Protostegidae is a family of
extinct marine turtles
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...
that lived during the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period. The family includes some of the largest sea turtles that ever existed. The largest, ''
Archelon'', had a head long. Like most sea turtles, they had flattened bodies and
flippers
Flipper may refer to:
Common meanings
*Flipper (anatomy), a forelimb of an aquatic animal, useful for steering and/or propulsion in water
*Alternate name for a swimfin, footwear that boosts human swimming efficiency
* Flipper (cricket), a type of ...
for front appendages; protostegids had minimal
shells like
leatherback turtles of modern times.
Anatomy
As some of the first
marine turtles
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...
, the protostegids set the general body plan for future species of sea turtles. They had a generally depressed turtle body plan, complete with four limbs, a short tail, and a large head at the end of a relatively short neck. Like other sea turtles, they possessed oar-like front appendages especially evolved for swimming in the open ocean. Similar to the still-
extant, possibly closely related
Dermochelyidae, protostegids possessed extremely reduced
carapaces. Some specimens had skeletal protrusions from their ribs almost wrapping around their bodies in place of a complete shell. Like modern sea turtles, protostegids had sharp beaks. One of the defining characteristics of the members of the family was their almost-disproportionately large heads. Specifically, some specimens of ''
Archelon'' have been found with heads long. In addition, the members of the family had somewhat reduced plastrons, as well.
Ecology
Trophic ecology
While all members of the family are
extinct,
palaeoecological studies on the members of the family have provided some insight into the ecological roles of the Protostegidae. Analysis of fossil organs of some protostegids has revealed entire stomachs containing fossilized
shellfish.
The turtles, in turn, are postulated to have been preyed upon by the major predators of the time. Fossil protostegids have been found with tooth impressions from the large
lamnid
The Lamnidae are the family of mackerel sharks known as white sharks. They are large, fast-swimming predatory fish found in oceans worldwide, though prefer environments with colder water. The name of the family is formed from the Greek word ''lam ...
sharks of the time.
Two specimens of ''
Protostega gigas
''Protostega'' ('first roof') is an extinct genus of sea turtle containing a single species, ''Protostega gigas''. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas (''Hesperornis'' zone, dated to 83.5 mill ...
'' have been discovered to have tooth marks from large
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s. In addition, teeth of the extinct shark ''
Cretoxyrhina mantelli
''Cretoxyrhina'' (; meaning 'Cretaceous sharp-nose') is an extinct genus of large mackerel shark that lived about 107 to 73 million years ago during the late Albian to late Campanian of the Late Cretaceous. The type species, ''C. mantelli'', is ...
'' have been found embedded in at least one ''Protostega'' skeleton.
Evolutionary history
The family's oldest members are ''
Desmatochelys padillai
''Desmatochelys'' (from Greek δεσμός ''desmos'' 'link' + χέλῡς ''khelus'' ‘tortoise’) is an extinct genus of sea turtles belonging to the family Protostegidae. This genus contains two known species, ''D. lowii'' and ''D. padillai' ...
'', known from the specimens recovered from the
Early Cretaceous Paja Formation of
Colombia and ''
Santanachelys gaffneyi
''Santanachelys gaffneyi'' is an extinct species of sea turtle. It is the only species in the genus ''Santanachelys'', which itself is a member of the extinct family Protostegidae. The species was first described from a 20-centimeter long fossi ...
'', known from a specimen excavated from
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
in 1998. The latter species first appeared during the Early Cretaceous. As an early sea turtle, ''Santanachelys'' had several unspecialized characteristics, such as distinguishable digits in its flipper-like arms. Later relatives' flippers were completely fused together for more efficient swimming.
As with most large
fauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...
of the
era, the Protostegidae died out during the events of the
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
that led to the extinction of the
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s.
The exact
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
position of protostegids among turtles is uncertain. Some phylogenetic studies determine the
leatherback turtles in the family
Dermochelyidae to be their closest living relatives, with both these families being
monophyletic.
Conversely, the phylogenetic analyses conducted by Joyce (2007) and Anquetin (2012), which included one protostegid species (''Santanachelys gaffneyi''), recovered the family as only distantly related to leatherback turtles. Joyce (2007) recovered Protostegidae as basal
eucryptodiran turtles lying outside the
crown group of
Cryptodira (the least inclusive clade containing all living cryptodirans) and closely related to ''
Solnhofia parsonsi'';
''Santanachelys'' had a similar phylogenetic position in the analysis conducted by Anquetin (2012), who stressed that future studies should include more protostegids to confirm this phylogenetic placement.
If confirmed, these results would prove that protostegids weren't close relatives of leatherback turtles (or in fact any living cryptodirans), but instead "represent an independent lineage of marine turtles that originated in the Late Jurassic".
The analyses conducted by Sterli (2010) and Sterli & de la Fuente (2011) recovered ''Santanachelys'' (and, presumably, the entirety of Protostegidae) as even more distantly related to living cryptodirans; it was found to be basal turtle lying outside the crown group of turtles (the least inclusive clade containing cryptodirans and
pleurodira
The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differen ...
ns).
A phylogenetic analysis conducted by Cadena and Parham (2015) recovered Protostegidae within the crown group of Cryptodira; specifically the family was recovered as belonging to Chelonioidea and more closely related to the
leatherback sea turtle than
cheloniids are.
Taxonomic history
In 1888, the Belgian zoologist
George Albert Boulenger published his classification of the
Testudinata
Testudinata is the group of all tetrapods with a true turtle shell. It includes both modern turtles (Testudines) and many of their extinct, shelled relatives (stem-turtles). Though it was first coined as the group containing turtles by Jacob Th ...
within the 9th edition of the ''
Encyclopædia Britannica
The (Latin for "British Encyclopædia") is a general knowledge English-language encyclopaedia. It is published by Encyclopædia Britannica, Inc.; the company has existed since the 18th century, although it has changed ownership various t ...
''. The genus ''Protostega'' was placed within the family
Sphargidae under the suborder
Athecae. A year or so later, the entire suborder was downgraded by
Karl Alfred von Zittel
Karl Alfred Ritter von Zittel (25 September 1839 – 5 January 1904) was a German palaeontologist best known for his ''Handbuch der Palaeontologie'' (1876–1880).
Biography
Karl Alfred von Zittel was born in Bahlingen in the Grand Duchy o ...
into a family within the
Cryptodira.
In 1994, Hirayama proposed a three-family subdivision of the
sea turtle superfamily based on
cladistic analysis; Protostegidae was given full, formal family status in the system, containing most of the
extinct genera, including ''
Archelon'', and a previously undescribed protostegid.
The unidentified specimen was fully described in 1998, as the species ''
Santanachelys gaffneyi
''Santanachelys gaffneyi'' is an extinct species of sea turtle. It is the only species in the genus ''Santanachelys'', which itself is a member of the extinct family Protostegidae. The species was first described from a 20-centimeter long fossi ...
''. The genus ''
Santanachelys
''Santanachelys gaffneyi'' is an extinct species of sea turtle. It is the only species in the genus ''Santanachelys'', which itself is a member of the extinct family Protostegidae. The species was first described from a 20-centimeter long f ...
'' was appended to the family after the new species was described. This specimen was later to be analyzed to be the family's oldest member.
References
Further reading
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{{Testudines
Prehistoric reptile families
Aptian first appearances
Maastrichtian extinctions
Taxa named by Edward Drinker Cope
Extinct turtles