Mollusca
From Palaeos
| LOPHOTROCHOZOA | |
| Taxonomy | Phylogeny |
Kingdom: Metazoa
|
Protostomia `--o Lophotrochozoa [or Spiralia] `--+--Bryozoa [Ectoprocta] |--Platyzoa `--o Trochozoa |==Coeloscleritophora† ("Procoelomata†") |--Nemertea [Nemertinea] |--o Eutrochozoa | |?-Tullimonsterida † | |--Mollusca | |--Hyolitha † | |--Sipuncula | `--Annelida `--o Brachiozoa |--Phoronida `--Brachiopoda |
Contents |
[edit] Introduction
The mollusks are a large and diverse group of soft-bodied unsegmented animals. Nearly 130,000 recent species are known, and some 35,000 fossil species (these latter are without doubt only a small fraction of those that have ever lived). They include many familiar animals, like snails, clams, squid, octopods, etc, as well as others not so well known. They range in size from microscopic forms to the giant squid (Architeuthis), and have a long and venerable history appearing during earliest Cambrian time, if not before.
| MOLLUSCA | |
| Taxonomy | Phylogeny |
Kingdom: Metazoa |
o Eutrochozoa `--o MOLLUSCA `-+--Solenogastres `--+--Caudofoveata `--o Testaria `--+--o Polyplacophora `--o Conchifera |--Tryblidiida |?-- Stenothecoida `--+?--Bivalvia (if molecular phylogeny) `==Helcionelloida (paraphyletic) | |--+-- †Yochelcionellidae | | `?-- Cephalopoda | |? -- †Criconarida | `-+?--Bivalvia (if transitional fossils) | `-- †Rostroconchia | `-- Scaphopoda `--+--†Paragastropoda `-- Gastropoda |
[edit] Physiology
All mollusks possess some or all of the following characteristics:
- a muscular fleshy foot which with gastropods (snails) is used to crawl along with but in cephalopods is modified into tentacles
- a visceral mass containing the digestive, excretory, and reproductive organs
- a mantle, usually two folds that enclose the gills or lungs, and also secretes the protective shell
- a radula, a tongue-like feeding organ, essentially a sort of built-in saw equipped with rows of microscopic teeth that is incredibly effective at scraping food off hard surfaces or grinding through any substance (some carnivorous snails are able to bore a whole through the shells of other mollusks)
- a special respiratory gill (the ctenidium)
- a shell made of calcium carbonate. This fossilizes easily, which explains the rich fossils record the mollusks possess.
Although not all mollusks share all these characteristics, generally all mollusks have at least some of them.
[edit] Molluscan Diversity
Despite sharing these common features, the mollusks are an incredibly diverse group. The following diagram shows the major classes, which as can be seen are all based on variation in this same body plan.
Figure from D. R. Prothero, 1998, Bringing Fossils to Life, McGraw-Hill, p. 277; from Clarkson, 1993 - this image from LAB 3 Chapter 8: Phylum Mollusca and Biological Diversity and Evolution through Time
This diagram presents a stylized relationship between the different molluscan groups, with all evolving from a "hypothetical ancestral mollusk". It is now known that molluscan phylogeny is a lot more complex than this.
[edit] Habitat
As one of the most successful forms of animal life, the mollusks have conquered almost every habitat and exist in all the oceans (from shallow tidal pools to the deepest trenches), in fresh water, and on land. The only environment they cannot cope with are very dry regions, as their moist skin is easily desiccated. Even so, the greatest diversity of mollusks species is to be found in marine environments.
[edit] Taxonomic History
When Linnaeus was formulating his system of binomial nomenclature the mollusks were grouped (along with most invertebrates) under Vermes, worms. The name Mollusca (from the Latin mollis "soft"), was first used by the great French zoologist Cuvier in 1798 to refer to cephalopods like squids and cuttlefish. It was later extended to include other organisms of this group, such as snails and bivalves.
[edit] Evolutionary History
The mollusks first appeared during the earliest Cambrian (Tommotian), at the very start of radiation of coelomate animals. As the Cambrian period progressed, many types appeared that soon died out. Although previously included among the conventional classes of mollusks it is increasingly argued that these represent distinct classes, early experiments in molluscan evolution, or transition forms. However, the exact number of extinct classes remains debatable.
By the Ordovician period the three main classes - Gastropoda, Bivalvia, and Cephalopoda - had increased greatly in number and diversity. This was a period of major evolutionary diversity for the phylum, and they became common in most marine ecosystems.
The Devonian saw bivalves invade freshwater, and the first land snails appeared during the Carboniferous. During the Devonian also, the ammonoids took over from the nautiloids as the dominant cephalopod group, and these creatures continued to flourish throughout the entire Mesozoic, living alongside species of bivalves and gastropods not very different to modern forms. (Oysters for example were common during the Jurassic and Cretaceous, while scallops date back to the Paleozoic). By mid Cretaceous most mollusks, like most invertebrates and fish, were essentially like modern forms, except for the cephalopods which were still represented largely by the Mesozoic ammonites and belemnites. The ammonites were decimated by the K-T extinction event, as were the reef-building rudist bivalves (although these were on the decline for some time), but the coleoids (soft-bodied squids, octopods etc) continued to evolve quite happily.
[edit] Systematics
Traditionally seven classes are recognized: Aplacophora (primitive worm-like shell-less forms), Polyplacophora (chitons, at one time included with the Aplacophora in the class Amphineura), Monoplacophora (primitive ancestral forms), Gastropoda (snails and slugs), Bivalvia (clams, mussels, oysters, etc), Scaphopoda (tooth and tusk shells), and Cephalopoda (squids, octopus, nautilus, cuttlefish), of which only Gastropoda, Bivalvia, and Cephalopoda have large numbers of species and a strong fossil record. However it is becoming increasingly recognized that there were more groups that have since died out.
With the falling out of favor of Linnaean methodology, recent work in molluscan systematics is done via cladistic analysis. The following is a recent cladogram showing molluscan relationships.
image and text by Ellen Thomas - from Biological Diversity and Evolution through Time
My own position, oft-stated, is that the Linnaean and cladistic methodologies are complementary rather than contradictory. Cladistics provides a means of drawing the family tree of life, while the Linnean hierarchy enables us to construct a useful, if arbitrary, arrangement for cataloging and comparative purposes. The following listing therefore represents a possible tally of molluscan classes (including extinct forms) that are known or have been proposed (note - not all experts agree as to the status of the various Paleozoic minor classes).
- Solenogastres (= Neomeniomorpha; Aplacophora in part) - no fossil record - very small creeping worm-like soft bodied shell-less forms.
- Caudofoveata (= Chaetodermomorpha; Aplacophora in part) - ?Silurian to Recent - very small deep sea worm-like soft bodied burrowing shell-less forms. The Silurian Acaenoplax, which resembles both Caudofoveata and Polyplacophora, may be an early form of this lineage.
- Polyplacophora (= Amphineura in part) - Late Cambrian to Recent - The chitons, which have eight plates or "valves" which are overlapping in post-Mesozoic forms. The foot is broad; the head reduced. The soft parts show some segmentation. The animal generally lives attached to rocks in the inter-tidal zone. Appear to be distantly related to the Solenogastres and Caudofoveata (as ancestors/cousins), and the Tryblidiida.
- Tryblidiida (= Monoplacophora sensu stricto, Tergomya in part) - Late Cambrian to Recent - Creeping cap-shelled forms with paired muscle attachments. The most primitive conchiferan group. A few dozen deep water recent species remain as "living fossils". As with the Polyplacophora the soft parts show some segmentation. Probably evolved from a soft-bodied ancestor, possibly Polyplacophora relationships.
- Stenothecoida (= Monoplacophora in part) - Early to Middle Cambrian - Small group of little known bivalved forms. There relationship with other molluscan groups is unclear, and it is not agreed whether they qualify as class status. No descendents, ancestors unknown.
- Helcionelloida (= Monoplacophora in part; paraphyletic) - Earliest to Late Cambrian - an ancestral lineage or grade. Consist of creeping or weakly infaunal forms with cap-shaped or spiral shell. Probably evolved from a soft-bodied ancestor, as the direction of shell coiling is opposite that of the Tryblidiida, which they predate in any case. Generally considered directly or indirectly the ancestors of all conchiferan mollusks other than the Tryblidiida.
- Bivalvia (= Pelecypoda) - Early Cambrian to Recent - mostly sedentary or burrowing forms with a shell of two valves hinged dorsally. The foot is generally hatchet-shaped (hence the name "pelecypod"); the head lacking; a very diverse group, includes epifaunal or infaunal forms, mostly marine but some freshwater species. Clams oysters, mussels, etc. Generally considered to have evolved from rostroconches, although this would relate them to helcionelloids, if not actually scaphopods and cephalopods, and the weight of current molecular and cladistic evidence indicates the bivalves diverged from the main Conchiferan lineage quite early.
- Paragastropoda - Early Cambrian to Devonian - Small group of superficially snail-like forms, and perhaps snail-like habits, distinguished by lack of torsion. Evolved from helcionelloids, no descendents. It is not even certain if this is a natural or a polyphyletic group.
- "Tergomya" in part (= Monoplacophora in part; polyphyletic) - Late Cambrian to Devonian - A probably artificial assemblage of creeping cap- or partly spiral shelled forms with single or paired muscle attachments. The Tryblidiida are usually included here but they are certainly a more basal group. Non-tryblidiidan Tergomya may have evolved probably several times from helcionelloids, although some may be related to the Tryblidiida. Some of the helcionelloid-descended forms would seem to be related to the gastropods.
- Gastropoda - Late Cambrian to Recent - Diverse group of shelled or shell-less forms, evolved from a spiral-shelled helcionelloid or helcionelloid-descended "tergomyan" ancestor that underwent torsion. The body is usually asymmetrical, with a distinct head, pair of eyes, and one or two pairs of tentacles. The shell is usually spirally-coiled, but may (in the case of limpets) be cap shaped, and in some species is vestigial or completely absent. The foot is broad and used to creep along the substrate. Includes marine, freshwater, and terrestrial species (the only mollusks to invade the land). Snails, slugs and their relatives.
- Rostroconchia - Early Cambrian to Late Permian - A Paleozoic group of infaunal forms. The shell consists of two valves joined in adults by an enclosed hinge. Evolved from helcionelloids. The posterior of the shell is usually produced into an elongate tube, presumably for inhalant and exhalent siphons. Originally considered to have given rise to both scaphopods and bivalves, as they share many similarities with both those groups. But relationships among early molluscan classes remain controversial.
- Scaphopoda - Devonian to Recent - small semi-infaunal forms, the tubular shell open at both ends and resembles a miniature elephant's tusk, with one end larger than the other. The foot is conical; there are no gills. Probably evolved from rostroconches. Shown to be related to the cephalopods.
- Cricoconarida (= Tentaculitoidea) - Middle Cambrian to Devonian - very small pelagic forms with tubular shells. They are thought to be related to the cephalopods, and seem to have born a number of arms.
- Cephalopoda - Latest Cambrian to Recent - diverse group of large intelligent predaceous forms, with a large external shell (most Paleozoic and Mesozoic forms, and current Nautilus), a vestigial internal shell, or (in the case of the octopus) the shell is lost altogether. The head is large with well-developed eyes, and armed with horny chitinous jaws and many arms or tentacles for grasping prey; the head fused to the foot (hence the name). Evolved from helcionelloids.
- Hyolitha - Earliest Cambrian to Permian - Enigmatic animals with small conical shells, shell-microstructure indicates close to mollusks but experts do not agree whether they actually were mollusks.
[edit] Alternative Phylogeny
<==Mollusca [Ancrypoda, Diasoma, Ganglioneura, Loboconcha, Monoplacophora] | i. s.: Amiskwia Walcott 1911 | `--A. sagittiformis Walcott 1911 | Eovasum | Paraseraphs | Velates | Katharina tunicata | Pupa strigosa | Zafra | |--Z. mitriformis Adams 1860 | |--Z. subvitrea (Smith 1879) | |--Z. validicosta (Habe 1960) | `--Z. zonata (Gould 1860) | Margaretes umbilicus | Modiolaria discors | Nutricola | Kelletia kelletii | *Bulla bulla | Cumingia tellenoides | Succinodon (n. d.) | Harmandia Rochebrune 1882 | Deltoideum delta | Bilobissa dilatata | Tegula brunnea | Liolophura japonica | Umbella d’Orbigny 1841 | Ctenoides floridanus | Coniconchia | |--Hyolitha | `--Tentaculoidea | Psammobia [Psammobiidae] | `--P. flabellum Viader 1951 |--+--Rostroconchia | `--Bivalvia `--Glossophora |--Archaeobranchia [Hyperstrophina, Paragastropoda] | |--Cyrtosoma | `--Khairkhaniidae `--Aculifera [Spiculata] |--+--Polyplacophora | `--+--Neopilina galatheae | `--Laevipilina antarctica Warén & Hain 1992 `--+--Acaenoplax Sutton, Briggs et al. 2001 | `--*A. hayae Sutton, Briggs et al. 2001 `--+--Solenogastres `--Caudofoveata [Chaetodermomorpha] | i. s.: Scutopus ventrolineatus `--Chaetodermatidae |--Chaetoderma nitidulum |--Falcidens Salvini-Plawen 1969 | |--F. lipuros | `--F. poias Scheltema 1995 `--Lepoderma chiastos (see below for synonymy)
Lepoderma chiastos [incl. Falcidens (Lepoderma) targotegulatus Salvini-Plawen 1992]
* Type species of genus indicated
[edit] References
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[edit] Credits
MAK020929; alternative phylogeny CKT071117
