Evolution of molluscs

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Anatomical diagram of a hypothetical ancestral mollusc Archimollusc-en.svg
Anatomical diagram of a hypothetical ancestral mollusc

The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously[ further explanation needed ] been compared to molluscs include Kimberella and Odontogriphus .

Contents

Fossil record

Yochelcionella water flow.png
The tiny Helcionellid fossil Yochelcionella is thought to be an early mollusc [1]
Neptunea despecta.jpg
Spirally coiled shells appear in many gastropods. [2]

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 538.8 to 485.4 million years ago. However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.

Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. [3] Kimberella , from about 555  million years ago, has been described by some paleontologists as "mollusc-like", [4] [5] but others are unwilling to go further than "probable bilaterian". [6] [7] There is an even sharper debate about whether Wiwaxia , from about 505  million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms. [6] [8] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510 million years ago are fragments of a genuinely mollusc-like radula. [9] This appears to contradict the concept that the ancestral molluscan radula was mineralized. [10]

However, the Helcionellids, which first appear over 540  million years ago in Early Cambrian rocks from Siberia and China, [11] [12] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites. [1] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults. [13]

Some analyses of helcionellids concluded these were the earliest gastropods. [14] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion characteristic of modern gastropods, that twists the internal organs so the anus lies above the head. [2] [15] [16]

Nautiloid septa n siphuncle 01.png
  = Septa
  = Siphuncle
Nautiloid septa n siphuncle 01.png
Septa and siphuncle in nautiloid shell

Volborthella , some fossils of which predate 530  million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella's classification is uncertain. [17] The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface. [18] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65  million years ago. [19] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today. [20]

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves. [21] [22] [23] [24] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago. [25] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event. [26] Even so, bivalves remain abundant and diverse.

The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life [27]

Phylogeny

Lophotrochozoa

Brachiopods

Bivalves

Monoplacophorans
("limpet-like", "living fossils")

Gastropods
(snails, slugs, limpets, sea hares)

Cephalopods
(nautiloids, ammonites, squid, etc.)

Scaphopods (tusk shells)

Aplacophorans
(spicule-covered, worm-like)

Polyplacophorans (chitons)

Halwaxiids

Wiwaxia

Halkieria

Orthrozanclus

Odontogriphus

A possible "family tree" of molluscs (2007). [28] [29] Does not include annelid worms as the analysis concentrated on fossilizable "hard" features. [28]

The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs, [5] [6] [8] [9] debates arise about the relationships between the classes of living molluscs. [7] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related. [30]

Molluscs are generally regarded members of the Lophotrochozoa, [28] a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla. [31] The diagram on the right summarizes a phylogeny presented in 2007.

Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs. [32] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm. [33] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments, [34] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized. [7] In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; but this is difficult to reconcile with the embryological origins of spicules. [32]

The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. [1] Eventually, the cuticle would have become mineralized, [1] using the same genetic machinery (the engrailed gene) as most other bilaterian skeletons. [33] The first mollusc shell almost certainly was reinforced with the mineral aragonite. [35]

The evolutionary relationships 'within' the molluscs are also debated, and the diagrams below show two widely supported reconstructions:

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses, [36] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups. [37]

However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded. [30] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated. [38] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree. [39] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question. [40]

Related Research Articles

<span class="mw-page-title-main">Gastropoda</span> Class of molluscs

Gastropods, commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda.

The radula is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus. The radula is unique to the mollusks, and is found in every class of mollusk except the bivalves, which instead use cilia, waving filaments that bring minute organisms to the mouth.

<i>Kimberella</i> Primitive Mollusc-like organism

Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

<span class="mw-page-title-main">Monoplacophora</span> Superclass of molluscs

Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell, inhabiting deep sea environments. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record, and were thought to have become extinct 375 million years ago.

<span class="mw-page-title-main">Coleoidea</span> Subclass of cephalopods

Coleoidea or Dibranchiata, is one of the two subclasses of cephalopods, containing all the various taxa popularly thought of as "soft-bodied" or "shell-less". Unlike its extant sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal shell called cuttlebone or gladius that is used for buoyancy or as muscle anchorage. Some species, notably incirrate octopuses, have lost their cuttlebone altogether, while in some it has been replaced by a chitinous support structure. A unique trait of the group is the ability to edit their own RNA.

<span class="mw-page-title-main">Tusk shell</span> Class of elephant tusk shell molluscs

The tusk shells or tooth shells, technically the Scaphopoda, are members of a class of shelled marine mollusc with worldwide distribution, and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range in length 0.5–18 cm (0.20–7.09 in). Members of the order Dentaliida tend to be larger than those of the order Gadilida.

<span class="mw-page-title-main">Halkieriid</span> Family of incertae sedis

The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.

<i>Odontogriphus</i> Genus of soft-bodied animals from middle Cambrian

Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization.

<i>Orthrozanclus</i> Extinct genus of Cambrian animals

Orthrozanclus is a genus of sea creatures known from two species, O. reburrus from the Middle Cambrian Burgess shale and O. elongata from Early Cambrian Maotianshan Shales. Animals in this genus were one to two centimeters long, with spikes protruding from their armored bodies. The placement of this genus into a specific family is not universally accepted.

<span class="mw-page-title-main">Halwaxiida</span> Proposed clade of extinct Lophotrochozoa

Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.

<span class="mw-page-title-main">Marine invertebrates</span> Marine animals without a vertebrate column

Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.

<span class="mw-page-title-main">Helcionellid</span> Extinct order of molluscs

Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about 540 to 530 million years ago in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician. These fossils are component of the small shelly fossils (SSF) assemblages.

<span class="mw-page-title-main">Mollusc shell</span> Exoskeleton of an animal in the phylum Mollusca

The molluscshell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea; many live on the land and in freshwater.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

<span class="mw-page-title-main">Mollusca</span> Phylum of invertebrate animals

Mollusca is the second-largest phylum of invertebrate animals, after Arthropoda; members are known as molluscs or mollusks. Around 76,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.

<i>Knightoconus</i> Extinct genus of molluscs

Knightoconus antarcticus is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica. It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.

<i>Volborthella</i> Extinct animal of uncertain classification

Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.

<span class="mw-page-title-main">Evolution of brachiopods</span> The origin and diversification of brachiopods through geologic time

The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.

The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.

<span class="mw-page-title-main">Nephrozoa</span> Clade of animals

Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The Ambulacraria are occasionally thought to be sister to the Xenacoelomorpha, forming the Xenambulacraria as basal Deuterostomia, or basal Bilateria invalidating Nephrozoa and Deuterostomia in multiple studies. The coelom, the digestive tract and excretory organs (nephridia), and nerve cords developed in the Nephrozoa. It has been argued that, because protonephridia are only found in protostomes, they cannot be considered a synapomorphy of this group. This would make Nephrozoa an improper name, leaving Eubilateria as this clade's name.

References

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