Metriorhynchus is an extinct genus of marine crocodyliform that lived in the oceans during the Middle to Late Jurassic. Metriorhynchus was named by the German palaeontologist Christian von Meyer in 1830. Metriorhynchus was a carnivore that spent much, if not its life out at sea. No Metriorhynchus eggs or nests have been discovered, so little is known of the reptile's life cycle, unlike other large marine reptiles of the Mesozoic, such as plesiosaurs or ichthyosaurs which are known to give birth to live young out at sea. Where Metriorhynchus mated, whether on land or at sea, is unknown; the name Metriorhynchus means "Moderate snout", is derived from the Greek Metrio- and -rhynchos. Fossil specimens referrable to Metriorhynchus are known from Middle-Late Jurassic deposits of England and Germany. Species in this genus were traditionally classed into two skull groups: longirostrine and brevirostrine. However, there has been some contention as to how many of these species are valid those from the Callovian.
All brevirostrine species have been transferred to the genera Purranisaurus and Suchodus. Eudes-Deslongchamps found there to be four Callovian species: M. superciliosus, M. moreli, M. blainvillei, M. brachyrhynchus. Andrews considered there to be seven valid species: M. superciliosus, M. moreli, M. brachyrhynchus, M. durobrivensis, M. cultridens, M. leedsi and M. laeve. Adams-Tresman using linear morphometrics however could only distinguish between the two skull groups, so she found there to be two species: M. superciliosus and M. brachyrhynchus. Vignaud however, considered there to be three Callovian species: M. superciliosus, M. brachyrhynchus and M. leedsi. Fragmentary remains attributed to Metriorhynchus are known from South America during the Bajocian and Bathonian. However, phylogenetic analysis has shown; the genera Purranisaurus and Suchodus have been considered junior synonyms of Metriorhynchus, Recent phylogenetic analyses however, do not support the monophyly of Metriorhynchus. Some of the longirostrine forms, however, do appear to form a natural group.
The cladogram presented below follows an analysis by Mark Young and Marco Brandalise de Andrade, published in November 2009. Cladogram after Cau & Fanti. Averaging 3 metres in length, Metriorhynchus was of a similar size to modern crocodiles. However, it had a streamlined body and a finned tail, making it a more efficient swimmer than modern crocodilian species. Metriorynchus had nasal salt glands which, like the salt glands of all other marine reptiles, were used to remove excess salt; this means that like Geosaurus it would have been able to "drink" salt-water and eat salty prey, such as cephalopods, without dehydrating. Metriorhynchus was a versatile and opportunistic predator, predating upon both the armoured ammonites and the fast moving fish. Though Metriorhynchus was an effective predator, it was vulnerable to predation from apex predators such as Liopleurodon which could grow to 6.39 meters in length. Since Metriorhynchus had lost its osteoderms, "armour scutes", to become more efficient swimmers it would have had little defense against larger marine predators.
List of marine reptiles Buffetaut E. 1982. "Radiation évolutive, paléoécologie et biogéographie des Crocodiliens mésosuchienes". Mémoires Societé Geologique de France 142: 1–88 Metriorhynchus fact files
Archosaurs are a group of diapsid amniotes whose living representatives consist of birds and crocodilians. This group includes all extinct dinosaurs, extinct crocodilian relatives, pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most recent common ancestor of living birds and crocodilians and all of its descendants, it includes two main clades: Pseudosuchia, which includes crocodilians and their extinct relatives, Avemetatarsalia, which includes birds and their extinct relatives. Archosaurs can be distinguished from other tetrapods on the basis of several synapomorphies, or shared characteristics, first found in a common ancestor; the simplest and most agreed synapomorphies of archosaurs include teeth set in sockets and mandibular fenestrae, a fourth trochanter. Being set in sockets, the teeth were less to be torn loose during feeding; this feature is responsible for the name "thecodont", which paleontologists used to apply to many Triassic archosaurs. Some archosaurs, such as birds, are secondarily toothless.
Antorbital fenestrae reduced the weight of the skull, large in early archosaurs, rather like that of modern crocodilians. Mandibular fenestrae may have reduced the weight of the jaw in some forms; the fourth trochanter provides a large site for the attachment of muscles on the femur. Stronger muscles allowed for erect gaits in early archosaurs, may be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic Permian-Triassic extinction event. There is some debate about; those who classify the Permian reptiles Archosaurus rossicus and/or Protorosaurus speneri as true archosaurs maintain that archosaurs first appeared in the late Permian. Some taxonomists classify both Archosaurus Protorosaurus speneri as archosauriforms; the earliest archosaurs were rauisuchians, such as Scythosuchus and Tsylmosuchus, both of which have been found from Russia and date back to the Olenekian. Synapsids were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event.
Few large synapsids survived the event, although one form, attained a widespread distribution soon after the extinction. Archosaurs became the dominant land vertebrates in the early Triassic. Fossils from before the mass extinction have only been found around the Equator, but after the event fossils can be found all over the world; the two most suggested explanations for this are: Archosaurs made more rapid progress towards erect limbs than synapsids, this gave them greater stamina by avoiding Carrier's constraint. An objection to this explanation is that archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of Lystrosaurus and other synapsids. Archosaurs have more efficient respiratory systems featuring unidirectional air flow. Dr. Peter Ward suggests this may have proven advantageous in a suspected drop in oxygen levels at the end of the Permian; the early Triassic was predominantly arid, because most of the earth's land was concentrated in the supercontinent Pangaea.
Archosaurs were better at conserving water than early synapsids because: Modern diapsids excrete uric acid, which can be excreted as a paste, resulting in low water loss as opposed to a more dilute urine. It is reasonable to suppose that archosaurs excreted uric acid, therefore were good at conserving water; the aglandular skins of diapsids would have helped to conserve water. Modern mammals excrete urea, which requires a high urinary rate to keep it from leaving the urine by diffusion in the kidney tubules, their skins contain many glands, which lose water. Assuming that early synapsids had similar features, e.g. as argued by the authors of Palaeos, they were at a disadvantage in a arid world. The same well-respected site points out that "for much of Australia's Plio-Pleistocene history, where conditions were similar, the largest terrestrial predators were not mammals but gigantic varanid lizards and land crocs."However, this theory has been questioned, since it implies synapsids were less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity and the fact that desert dwelling mammals as well adapted in this department as archosaurs, some cynodonts like Trucidocynodon were large sized predators.
Since the 1970s, scientists have classified archosaurs on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot; the Pseudosuchia appeared early in the Triassic. In their ankles, the astragalus was joined to the tibia by a suture and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some crurotarsans developed erect limbs. Modern crocodilians are crurotarsans that can walk with their lim
Mesoeucrocodylia is the clade that includes Eusuchia and crocodyliforms placed in the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, continues to the present day, it was long known that Mesosuchia was an evolutionary grade, a hypothesis confirmed by the phylogenetic analysis of Benton and Clark which demonstrated that Eusuchia was nested within Mesosuchia. Due to the paraphyly of Mesosuchia, Mesoeucrocodylia was erected to replace Mesosuchia. Several anatomical characteristics differentiate Mesoeucrocodylia from the other crocodylomorph clades; the frontal bones of the skull are fused together for example. Mesoeucrocodylians possess something of a secondary palate, formed by the posterior extension of sutured palatine bones; the otic aperture of the members of this clade is blocked posteriorly by the squamosal bone. Below is a cladogram from Calvo. Mesoeucrocodylia portal
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, snakes, lizards and their extinct relatives. The study of these traditional reptile orders combined with that of modern amphibians, is called herpetology; because some reptiles are more related to birds than they are to other reptiles, the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade. For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living Diapsids; the earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became adapted to life on dry land. Some early examples include Casineria. In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs and sauropods, as well as many species of theropods, including troodontids, dromaeosaurids and abelisaurids, along with many Crocodyliformes, squamates.
Modern non-avian reptiles inhabit all the continents except Antarctica, although some birds are found on the periphery of Antarctica. Several living subgroups are recognized: Testudines, 350 species. Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades – the fetus develops within the mother, contained in a placenta rather than an eggshell; as amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm to the saltwater crocodile, Crocodylus porosus, which can reach 6 m in length and weigh over 1,000 kg.
In the 13th century the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, assorted amphibians, worms", as recorded by Vincent of Beauvais in his Mirror of Nature. In the 18th century, the reptiles were, from the outset of classification, grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturæ; the terms "reptile" and "amphibian" were interchangeable, "reptile" being preferred by the French. Josephus Nicolaus Laurenti was the first to formally use the term "Reptilia" for an expanded selection of reptiles and amphibians similar to that of Linnaeus. Today, the two groups are still treated under the same heading as herptiles, it was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians and mammals.
The British anatomist Thomas Henry Huxley made Latreille's definition popular and, together with Richard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", including dinosaurs and the mammal-like Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into mammals and ichthyoids, he subsequently proposed the names of Ichthyopsida for the latter two groups. In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, that reptiles and mammals were united by the amniotic egg; the terms "Sauropsida" and "Theropsida" were used again in 1916 by E. S. Goodrich to distinguish between lizards and their relatives on the one hand and mammals and their extinct relatives on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, other features, such as the structure of the forebrain.
According to Goodrich, both lineages evolved from an earlier stem group, Protosauria in which he included some animals today considered reptile-like amphibians, as well as early reptiles. In 1956, D. M. S. Watson observed that the first two groups diverged early in reptilian history, so he divided Goodrich's Protosauria between them, he reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included Procolophonia, Millerosauria, Squamata, Rhynchocephalia
Sebecus is an extinct genus of sebecid crocodylomorph from Eocene of South America. Fossils have been found in among others Patagonia. Like other sebecosuchians, it was terrestrial and carnivorous; the genus is represented by a single species, the type S. icaeorhinus. Several other species have been referred to Sebecus, but were reclassified as their own genera. Named by American paleontologist George Gaylord Simpson in 1937, Sebecus was one of the first known sebecosuchians. Simpson described the type species, S. icaeorhinus, from a fragmented skull and lower jaw found in the Sarmiento Formation. The specimen was discovered by the American Museum of Natural History's First Scarritt Expedition to Patagonia. Teeth had been known since 1906 when Argentine paleontologist Florentino Ameghino associated them with carnivorous dinosaurs; the more complete material found by Simpson established that the new animal was a crocodyliform. Although Simpson's fossil was considered one of the best finds of the expedition, Simpson described the genus only in 1937.
He noted its unusual ziphodont dentition in which the teeth were laterally serrated. Simpson was preparing a more detailed monograph on the genus, but entered the United States Army before its completion. Another American paleontologist, Edwin Harris Colbert, completed Simpson's work describing the genus and placing it in a new family, Sebecidae. Colbert placed Sebecus and the Cretaceous baurusuchid Baurusuchus in the suborder Sebecosuchia, as both had deep snouts and ziphodont teeth; the name Sebecus is a Latinisation of the crocodile god of ancient Egypt. Sebek was considered "champsa" in crocodilian nomenclature; the specific name icaeorhinus of the type species is derived from the Greek words εικαίοs and ρύγχος. Εικαίοs means "random" or "not according to plan" and ρύγχος means "snout", in reference to the animal's unusually deep snout. In 1965, American paleontologist Wann Langston, Jr. named a second species, S. huilensis, from the Miocene Honda Group at the La Venta locality in Colombia.
S. huilensis was named on the basis of skull fragments. The deposits are Laventan in age, extending the range of the genus into the Neogene by around 40 million years. In 1977, remains were described from the Miocene of Peru. A third species of Sebecus, S. querejazus, was named in 1991 from the early Paleocene Santa Lucia Formation in Bolivia. This extended the range of Sebecus back to the beginning of the Paleogene, soon after the Cretaceous–Paleogene extinction event. In 1993, Gasparini et al. described Sebecus carajazus. This was not a fourth species but lapsus calami, of Sebecus querejazus. A 2007 study of sebecids reclassified several species; the two species S. huilensis and S. querejazus were given their own genera and Langstonia, respectively. Langstonia huilensis, named after Langston, was distinguished from Sebecus by its narrower snout and spaced teeth. Zulmasuchus querejazus, named after Zulma Gasparini, one of the authors of the study, differs from Sebecus in its wider snout; the postcranial skeleton of S. icaeorhinus was unknown until Pol et al. described postcranial remains of several individuals of this species, including a articulated specimen MPEF-PV 1776 with anterior region of the dentary and most of the postcranial skeleton preserved.
Estimates of total body length and mass of MPEF-PV 1776 vary from 2.2 to 3.1 m, from 52.2 to 113.5 kg, respectively. The postcranial skeleton of Sebecus provides additional evidence of its terrestriality, its limbs femora, were proportionally longer than limbs of living crocodilians. Unusual among crocodyliforms, Sebecus has a narrow snout; the nares, or nostrils, open anteriorly at the tip of the snout. While most crocodilians have flat skulls that are raised near the eyes and postorbital region behind the eyes, the skull of Sebecus is level; the great depth of the snout makes most of the length of its upper margin level with the margin of the orbits, or eye sockets. The supratemporal fenestrae, two holes on the skull table, are small. Laterally compressed, or ziphodont teeth, are characteristic of other sebecosuchians. Although the teeth vary in size, they are homodont. At the tips of the upper and lower jaws, the teeth are rounder in cross-section; the fourth dentary tooth is raised in the lower jaw to form an effective canine.
The foremost teeth of the lower jaw are lower than the fourth tooth. At the tip of the jaw the first dentary tooth directed forward; the teeth of the upper and lower jaws form an alternate pattern to allow the jaw to close tightly. A notch is present between the maxilla and premaxilla bones of the upper jaw, accommodating the fourth dentary tooth when the jaw is closed; the procumbent first dentary teeth fit between the second premaxillary teeth. This close fit allows the serrated edges of the teeth shear with one another; the articulation between the articular and quadrate bones at the jaw joint is well developed. Along with the broad downturned "wings" formed by the pterygoid and ectopterygoid bones at the bottom of