Tyto robusta
Encyclopedia
Tyto robusta was a prehistoric barn-owl. It lived at what is now Monte Gargano
in Italy, and was an island throughout much of the Neogene
when sea level
s were higher. The owl's remains date back to the Miocene
-Pliocene
boundary 5.5 to 5 million years ago. The fossil bones are about 60% as long again as a modern Barn Owl
, giving a total length of about 50–65 cm for T. robusta. This owl provides an interesting case study of evolution
and insular gigantism.
ally with the gigantic relative, Tyto gigantea
, which at some 220% of the size of the Barn Owl
grew to some 70–85 cm in length—larger than a Eurasian Eagle-owl and maybe up to twice as heavy. Like other apex predator
s they can be presumed to have occurred at low population densities
. Being apparently endemic to Gargano Island and perhaps some other islands in today's Apennines, where one or two huge eagle
s and at least at times a normal-sized barn-owl also occurred (though no large mammal
ian carnivores), these owls raise the question how minimum viable population
s could be maintained long enough for them to live and prosper.
It is extremely commonplace for larger predatory birds such as Falconiformes
and Strigiformes to evolve
differently-sized sexes. Namely, the females being considerably larger with little or no overlap in size between sexes. This allows them to hunt for different-sized prey, allowing a larger, more resilient population to live off a land. But this was apparently not the case here, as there is a reasonably large sample of bones of each of the 3 owl taxa to display the natural and sexual variation within each.
In the oldest fossil samples, no giant owls were found to accompany them, whereas the latest seemed to a range of owls attributed to all 3 species. The small owls supposedly were replaced by or evolved into T. robusta from some point onwards, and T. gigantea appeared later, supposedly alongside it as well as a recolonization by Continental European stock. That this was by no means a very parsimonious assumption
was already noted in the original descriptions of the remains assigned to T. robusta:
More recently, it was noted that the smaller barn-owl of Gargano was not Tyto sanctialbani, which inhabited Europe north of the Alps
in the latter Miocene, but its Mediterranean relative Tyto balearica, which was a sister species or chronospecies
of or a southward divergence from T. sanctialbani and decidedly more robust. In addition, it appeared as if the Tyto of the youngest samples fell into 2, not 3 size groups.
Presumably, the island was initially colonized by T. balearica whose core range eventually extended from the Apennines westwards to Iberia
; they might have become cut off from the main population when the Mediterranean reflooded at the end of the Messinian salinity crisis
6 million years ago. The entire population evolved into larger T. robusta, which were still close enough in size to exclude T. balearica from maintaining a constant presence alongside the larger owls. By and by, much of the fauna
of Gargano Island, in a dramatic case of evolution according to Cope's Rule
, became larger and larger; the barn-owls were not an exception as there was larger and larger prey accessible only to large individuals of each successive generation.
According to the revised theory, T. robusta had entirely evolved into T. gigantea eventually, and the prey preferences of these were so that the plentiful smaller mammals were again less intensely preyed upon, permitting a resident population of the now well-distinct T. balearica to recolonize Gargano Island. The remains initially presumed to be "late" T. robusta would actually be large T. balearica. Jirí Mlíkovský who proposed this solution does not use paleosubspecies in his taxonomic treatments and synonymized T. robusta with T. gigantea. But the former was actually described a few pages before the latter in the same work, and thus the synonymy would be the other way around. In any case, if these owls are considered paleosubspecies, the largest and latest form would be called Tyto robusta gigantea.
However, with more material having become available in the last few years, it is possible to better determine which individual bone belonged to what owl taxon
, and it seems as if the giant barn-owls may indeed be separable as distinct species; the possibility of only a single large to huge population with marked sexual dimorphism
existing at any one time cannot be fully excluded though. Identification of the small barn owl species as T. balearica and not T. sanctialbani was confirmed on the other hand.
period, it was not too dissimilar from today, albeit a bit warmer and more humid. Conditions must have been more extreme in the Mediterranean basin though, as the region had been drastically affected by the Messinian Salinity Crisis
. In any case, the Gargano giant barn-owls must have lived in habitat not too different from that of the Caribbean
giant barn-owls of the Quaternary
: semihumid to semiarid—perhaps seasonally arid—open country tending to karst
ic, and vegetated mainly with shrubs and small trees, perhaps dotted with copses of larger trees. The owls would probably have sought sheltered places to roost during the day, but for nesting barn-owls prefer crevices or hollows, and those of sufficient size would probably have been more plentiful in the broken ground than in the smallish trees of the region. Like some barn-owls today, the huge forms of Gargano Island might thus have been ground-nesters.
These owls would have ruled Gargano's nightly skies; at daytime the no less impressive Garganoaetus eagles took over. As the larger land animals of Gargano Island are impressively well-known, it is possible to give a quite comprehensive list of the animals the giant owls ate even in the absence of fossil owl pellets providing direct evidence. Available prey included:
In addition, the smaller rodent
s (Mikrotia spp. and Apodemus gorafensis mice
and a Cricetus hamster) might have been taken on occasion (or preferably by T. robusta, whatever its taxonomic and systematic status). It is unlikely that ponderous owls on an island abundant in mammalian prey would evolve good bird-hunting skills, but theoretically the local pigeons (Columba
omnisanctorum or C. pisana) as well as the smaller Tyto balearica might have been possible prey also.
Gargano
Gargano is a historical and geographical Italian sub-region situated in Apulia, consisting of a wide isolated mountain massif made of highland and several peaks and forming the backbone of the Gargano Promontory projecting into the Adriatic Sea. The high point is Monte Calvo at . Most of the upland...
in Italy, and was an island throughout much of the Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
when sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
s were higher. The owl's remains date back to the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
-Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
boundary 5.5 to 5 million years ago. The fossil bones are about 60% as long again as a modern Barn Owl
Barn Owl
The Barn Owl is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as Common Barn Owl, to distinguish it from other species in the barn-owl family Tytonidae. These form one of two main lineages of living owls, the other being the typical...
, giving a total length of about 50–65 cm for T. robusta. This owl provides an interesting case study of evolution
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...
and insular gigantism.
Evolution, taxonomy and systematics
It was found sympatricSympatry
In biology, two species or populations are considered sympatric when they exist in the same geographic area and thus regularly encounter one another. An initially-interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation...
ally with the gigantic relative, Tyto gigantea
Tyto gigantea
Tyto gigantea is an extinct barn owl from what is now Gargano, Italy, dating back to the late Miocene. From its remains, T. gigantea is suggested to have been as large as, or larger than the Eurasian Eagle-owl, Bubo bubo....
, which at some 220% of the size of the Barn Owl
Barn Owl
The Barn Owl is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as Common Barn Owl, to distinguish it from other species in the barn-owl family Tytonidae. These form one of two main lineages of living owls, the other being the typical...
grew to some 70–85 cm in length—larger than a Eurasian Eagle-owl and maybe up to twice as heavy. Like other apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
s they can be presumed to have occurred at low population densities
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
. Being apparently endemic to Gargano Island and perhaps some other islands in today's Apennines, where one or two huge eagle
Eagle
Eagles are members of the bird family Accipitridae, and belong to several genera which are not necessarily closely related to each other. Most of the more than 60 species occur in Eurasia and Africa. Outside this area, just two species can be found in the United States and Canada, nine more in...
s and at least at times a normal-sized barn-owl also occurred (though no large mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian carnivores), these owls raise the question how minimum viable population
Minimum Viable Population
Minimum viable population is a lower bound on the population of a species, such that it can survive in the wild. This term is used in the fields of biology, ecology, and conservation biology...
s could be maintained long enough for them to live and prosper.
It is extremely commonplace for larger predatory birds such as Falconiformes
Falconiformes
The order Falconiformes is a group of about 290 species of birds that comprises the diurnal birds of prey. Raptor classification is difficult and the order is treated in several ways.- Classification problems :...
and Strigiformes to 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...
differently-sized sexes. Namely, the females being considerably larger with little or no overlap in size between sexes. This allows them to hunt for different-sized prey, allowing a larger, more resilient population to live off a land. But this was apparently not the case here, as there is a reasonably large sample of bones of each of the 3 owl taxa to display the natural and sexual variation within each.
In the oldest fossil samples, no giant owls were found to accompany them, whereas the latest seemed to a range of owls attributed to all 3 species. The small owls supposedly were replaced by or evolved into T. robusta from some point onwards, and T. gigantea appeared later, supposedly alongside it as well as a recolonization by Continental European stock. That this was by no means a very parsimonious assumption
Occam's razor
Occam's razor, also known as Ockham's razor, and sometimes expressed in Latin as lex parsimoniae , is a principle that generally recommends from among competing hypotheses selecting the one that makes the fewest new assumptions.-Overview:The principle is often summarized as "simpler explanations...
was already noted in the original descriptions of the remains assigned to T. robusta:
"If one assumes that the large barn-owls on Gargano would have arisen from multiple colonizations [...], T. robusta would have to be separated into two distinct species. The medium-sized form of the first colonist cannot be the same as the middle-sized form of the second colonist. As there are no morphological findings to support this assumption, it was refrained from separating [the T. robusta material into] two species. In how far one can actually expect morphological differences in forms so closely related is debatable."
More recently, it was noted that the smaller barn-owl of Gargano was not Tyto sanctialbani, which inhabited Europe north of the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
in the latter Miocene, but its Mediterranean relative Tyto balearica, which was a sister species or chronospecies
Chronospecies
A chronospecies describes a group of one or more species derived from a sequential development pattern which involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. This sequence of alterations eventually produces a population which is physically,...
of or a southward divergence from T. sanctialbani and decidedly more robust. In addition, it appeared as if the Tyto of the youngest samples fell into 2, not 3 size groups.
Presumably, the island was initially colonized by T. balearica whose core range eventually extended from the Apennines westwards to Iberia
Iberian Peninsula
The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...
; they might have become cut off from the main population when the Mediterranean reflooded at the end of the Messinian salinity crisis
Messinian salinity crisis
The Messinian Salinity Crisis, also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene...
6 million years ago. The entire population evolved into larger T. robusta, which were still close enough in size to exclude T. balearica from maintaining a constant presence alongside the larger owls. By and by, much of the fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
of Gargano Island, in a dramatic case of evolution according to Cope's Rule
Cope's rule
Cope's rule states that population lineages tend to increase in body size over evolutionary time. While the rule has been demonstrated in many instances, it does not hold true at all taxonomic levels, or in all clades...
, became larger and larger; the barn-owls were not an exception as there was larger and larger prey accessible only to large individuals of each successive generation.
According to the revised theory, T. robusta had entirely evolved into T. gigantea eventually, and the prey preferences of these were so that the plentiful smaller mammals were again less intensely preyed upon, permitting a resident population of the now well-distinct T. balearica to recolonize Gargano Island. The remains initially presumed to be "late" T. robusta would actually be large T. balearica. Jirí Mlíkovský who proposed this solution does not use paleosubspecies in his taxonomic treatments and synonymized T. robusta with T. gigantea. But the former was actually described a few pages before the latter in the same work, and thus the synonymy would be the other way around. In any case, if these owls are considered paleosubspecies, the largest and latest form would be called Tyto robusta gigantea.
However, with more material having become available in the last few years, it is possible to better determine which individual bone belonged to what owl taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, and it seems as if the giant barn-owls may indeed be separable as distinct species; the possibility of only a single large to huge population with marked sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
existing at any one time cannot be fully excluded though. Identification of the small barn owl species as T. balearica and not T. sanctialbani was confirmed on the other hand.
Ecology
The fossil owl bones are of Gargano are found in terra rossa ("red earth") paleokarst infills. As the global climate moved towards the start of the last ice ageIce age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
period, it was not too dissimilar from today, albeit a bit warmer and more humid. Conditions must have been more extreme in the Mediterranean basin though, as the region had been drastically affected by the Messinian Salinity Crisis
Messinian salinity crisis
The Messinian Salinity Crisis, also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene...
. In any case, the Gargano giant barn-owls must have lived in habitat not too different from that of the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
giant barn-owls of the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
: semihumid to semiarid—perhaps seasonally arid—open country tending to karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
ic, and vegetated mainly with shrubs and small trees, perhaps dotted with copses of larger trees. The owls would probably have sought sheltered places to roost during the day, but for nesting barn-owls prefer crevices or hollows, and those of sufficient size would probably have been more plentiful in the broken ground than in the smallish trees of the region. Like some barn-owls today, the huge forms of Gargano Island might thus have been ground-nesters.
These owls would have ruled Gargano's nightly skies; at daytime the no less impressive Garganoaetus eagles took over. As the larger land animals of Gargano Island are impressively well-known, it is possible to give a quite comprehensive list of the animals the giant owls ate even in the absence of fossil owl pellets providing direct evidence. Available prey included:
- DeinogalerixDeinogalerixDeinogalerix , was a genus of the order Erinaceomorpha, which lived in Italy in the Late Miocene. The genus was apparently endemic to what was then Gargano Island, today's Gargano peninsula...
—"hairy hedgehogs" that ranged from weasel- to dog-sized. The giant D. koenigswaldi with a skull of c.20 cm length could still have been preyed upon by the largest barn-owl females, though probably not on a regular basis as being the terrestrial apex predator it was a fierce animal and not easily subdued. - Smaller species of the HoplitomeryxHoplitomeryxThe extinct five-horned prongdeer Hoplitomeryx matthei with its sabrelike upper canines lived on the former Gargano Island during the Miocene and the Early Pliocene, now a peninsula on the east coast of South Italy....
—"prongdeer", five-horned sabre-toothed even-toed ungulateEven-toed ungulateThe even-toed ungulates are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates such as horses....
s. Barn-owls generally do not feed on carrionCarrionCarrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...
, their beak and head shapes being ill-suited for tearing apart prey too large to manipulate with their feet also. Thus the red deerRed DeerThe red deer is one of the largest deer species. Depending on taxonomy, the red deer inhabits most of Europe, the Caucasus Mountains region, Asia Minor, parts of western Asia, and central Asia. It also inhabits the Atlas Mountains region between Morocco and Tunisia in northwestern Africa, being...
-sized largest Hoplitomeryx do not seem to have been part of the giant owls' prey. - Paralutra garganensis—a species of otterOtterThe Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....
endemic to Gargano Island. - ProlagusProlagusProlagidae is an extinct family within the order of lagomorphs, which also includes the Leporidae . One genus, Prolagus, is recognised within the family. This family as Prolaginae was previously considered a subfamily of Ochotonidae. Prolagus first appeared in the Early Miocene in Europe, where it...
imperialis and P. apricenicus—huge pikaPikaThe pika is a small mammal, with short limbs, rounded ears, and short tail. The name pika is used for any member of the Ochotonidae, a family within the order of lagomorphs, which also includes the Leporidae . One genus, Ochotona, is recognised within the family, and it includes 30 species...
s endemic to Gagano Island. - Stertomys laticrestatus—a dormouseDormouseDormice are rodents of the family Gliridae. Dormice are mostly found in Europe, although some live in Africa and Asia. They are particularly known for their long periods of hibernation...
of large size. Apparently a monotypicMonotypicIn 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...
genus. - Hattomys—3 species of giant hamsterHamsterHamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....
, at least some of which probably also occurred elsewhere.
In addition, the smaller rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s (Mikrotia spp. and Apodemus gorafensis mice
Murinae
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. This subfamily is larger than all mammal families except the Cricetidae and Muridae, and is larger than all mammal orders except the bats and the remainder of the...
and a Cricetus hamster) might have been taken on occasion (or preferably by T. robusta, whatever its taxonomic and systematic status). It is unlikely that ponderous owls on an island abundant in mammalian prey would evolve good bird-hunting skills, but theoretically the local pigeons (Columba
Columba (genus)
The large bird genus Columba comprises a group of medium to large stout-bodied pigeons, often referred to as the typical pigeons. The terms "dove" and "pigeon" are used indiscriminately for smaller and larger Columbidae, respectively...
omnisanctorum or C. pisana) as well as the smaller Tyto balearica might have been possible prey also.