| Belemnites | |
|---|---|
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| The Early JurassicPassaloteuthis bisulcata showing soft anatomy | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Superorder: | †Belemnoidea |
| Order: | †Belemnitida Zittel, 1895 |
| Suborders | |
Belemnitida (orbelemnites) is anextinctorder ofsquid-likecephalopods that existed from theLate Triassic toLate Cretaceous (And possibly theEocene[4][5]). Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most to the tip, the tongue-shaped pro-ostracum, the conicalphragmocone, and the pointy guard. Thecalcitic guard is the most common belemnite remain. Belemnites, in life, are thought to have had 10 hooked arms and a pair of fins on the guard. Thechitinous hooks were usually no bigger than 5 mm (0.20 in), though a belemnite could have had between 100 and 800 hooks in total, using them to stab and hold onto prey.
Belemnites were an important food source for many Mesozoic marine creatures, both the adults and the planktonic juveniles and they likely played an important role in restructuring marine ecosystems after theTriassic–Jurassic extinction event. They may have laid between 100 and 1,000 eggs. Some species may have been adapted to speed and swam in the turbulentopen ocean, whereas others resided in the calmerlittoral zone (nearshore) and fed off the seafloor. The largest belemnite known,Megateuthis elliptica, would have measured up to 3.11 metres (10.2 ft) in total body length.[6]
Belemnites werecoleoids, a group that includes squid andoctopuses, and are often grouped into the superorderBelemnoidea, though the higher classification of cephalopods is volatile and no clear consensus exists on how belemnites are related to modern coleoids. Guards can give information on the climate, habitat, and carbon cycle of the ancient waters they inhabited. Guards have been found since antiquity and have become part of folklore.[7][8]
The belemnite cone is composed of three parts. Going from arms to tip, these are the tongue-shaped pro-ostracum; the conical, chamberedphragmocone; and the spear-shaped guard at the very tip.[9][10] The guard is attached to the phragmocone in a socket called the alveolus.[10][11] The cone, in life, would have been encased in muscle andconnective tissue. They hadcalcite guards,[12] andaragonite pro-ostraca and phragmocones,[9] though a few belemnites also had aragonite guards,[13] and the alveolar side of the guards ofbelemnitellids may have also been of aragonite.[11] The pro-ostracum probably supported the soft parts of the belemnite, similar to thegladius of squid, and completely surrounded the phragmocone.[10][14]
The phragmocone was divided bysepta into chambers, much like the shells of cuttlefish andnautiluses.[14] The chambered phragmocone was probably thecenter of buoyancy, so was positioned directly above thecenter of mass for stability purposes. Concerning buoyancy, belemnites may have behaved much like modernram's horn squid, having the chambers of the phragmocone flooded and slowly releasing more seawater via thesiphuncle tube as the animal increases in size and weight over its lifetime to maintainneutral buoyancy.[9] At the tip of the phragmocone beneath the guard is a tiny, cup-likeprotoconch, the remains of theembryonic shell.[1][14]
The dense guard probably served tocounterbalance the weight of the soft parts in themantle cavity near the arms on the opposite end of the animal,analogous to thecamera ofnautiloids. This would have allowed the animal to move horizontally through the water.[9][10] The guard may have also served to cut through waves while swimming at the surface, though modern cephalopods generally stay completely submerged. Though unlikely, fossilization possibly increased the perceived density of the guard, and it may have been up to 20% more porous in life. Fins may have been attached to the guard, or the guard may have lent support for large fins. Including arms, guards could have accounted for one-fifth to one-third of the total length of a belemnite.[9]
Belemnites had aradula – the "tongue" embedded in thebuccal mass, the first part of agastropod digestive system – similar to open-ocean predatory cephalopods. The radula had rows of seven teeth, consistent with modern predatory squid. Thestatocysts – which give a sense ofbalance and function much like thecochlea of the ear – were large, much like in modern fast-moving squid.[12] Like other cephalopods, the skin was likely thin and slippery. The eyeballs were likely thicker, stronger, and more convex than in other cephalopods.[15]
The mantle cavity of cephalopods serves to contain the gills,gonads, and other organs; also, water is siphoned into and expelled out of the mantle cavity via a tube opening near the arms of the animal, thehyponome, forjet propulsion. Though the hyponome was well-developed in belemnites,[12] the phragmocone was large, implying a small mantle cavity and thus less efficient jet propulsion. Like some modern squid, belemnites may have mainly used large fins to coast alongcurrents.[9] TwoAcanthoteuthis specimens with preserved soft-anatomy elements had a pair of rhomboid fins near the top of their guards; however, the specimens had different-sized fins, possibly owing tosexual dimorphism, age, or distortion during fossilization. These specimens appeared to have had similar adaptations to modern squid for speed, and may have been able to reach similar maximum speeds of 1.1 to 1.8 km/h (0.68 to 1.12 mph) like modern migratingTodarodesflying squid.[12]
Belemnites had 10 hooked arms of more or less equal length with suckers.[13] The hooks were rarely larger than 5 mm (0.20 in), and increased in size toward the midsection of the arm, possibly because the midsection is where maximum power could be exerted when grabbing, or bigger hooks on the extremities of the arm increased the risk of losing the arm. Having two rows of hooks covering the entire breadth of the arm, a belemnite could have had between 100 and 800 hooks in total.[16][17] Some hooks have a spur just above the base, but this may be a distortion from fossilization or preparation of the material.[18] Thechitinous hooks are subdivided into three sections: The base - which can be either flat or concave - the shaft - which projects either upward at an incline either straight or bent - and the uncinus - which can be hook- orsaber-like.[17] Overall, they were fish-hook shaped, and probably only the uncinus was exposed.[16]
Different hook shapes were probably specialized for certain tasks,[19][20][21] for example, a strongly hooked uncinus was designed to stab prey at a constant angle. It would force and sink in deeper if the prey tried to move away from the belemnite. Hook shapes and forms vary from species to species. InChondroteuthis, large hooks were common near the mouth, and were either used for surrounding small prey or ramming into large prey, but these large hooks were not present in a small specimen, indicating it was either a juvenile, and the development of different hooks coincided with a difference in prey selection, or the specimen was a female and the hooks were used by males for male-on-male combat or during copulation. In modern hook-bearing squid species, only matured males have hooks, indicating a reproductive purpose. The hooks, being analogous to suckers, possibly could move.[17]
The males, like in modern squid, probably had one or twohectocotyli - long, modified arms used in copulation or combat with other males. Instead of several hooks, the hectocotyli feature a pair of enlarged hooks—mega-onychites—to latch onto the female at a safe distance to prevent getting stuck with one of her hooks. Like squid, the positioning of the mega-onychites could have been either at the tip or origin of the arm depending on the species. Copulation probably involved the male depositingspermatophores into the female's internal mantle chamber.[16]
Like other cephalopods, belemnites may have laid floating egg masses,[14] and a single female may have laid between 100 and 1,000 eggs.[22] Hatchlings were either miniature forms of adults or went through a larval stage. According to the latter model, the egg was formed by the protoconch and a single-layered shell wall. During the larval stage, the protoconch became internal and the guard began to form. The embryo ofPassaloteuthis, the most well-studied among belemnite embryos, had a protoconch, a developing guard, and a solid guard. The developing guard tightly surrounded the protoconch. The embryonic shell consisted of an ovoid protoconch and several chambers. The protoconch had two layers, and several compartments - called "protoconch pockets" - formed between the layers, which may have stored gas or liquid in life to stay buoyant. The protoconch and guard were probably made of chitin, a protective material that may have allowed the embryo to survive at greater depths and colder temperatures, develop into adults faster, and allow juveniles and adults to venture into deeper waters.[23] Further, the protoconch would have allowed them to form limbs before reaching the phragmocone stage, and thus inhabit the open ocean earlier. These may have allowed belemnites to colonize a range of habitats across the world.[23][24]
Much like in cuttlefish, nautiluses, andammonites, the number and successive size of the chambers of the phragmocone are used to analyze the growth of an individual over its life. Successive belemnite chambers tend to increase in size exponentially. Unlike other cephalopods, no decreasing trend of chamber size is seen in the earliest stages. The decreasing trend generally coincides with hatching, meaning embryonic belemnites had no or few chambers and hatched only with a protoconch. The phragmocone, thus, developed after hatching. Ammonites are thought to have done the same, implying a similar reproductive strategy, and, considering both reachedcosmopolitan distributions, a rather efficient one. Belemnite hatchling protoconches are estimated to have been generally around 1.5 to 3 mm (0.059 to 0.118 in).[14]
The guards ofMegateuthis elliptica are the largest among belemnites, measuring 60 to 70 cm (24 to 28 in) in length[25] and up to 50 mm (2.0 in) in diameter.[26] The CretaceousNeohibolites is one of the smallest known with a guard length of around 3 cm (1.2 in).[27] In the New ZealandBelemnopsis, four major annual growth stages were preserved in the guard, giving belemnites a lifespan of about three to four years.[28] Themesohibolitid belemnites, using the same methods, had a lifespan of about a year.[29] InMegateuthis, the guard was demonstrated to have fully developed after one or two years, and growth spurts followed thelunar cycle.[30]
_guard_with_a_zigzag-like_deformation_-_photo_vs._MRI.png)
Belemnite guards have sometimes been found with fractures with signs of healing. These have been interpreted in the past as evidence of digging, with belemnites using their guard to dig up prey on the seafloor; belemnites are now generally interpreted to have been open ocean predators. A deformed, zigzag-like guard of aGonioteuthis was likely the result of a failed predation attempt. Two otherGonioteuthis guard specimens exhibit a double-pointed tip, probably stemming from some traumatic event. One belemnite guard also presents a double-pointed tip, with one of the points projecting higher than the other, probably a sign of an infection or settlement of a parasite. ANeoclavibelus guard features a large growth on the side likely stemming from a parasitic infection. AHibolithes guard shows a large ovoid bubble near the base, likely deriving from a parasiticcyst.[31] AGoniocamax guard has several blister-like formations, thought to have come from apolychaeteflatworm infection.[32]
The calcitic guards were desirable habitats for boring parasites indicated by the diversity oftrace fossils left on some guards, including the spongeEntobia, wormTrypanites, and barnacleRogerella.[33][34]
Belemnites, beingcoleoids, derive from theorthoconic (conical)DevonianbelemnoidorderAulacocerida, which, in turn, is derived from the DevonianBactritida.[35] Belemnites were traditionally thought to have evolved in northern Europe in theHettangian stage of theEarly Jurassic 201.6–197million years ago (mya) and later spread to the rest of the world by thePliensbachian stage 190 mya. However, the 2012 discovery of early Asian forms classified into thefamilySinobelemnitidae[1]—now moves this to around 234 mya in theCarnian stage of theLate Triassic. Belemnites probably originated in the Asian part of thePanthalassic Ocean around the eastern coasts of the ancient continent ofLaurasia in a cephalopodradiation, alongside the octopus-likePrototeuthina and the belemnoidPhragmoteuthida.[36][26] A dubiousPermian occurrence, thePalaeobelemnopsidae, was reported from Southern China.[37] By the Early Jurassic, belemnites were probably quite common, having spread out into the western Laurasian coasts andGondwanan waters to the south.[38][26]
Guard shapes in the early Jurassic ranged from conical to spearheaded, but spearheaded became more prevalent as the Jurassic progressed. This was probably due to pressure to become more streamlined and increase swimming efficiency,coevolving with increasingly faster predators and competitors. Their early evolution and apparent abundance were likely important in reconstructing marine ecosystems after theTriassic–Jurassic extinction event, providing an ample food source for marine reptiles and sharks.[38]
The Belemnoidea, as a group, seemed to feature a reduction of the projection of the otherwise-conical phragmocone into the pro-ostracum. That of the most ancient order Aulacocerida is orthoconic (none projects), Phragmoteuthida three-quarters projects, Belemnitida a quarter, and the most developedDiplobelida an eighth.[39]
The first mention of belemnites in writing comes from the Greek philosopherTheophrastus, who lived in the 4th and 3rd centuryBCE, in his bookDe Animalibus Quæ Dicuntur Invidere who described it aslyngurium,lynx urine which had been buried and solidified.Pliny the Elder, in the first century CE, did not believe in lyngurium and called the gemstone a belemnite for the first time—though not recognizing it as a fossil.[40] The name is fromAncient Greek βέλεμνονbélemnon meaning dart for the guard's shape.[41][15] Subsequent authors either considered it to be lyngurium oramber. The first mention of a belemnite representing a fossil was made in 1546 by GermanmineralogistGeorgius Agricola, and subsequent authors gave several hypotheses to its nature in life, including them beingshellfish,sea urchin spines,sea cucumbers,coral polyps, or some internal shell.[40]
In 1823, English naturalistJohn Samuel Miller classified belemnites as cephalopods,[40] comparing the newly discovered phragmocone remains to that of a nautilus, and concluding a resemblance toSepiacuttlefish. He also erected thegenusBelemnites with 11 species.[41] This classification was confirmed when the first impressions of belemnite soft body anatomy were described by English paleontologistRichard Owen in 1844.[15] In 1895, German paleontologistKarl Alfred Ritter von Zittel organized thecladeBelemnoidea and included the familiesBelemnitidae,Asteroconites, andXiphoteuthis.[42]
The guard—also known as the rostrum, scabbard, gaine, and sheath[42]—is the part of the animal most likely to be fossilized.[9][35] Guards are difficult to distinguish at the species level, and, consequently,synonyms are common and inflate the group's apparent diversity.[38] Preserved hooks can be used to distinguish belemnite species as each species has unique hook shapes. However,scolecodontsegmented worm fossils have been mistaken for belemnite hooks and vice versa.[18] Preserved fossil guards are used to measure the ancientisotopic signature of the waters the individual inhabited in life, which gives information on the climate, habitat, andcarbon cycle.[12][43]
Belemnites werecephalopods. Having no outer shells, they are classified into thesubclass Coleoidea.[12] In 1994, American geologist Peter Doyle defined Coleoidea as composed of three superorders:Decapodiformes (squid andcuttlefish),Octopodiformes (octopuses), and Belemnoidea; with Belemnoidea containing the ordersAulacocerida,Diplobelida, and Belemnitida. Also, the orderPhragmoteuthida is sometimes believed to be asister group to Belemnoidea, but Doyle considered it to be astem-group to Decapodiformes and Octopodiformes.[44]
| Coleoidea |
| Classification of Coleoidea according to Doyle 1994[44] |
However, the higher classification of cephalopods is volatile with no clear consensus. Coleoidea is sometimes divided intoNeocoleoidea (containing all modern cephalopods) andPaleocoleoidea (containingBelemnoidea), so belemnites would be asister group of modern cephalopods. However, this grouping is probablyparaphyletic—it does not contain a common ancestor and all its descendants—and, thus, invalid.[45] According to some authors, belemnites were astem-group of Decapodiformes:
| Cephalopoda |
| Top: Belemnitida outside Decapodiformes.[46] Bottom: Belemnitida as astem-group of Decapodiformes[12] |
According to the "belemnoid root-stock theory", belemnoids gave rise to modern coleoids sometime in the Mesozoic, with octopuses deriving from Phragmoteuthida and squid from Diplobelida, making Belemnoidea paraphyletic. ThespirulidLongibelus could be atransitional species between belemnoids and squid.[39] However, molecular evidence suggests that the squid and octopus lineage diverged from Belemnoidea in the Permian.[47]
| Coleoidea |
| "Belemnoid root-stock theory"[39][48] |
Theorder Belemnitida is amonophyletic taxon, consisting of a common ancestor and all of its descendants, and is characterized by the possession of ten hooked appendages, a multilayered outer wall of the phragmocone, and aseptum between the pro-ostracum and the phragmocone. Belemnitida is separated into twosuborders:Belemnitina andBelemnopseina, though a third possible suborder may exist with Sinobelemnitidae. The Belemnopseina guards have a groove on their alveolus, whereas the Belemnitina have a groove at their apex. The grooves probably corresponded to blood vessels.[36] Another suborder,Belemnotheutina, is also proposed, whose members have an aragonitic guard in contrast to the calcitic guards of other belemnites. Aragonitic guards are usually only seen in the ancestral Aulacocerida belemnoids, and Belemnotheutina may represent a transitional stage between the two orders, though some believe Belemnitida derived from Phragmoteuthida which derived from Aulacocerida.[13]
Belemnitida genera[49] |
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Belemnite remains are found in what werelittoral (nearshore) and mid-shelf zones.[28] To hunt, they may have quickly or stealthily grabbed prey, maintaining a grip with the hooks, and then dove down to eat.[15] It is traditionally thought they resided on the shelf their entire life,[28] and preyed oncrustaceans and othermollusks.[35] Belemnites with slender guards may have been better swimmers than those with more massive guards, with the former having dived into deeper waters and hunted in the open ocean; and the latter restricted to the nearshore and fed from the seafloor.[28] Broadly speaking, they may have preferred temperatures of 12–25 °C (54–77 °F), and, like modern squid, warmer waters may have heightened theirmetabolism, increasing birth and growth rates, but also decreasing lifespan.[38] It has been suggested that most belemnite species werestenothermic, inhabiting only a narrow range of temperatures, thoughNeohibolites had a cosmopolitan distribution during theCretaceous Thermal Maximum, a period of dramatic increase in global temperatures.[50]
Belemnites were likely an abundant and important food source to many sea-going creatures of the Mesozoic. Belemnite hook remains have been found in the stomach contents ofcrocodilians,plesiosaurs, andichthyosaurs; and thecoprolite remains of ichthyosaurs and the extinctthylacocephalan crustaceans.[16] Some animals may have only eaten the heads, leaving the phragmocone and guards, however, the guards of around 250Acrocoelites were found in the stomach of a 1.6 m (5 ft 3 in)Hybodus shark, and a fragment in anOxford Clay marine crocodile, meaning they were eaten whole. It may be that they were to regurgitate the indigestible matter later, similar to the modernsperm whale.[51] To defend themselves, belemnites likely were able to eject a cloud of ink.[35]
The abundant planktonic belemnite larvae, along with planktonic ammonite larvae, likely formed the base of Mesozoicfood webs, serving a greater ecological function than the adults. Giantpachycormid fish are thought to have been the mainfilter feeders of the time, occupying the sameniche as modernbaleen whales.[22]
Large accumulations of guards are commonly found and have been nicknamed "belemnite battlefields". The most quoted explanation is that belemnites weresemelparous and died shortly afterspawning, much like modern coleoids which migrate from the ocean to the shelf area. In battlefields comprising both adults and juveniles—as the former model would consist entirely of adults—large groups of belemnites may have been killed by volcanism, changes in salinity or temperature,harmful algal blooms (and, thereby,anoxia), or mass stranding. Another popular theory is that the guards were simply moved or redeposited by ocean currents into large aggregations. Some battlefields may be regurgitated indigestible matter from a predator.[51]
Squid and octopuses diversified and began tooutcompete belemnites by theLate Jurassic toEarly Cretaceous.[50][52] Belemnites declined through the Late Cretaceous, and their range became more restricted to the polar regions; the southern populations became extinct in the early Maastrichtian, and the last belemnites—of the family Belemnitellidae—inhabited what is now northern Europe.[4] They finally became extinct in theCretaceous–Paleogene extinction event, around 66 mya, where, like in ammonites, it is thought the protoconch of embryos could not survive the ensuing acidification of the oceans.[14] However, the dubious genusBayanoteuthis is reported from theEocene, though this is often excluded from Belemnitida.[4][5]
Following the extinction of the belemnites at the end of the Cretaceous,holoplanktonic gastropods, namelysea butterflies, replaced planktonic belemnite larvae at the base of the food chain.[22]
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| The symbol of the god Min inhieroglyphs | ||
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Belemnite guards have been known since antiquity, and much folklore has evolved since.[53]The symbol of the Egyptian godMin has been described, among others, as two fossil belemnites.[54]Before belemnites were identified as fossils, it was believed the guards were some gemstones, namely lyngurium and amber.[40] After a thunderstorm, guards would sometimes be left exposed in the soil, explained aslightning bolts thrown from the sky. This belief persists in parts of rural Britain. In Germanic folklore, belemnites are known by at least 27 different names, such asFingerstein ("finger stone"),Teufelsfinger ("Devil's finger"), andGespensterkerze ("ghostly candle").[53] In Southern England, the pointy guards were used to curerheumatism, ground up to cure sore eyes (which only aggravated the problem), and, in Western Scotland, put into water to curedistemper in their horses.[55]
Belemnitella was declared thestate fossil of Delaware on 2 July 1996.[56]
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