and garlic, but the Liliaceae have been split and the onion-like plants are now in the family Amaryllidaceae and...
" but this seems a most unlikely host plant for Belidae
Belidae
Belidae is a family of weevils, called belids or primitive weevils because they have straight antennae, unlike the "true weevils" or Curculionidae which have elbowed antennae...
; in any case it has never again been found on an Asparagus species but only on Putterlickia pyracantha (Celastraceae
Celastraceae
The Celastraceae , is a family of about 90-100 genera and 1,300 species of vines, shrubs and small trees, belonging to the order Celastrales...
). Hispodes spicatus has not been recorded from any other plant than Rhoicissus tridentata (Vitaceae
Vitaceae
Vitaceae are a family of dicotyledonous flowering plants including the grapevine and Virginia creeper. The family name is derived from the genus Vitis...
). And though the Knysna-Amatole montane forests hold the conifers Podocarpus
Podocarpus
Podocarpus is a genus of conifers, the most numerous and widely distributed of the podocarp family Podocarpaceae. The 105 species of Podocarpus are evergreen shrubs or trees from 1-25 m in height...
and African cypress (Widdringtonia), which seem plausible host plants considering what other Belidae feed on, the Rosidae
Rosidae
Under the International Code of Botanical Nomenclature , Rosidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used; the only requirement being that it includes the family Rosaceae....
P. pyracantha and R. tridentata actually grown in shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
by habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
interspersed with the actual montane forest. It may be that H. spicatus larvae feed - atypically for the family - on flower buds, as they have not been found in immature fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
s which, apart from wood, are the other known food-type of Belidae larvae. More probably, they feed on the decaying wood like most of their relatives.
Presumably, the original host plants of belid larvae were conifers, namely Araucariaceae
Araucariaceae
Araucariaceae, commonly referred to as araucarians, is a very ancient family of coniferous trees. It achieved its maximum diversity in the Jurassic and Cretaceous periods, when it was distributed almost worldwide...
. From there the Metrioxenini shifted to palms. The African lineages might have host-shifted to the Rosidae shrubs directly (from conifers) or indirectly (via palms, which used to occur in theior present range but eventually disappeared). This issue could be resolved by checking whether the Pondoland Palm (Jubaeopsis caffra), the only survivor from the original palmtree flora of Southern Africa, is host to a metrioxenine.
Systematics and evolution
The three currently recognized living generaGenera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
of Metrioxenini are occasionally assigned to subtribes, that seem almost superfluous, as it seems that the presently monotypic
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
Metrioxenina simply represent a more basal lineage and the Afrocorynina (Afrocorynus and Hispodes) a more advanced one. But the relationship among the genera is not resolved with complete certainty. Indeed, Metrioxena was sometimes placed in the Oxycoryninae
Oxycoryninae
Oxycoryninae are subfamily of primitive weevils of the family Belidae, but sometimes treated as a distinct family Oxycorynidae. Like in other belids, their antennae are straight, not elbowed as in the true weevils , and their larvae feed on the wood of diseased or dying plants or on deadwood or...
and the Afrocorynina elevated to full tribe status, but this seems to be in error. It is possible that Metrioxena iwill eventually be split up into several genera (e.g. Prometrioxena). Hispodes on the other hand is occasionally separated in a subtribe of Hispodini, which Marvaldi and co-writers 2006 find to be an unwarranted arrangement, since it would render all subtribes monotypic and thus redundant.
The fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record shows that the Metrioxenini were well distinct by the mid-Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
, about 50 million years ago (mya). The belids as a whole are of Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
origin, and the Metrioxenini must thus have evolve
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
d in the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
or perhaps Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
, roughly some 100-60 mya.
There are three fossil species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of Metrioxenini known to date: Paltorhynchus bisulcatus of the Early Eocene Green River Formation
Green River Formation
The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in winter. Each pair of layers is called a varve and...
and "Paltorhynchus" narwhal from the latest Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
or earliest Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
of the Florissant Fossil Beds
Florissant Fossil Beds National Monument
Florissant Fossil Beds National Monument is a United States National Monument in Teller County, Colorado, that is noted for its fossils. It is located in a mountain valley just west of Pikes Peak and holds spectacular remnants of prehistoric life...
are both from North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. If the latter species actually belongs in the modern genus Metrioxena, since it is the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
of Paltorhynchus, that name would become a junior synonym of Metrioxena and a new genus would have to be established for the more primitive "P." bisulcatus. That species might represent another genus of the Metrioxenina or a still more basal member of the tribe. Metrioxena electrica known from Baltic
Baltic region
The terms Baltic region, Baltic Rim countries, and Baltic Rim refer to slightly different combinations of countries in the general area surrounding the Baltic Sea.- Etymology :...
amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...
and of comparable age to the North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n fossils, is sometimes separated in Archimetrioxena.