Misplaced Pages

#672327

69-1116: Geologic formation in Nunavut, Canada Fram Formation Stratigraphic range : Upper Devonian ( Frasnian ) PreꞒ Ꞓ O S D C P T J K Pg N [REDACTED] Photograph of rocks of the Fram Formation, indicating where the Qikiqtania (NV0401) and Tiktaalik (NV2K17) specimens have been found Type Geological formation Unit of Okse Bay Group Underlies Hell Gate Formation Overlies Hecla Bay Formation Thickness 10 to 20 meters Lithology Primary Siltstone Other Sandstone Location Coordinates 77.164917, -86.269283 Region [REDACTED]   Nunavut Country [REDACTED]   Canada Extent Ellesmere Island [REDACTED] The discovery site of Tiktaalik and Laccognathus embryi fossils in

138-452: A Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps: The first officially described Silurian placoderm is an antiarch, Shimenolepis , which is known from distinctively ornamented plates from Hunan , China. It

207-652: A close relative of Tiktaalik , were also found in the formation. The Fram Formation is a Middle to Upper Devonian clastic wedge forming an extensive continental facies consisting of sediments derived from deposits laid down in braided stream systems that formed some 375 million years ago, at a time when the North American craton (" Laurentia ") was straddling the equator. Paleobiota [ edit ] Vertebrates [ edit ] Genus Species Location Material Notes Images Asterolepidoidei Indeterminate NV0207 locality, near

276-539: A feature shared by birds and some ichthyosaurs . Early arthrodires, such as the genus Arctolepis , were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus , was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm. Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young. Antiarchi ("opposite anus") were

345-468: A group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, acanthoracids resembled scaly chimaeras or small, scaly arthrodires with blunt rostrums . They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in

414-513: A mosaic of tubercles. Like Stensioella heintzi , the pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms, they may be a group more advanced than the ptyctodonts . As such, placoderm experts consider Pseudopetalichthyida to be

483-407: A movable joint between armour surrounding the head and body. As the lower jaw moved down, the head shield moved, allowing for a larger opening. All arthrodires, save for Compagopiscis , lacked teeth, and used instead the sharpened edges of a bony plate, termed a "tooth plate," as a biting surface ( Compagopiscis had true teeth in addition to tooth plates). The eye sockets are protected by a bony ring,

552-665: A partial thoracic skeleton (NUFV 1294) and of a posterior median dorsal plate (NUFV 1293). A placoderm . [REDACTED] Eusthenopteron E. jenkinsi NV2K17 locality. A tristichopterid . [REDACTED] Holoptychius H. bergmanni NV2K17 locality (77º09.895’N, 86º16.157’W) near the eastern arm of Bird Fiord. A holoptychiid . Laccognathus L. embryi Specimens from at least 22 individuals known. A holoptychiid . [REDACTED] Qikiqtania Q. wakei Site NV0401 (77° 10.235′ N, 86° 11.279′ W) Specimen preserving

621-504: A primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist Philippe Janvier , as well as other paleontologists, has suggested that Stensioella is not a placoderm, but instead is a holocephalian . If this is true, then the holocephalians diverged from sharks before the Chondrichthyan Devonian radiation. Critics of Janvier's position say that aside from

690-775: A proxy for the age at which a surface, such as an alluvial fan, was created. Burial dating uses the differential radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure. Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, including optically stimulated luminescence (OSL), cathodoluminescence (CL), and thermoluminescence (TL). Thermoluminescence and optically stimulated luminescence are used in archaeology to date 'fired' objects such as pottery or cooking stones and can be used to observe sand migration. Incremental dating techniques allow

759-487: A reference for newly obtained poles for the rocks with unknown age. For paleomagnetic dating, it is suggested to use the APWP in order to date a pole obtained from rocks or sediments of unknown age by linking the paleopole to the nearest point on the APWP. Two methods of paleomagnetic dating have been suggested: (1) the angular method and (2) the rotation method. The first method is used for paleomagnetic dating of rocks inside of

SECTION 10

#1732798718673

828-484: A skinny Gemuendina with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the rhenanids . It is tentatively placed within Placodermi as

897-794: Is also correct to say that fossils of the genus Tyrannosaurus have been found in the Upper Cretaceous Series. In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit – such as the Hell Creek deposit where the Tyrannosaurus fossils were found – but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time. Placodermi Placoderms (from Greek πλάξ ( plax , plakos ) ' plate ' and δέρμα ( derma ) 'skin') are vertebrate animals of

966-500: Is also often used as a dating tool in archaeology, since the dates of some eruptions are well-established. Geochronology, from largest to smallest: It is important not to confuse geochronologic and chronostratigraphic units. Geochronological units are periods of time, thus it is correct to say that Tyrannosaurus rex lived during the Late Cretaceous Epoch. Chronostratigraphic units are geological material, so it

1035-461: Is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloging and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted. Both disciplines work together hand in hand, however, to

1104-415: Is likely monophyletic . The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. They eventually outcompeted the previously dominant marine arthropods (e.g. eurypterids ) and cephalopod molluscs (e.g. orthocones ), producing some of the first and most infamous vertebrate apex predators such as Eastmanosteus , Dinichthys and

1173-598: Is not clear, as none of the fossil specimens found have preserved mouth parts. Pseudopetalichthyida ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata in Hunsrück , Germany. Like Stensioella heintzi , and the Rhenanida , the pseudopetalichthids had armour made up of

1242-532: The Ar/ Ar dating method can be extended into the time of early human life and into recorded history. Some of the commonly used techniques are: A series of related techniques for determining the age at which a geomorphic surface was created ( exposure dating ), or at which formerly surficial materials were buried (burial dating). Exposure dating uses the concentration of exotic nuclides (e.g. Be, Al, Cl) produced by cosmic rays interacting with Earth materials as

1311-468: The Gogo Formation of Western Australia, had streamlined, bullet-shaped head armor, and Amazichthys , with morphology like that of other fast-swimming pelagic organisms , strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the sea floor. Some placoderms were herbivorous, such as

1380-565: The class Placodermi , an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods . While their endoskeletons are mainly cartilaginous , their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species . Placoderms were among the first jawed fish (their jaws likely evolved from

1449-508: The hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward. A study on Kolymaspis showcases that

SECTION 20

#1732798718673

1518-1211: The "lower jaws, partial left upper jaw and palate in articulation, gulars, ceratohyals, an articulated left pectoral fin and articulated scales from the dorsal midline, flank and lateral line series" An elpistostegalian . [REDACTED] Tiktaalik T. roseae Site NV2K17, southern Ellesmere Island "Based on a suite of specimens (NUFV 108–135) from a single locality (NV2K17). Three specimens (NUFV 108–110) preserve skulls, pectoral girdles and fins in articulation." A stegocephalian . [REDACTED] See also [ edit ] [REDACTED] Earth sciences portal [REDACTED] Paleontology portal [REDACTED] Canada portal List of fossiliferous stratigraphic units in Nunavut Escuminac Formation , another Frasnian-aged formation known for its sarcopterygian fossils. References [ edit ] ^ Downs, Jason P.; Daeschler, Edward B.; Lo, Nathanael; Carey, Emily N.; Shubin, Neil H. (October 2019). "Asterolepis alticristata n. sp. (Antiarchi) from

1587-724: The Academy of Natural Sciences of Philadelphia. 162 (1): 47–59. doi:10.1635/053.162.0104. ISSN 0097-3157. ^ Jason P. Downs; Edward B. Daeschler; Farish A. Jenkins Jr.; Neil H. Shubin (2011). "A new species of Laccognathus (Sarcopterygii, Porolepiformes) from the Late Devonian of Ellesmere Island, Nunavut, Canada". Journal of Vertebrate Paleontology. The Society of Vertebrate Paleontology. 31 (5): 981–996. doi:10.1080/02724634.2011.599462 ^ Stewart, Thomas A.; Lemberg, Justin B.; Daly, Ailis; Daeschler, Edward B.; Shubin, Neil H. (2022-07-20). "A new elpistostegalian from

1656-441: The Fram Formation on Ellesmere Island. The Fram Formation is an Upper Devonian ( Frasnian ) sequence of rock strata on Ellesmere Island that came into prominence in 2006 with the discovery in its rocks of examples of the transitional fossil , Tiktaalik , a sarcopterygian or lobe-finned fish showing many tetrapod characteristics. Fossils of Laccognathus embryi , a porolepiform lobe-finned fish, and Qikiqtania ,

1725-484: The Late Devonian of the Canadian Arctic". Nature. doi:10.1038/s41586-022-04990-w. ISSN 0028-0836 . ^ Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins Jr, 2006. "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan" Nature , April 2006, pp 757ff (pdf file) Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins Jr, 2006. "A Devonian tetrapod-like fish and

1794-499: The Middle Devonian of Australia, suggesting that the basipterygia were used in copulation. The placoderm claspers are not homologous with the claspers in cartilaginous fishes . The similarities between the structures has been revealed to be an example of convergent evolution . While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in

1863-565: The Middle to Late Devonian arthrodire Holonema , and some were planktivores , such as the gigantic arthrodire Titanichthys , various members of Homostiidae , and Heterosteus . Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing , Kimberley , Western Australia, of a small female placoderm, about 25 cm (10 in) in length, which died in

1932-627: The Upper Devonian (Frasnian) of Nunavut, Canada, and a report on the antiarch diversity of the Fram Formation" . Geodiversitas . 41 (1): 679–698. doi : 10.5252/geodiversitas2019v41a19 . ISSN   1280-9659 . ^ Fossil pollen suggest that these rocks lie within the early and middle Frasnian Stage of the Devonian. ^ Jason P. Downs; Edward B. Daeschler; Alison M. Long; Neil H. Shubin (2018). " Eusthenopteron jenkinsi sp. nov. (Sarcopterygii, Tristichopteridae) from

2001-430: The Upper Devonian of Nunavut, Canada, and a review of Eusthenopteron taxonomy" (PDF) . Breviora . 562 : 1–24. doi : 10.3099/MCZ44.1 . S2CID   51883892 . ^ Downs, Jason P.; Daeschler, Edward B.; Jenkins, Farish A.; Shubin, Neil H. (2013-03). " Holoptychius bergmannisp. nov. (Sarcopterygii, Porolepiformes) from the Upper Devonian of Nunavut, Canada, and a Review of Holoptychius Taxonomy ". Proceedings of

2070-409: The amount of radioactive decay of a radioactive isotope with a known half-life , geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With

2139-418: The anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to

Fram Formation - Misplaced Pages Continue

2208-448: The antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus , Bothriolepis , known from over 100 valid species. The vast majority of placoderms were predators , many of which lived at or near the substrate . Many, primarily the arthrodires , were active, nektonic predators that dwelled in

2277-436: The construction of year-by-year annual chronologies, which can be fixed ( i.e. linked to the present day and thus calendar or sidereal time ) or floating. A sequence of paleomagnetic poles (usually called virtual geomagnetic poles), which are already well defined in age, constitutes an apparent polar wander path (APWP). Such a path is constructed for a large continental block. APWPs for different continents can be used as

2346-473: The date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire Incisoscutum ritchei , also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability. The males reproduced by inserting a long clasper into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis and Cowralepis , both from

2415-474: The environmental catastrophes of the Late Devonian and end-Devonian extinctions . The earliest identifiable placoderm fossils are of Chinese origin and date to the early Silurian . At that time, they were already differentiated into antiarchs and arthrodires , as well as other, more primitive, groups. Earlier fossils of basal placoderms have not yet been discovered. The Silurian fossil record of

2484-521: The equally enigmatic Pseudopetalichthyida . These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten . Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giant apex predators to detritus -nibbling bottom dwellers . They had

2553-483: The evolution of the tetrapod body plan" Nature , April 2006, pp 757ff (pdf file) Retrieved from " https://en.wikipedia.org/w/index.php?title=Fram_Formation&oldid=1220738835 " Categories : Devonian System of North America Fluvial deposits Devonian southern paleotropical deposits Upper Devonian Series Hidden categories: Articles with short description Short description matches Wikidata Geochronology Geochronology

2622-416: The exception of the radiocarbon method , most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope. Two or more radiometric methods can be used in concert to achieve more robust results. Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and

2691-449: The exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other gnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase. Placoderms also share certain anatomical features only with

2760-489: The extinct and related acanthothoracids , and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish . On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, but holocephalians or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that

2829-693: The extinction event at the Frasnian – Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into the Carboniferous . The earliest studies of placoderms were published by Louis Agassiz , in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms . Some naturalists even suggested that they were shelled invertebrates or even turtle -like vertebrates. In

Fram Formation - Misplaced Pages Continue

2898-648: The first pair of gill arches ), as well as the first vertebrates to have true teeth . They were also the first fish clade to develop pelvic fins , the second set of paired fins and the homologous precursor to hindlimbs in tetrapods . 380-million-year-old fossils of three other genera, Incisoscutum , Materpiscis and Austroptyctodus , represent the oldest known examples of live birth . Placoderms are thought to be paraphyletic , consisting of several distinct outgroups or sister taxa to all living jawed vertebrates , which originated among their ranks. In contrast, one 2016 analysis concluded that Placodermi

2967-400: The jawless osteostracans ; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes. Currently, Placodermi are divided into eight recognized orders . There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella ; the other is

3036-520: The late 1920s, Dr. Erik Stensiö , at the Swedish Museum of Natural History in Stockholm , established the details of placoderm anatomy and identified them as true jawed fishes related to sharks . He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in wax . Once

3105-515: The limbs is still not perfectly understood, but most hypothesize that they helped their owners pull themselves across the substrate, as well as allowing their owners to bury themselves into the substrate. Brindabellaspis (" Brindabella's shield") was a long-snouted placoderm from the Early Devonian . When it was first discovered in 1980, it was originally regarded as a weejasperaspid acanthothoracid due to anatomical similarities with

3174-421: The males having pelvic claspers and possibly claspers on the head as well. Rhenanida (" Rhine fish") were flattened, ray-like , bottom-dwelling predators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to

3243-477: The massive Dunkleosteus . Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-Devonian Hangenberg event 358.9 million years ago, leaving the niches open for the osteichthyan and chondrichthyan survivors who subsequently radiated during the Carboniferous . Many placoderms, particularly the Rhenanida , Petalichthyida , Phyllolepida , and Antiarchi , were bottom-dwellers. In particular,

3312-401: The mid-Devonian extinction event. Petalichthyida ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the Early Devonian and were found throughout the world. The petalichthids Lunaspis and Wijdeaspis are among the best known. There

3381-450: The middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavities and were made of both bone and dentine . However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to

3450-711: The mouth of Vendom Fiord . A placoderm . Asterolepis A. alticristata NV2K17 locality. Numerous specimens. A placoderm . A. cf. radiata NV2K17 locality. Multiple plates. A placoderm . A. sp. indet. Locality NV0205. Anterior median dorsal plate (NUFV 1287). A placoderm . Bothriolepididae Sp. indet. NV0402, NV2K15, NV2K05, NV2K01 & NV2K07 localities. Partial cephalic skeleton (NUFV 1292) & several plates. A placoderm . Bothriolepis B. sp. indet. NV2K17 & NV0402 localities. Anterior median dorsal plate (NUFV 847), visceral impressions of

3519-425: The other species found at the same locality. According to Philippe Janvier , anatomical similarities in the brain of Brindabellaspis stensioi and the brain of a jawless fish suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placoderm incertae sedis have also been inserted in the order. Phyllolepida ("leaf scales") were flattened placoderms found throughout

SECTION 50

#1732798718673

3588-434: The placoderms is both literally and figuratively fragmented. Until the discovery of Silurolepis (and then, the discoveries of Entelognathus and Qilinyu ), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of

3657-456: The point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming sinuous , particularly with later forms. The pair of pectoral fins were modified into a pair of caliper -like, or arthropod -like limbs. In primitive forms, such as Yunnanolepis , the limbs were thick and short, while in advanced forms, such as Bothriolepis , the limbs were long and had elbow-like joints. The function of

3726-459: The point where they share the same system of naming strata (rock layers) and the time spans utilized to classify sublayers within a stratum. The science of geochronology is the prime tool used in the discipline of chronostratigraphy , which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies . By measuring

3795-476: The process of giving birth to a 6 cm ( 2 + 1 ⁄ 2  in) offspring and was fossilized with the umbilical cord intact. The fossil, named Materpiscis attenboroughi (after scientist David Attenborough ), had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young (" viviparous "), pushing

3864-426: The same age and of such distinctive composition and appearance that, despite their presence in different geographic sites, there is certainty about their age-equivalence. Fossil faunal and floral assemblages , both marine and terrestrial, make for distinctive marker horizons. Tephrochronology is a method for geochemical correlation of unknown volcanic ash (tephra) to geochemically fingerprinted, dated tephra . Tephra

3933-700: The same continental block. The second method is used for the folded areas where tectonic rotations are possible. Magnetostratigraphy determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale. The polarity timescale has been previously determined by dating of seafloor magnetic anomalies, radiometrically dating volcanic rocks within magnetostratigraphic sections, and astronomically dating magnetostratigraphic sections. Global trends in isotope compositions, particularly carbon-13 and strontium isotopes, can be used to correlate strata. Marker horizons are stratigraphic units of

4002-565: The scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the Devonian . During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater . But this diversity ultimately suffered many casualties during

4071-533: The second most successful order of placoderms known, after the Arthrodira . The order's name was coined by Edward Drinker Cope , who, after incorrectly identifying the first fossils as being those of an armored tunicate , mistakenly thought the eye-hole was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to

4140-561: The similarities between these two groups are superficial. The major differences were that holocephalians have shagreen on their skin, while ptyctodontids do not; the armoured plates and scales of holocephalians are made of dentine , while those of ptyctodontids are made of bone; the craniums of holocephalians are similar to sharks, while those of ptyctodontids are similar to those of other placoderms; and, most importantly, that holocephalians have true teeth, while ptyctodonts have beak-like tooth plates. Ptyctodontids were sexually dimorphic , with

4209-604: The sister group of the Arthrodires + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy . Stensioellida ("[Heintz's] little Stensio ") contains another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany. Stensioella was a thin fish that, when alive, looked vaguely like an elongated ratfish , or

SECTION 60

#1732798718673

4278-450: The solidified plates of "advanced" placoderms, such as antiarchs and arthrodires . However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their "mosaics" come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species. Acanthothoraci ("spine chests") were

4347-412: The specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with sharks ; however, as more fossils were found, placoderms were accepted as a sister group of chondrichthyans . Much later,

4416-516: The teeth of other gnathostomes. One of the largest known arthrodires, Dunkleosteus terrelli , was 3.5–4.1 metres (11–13 ft) long, and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus , both of

4485-419: The top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the phyllolepids may have been blind. Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have a strong but superficial resemblance to modern day chimaeras . Their armour was reduced to a pattern of small plates around the head and neck. Like

4554-455: The vertebrate shoulder girdle evolved from gill arches. It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks , given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from

4623-400: The world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such as stargazers or flatfish , which have eyes on

4692-472: Was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida", named after the first discovered species there, Quasipetalichthys haikouensis . Soon after the petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that pursued or ambushed smaller fish. Their diet

4761-440: Was originally considered to be from the late Llandovery , although later study reconsidered its age at Ludfordian . Shimenolepis plates are very similar to the early Devonian yunnanolepid Zhanjilepis , also known from distinctively ornamented plates. In 2022, Xiushanosteus is described from complete fossils from Telychian , late Llandovery of Chongqing , China. Paleontologists and placoderm specialists suspect that

#672327