BELLEROPHONTIDA

A phylogeny of the mollusks:

Bellerophontida is an extinct (Early Cambrian-Early Triassic) clade (or grade?) of planispiral-shelled univalved mollusks with paired muscle scars: hence, they are "monoplacophorans" in the broad paraphyletic sense. Presence of nacreous deposits on the outside surface of the shells suggests that the mantle extended out over the shell on a regular basis, as in some modern gastropods.

The superficially-snail-like appearance has meant that paleontologists have long considered these gastropod allies (or even stem-gastropods). This is certainly possible, but without soft tissue information it is difficult to state for certain.

Bellerophonts are the archetypical "dead clade walking": they survive the Permo-Triassic mass extinction, but at greatly reduced diversity. That diversity continues to decline, and by the end of the Early Triassic a few million years later they all disappear.

A phylogeny of the gastropods:

Snails (including slugs) are the most diverse group of mollusks, with 80% of living species (~75,000). Over 15,000 fossil species are known. In stark contrast to all the animals we have encountered in the class so far, this clade includes fully-terrestrial forms.

Gastropods (Cambrian to Holocene) are characterized by:

• Univalve, conical, often coiled shells
• Cephalization: head contains sense organs such as eyes and tentacles
• Torsion: during development, body rotates counter-clockwise
  • Veliger ("winged") larvae begin untorted
  • As it grows, body (with digestive, nervous, & other systems) swings around
  • The adaptive significance of torsion is not obvious, but one potential advantage is that the torted body (with its mantle cavity facing anteriorly) allows the head to retract into the space first, and allows for the evolution of an operculum on the dorsal posterior end that would be the last thing pulled into safety, sealing off the snail.
  • Torsion is NOT the same thing as the spiral of the shell
  • It appears that a relatively simple mutation causes asymmetrical development of muscles in the embryo, producing torsion

Gastropod systematics has been plagued by paraphyletic groups for many decades, as well as confusion over which Paleozoic members to include. On recent proposal is used here (see the cladogram above), but do not consider it set in stone. This system removes many classic paraphyletic groupings.

Gastropodiformes includes the crown-group (Gastropoda proper) as well as Cambrian taxa such as Chippewaella and Strepsodiscus, previously considered to be bellerophonts.

Crown gastropods include two major divisions: Eogastropoda and a clade comprised of Cambrian Strepsodiscus (the oldest known crown-gastropod) and Orthogastropoda

Eogastropods contain the extant clade Patellogastropoda (the true limpets), characterized largely by the retention of primitive features:

• Univalved, conical by uncoiled shells
• Epifaunal grazing habit, particularly on rocky substrates
• Radulae resembling those of polyplacophorans and scaphopods
  • It is worth noting that limpet radulae are among the strongest biological materials known, with five times the tensile strength of spider silk. The individual teeth incorporate goerthite (the iron-bearing hydroxide FeO(OH) into their matrix.
• Primitive eye morphology, two aspidobranch (bipectinate) gills, two nephridia
• Nevertheless, some soft-tissue features outside the scope of this course, as well as DNA, unites the clade of patellogastropods

Several extinct taxa have been allied to the patellogastropods as Eogastropoda. These seem to include the curious Upper Cambrian-to-Devonian Macluritoidea. These large, heavy shelled forms have one entirely flat surface: they may have actually been sedentary animals, and so unlike the majority of other gastropods. (It is noteworthy that some paleontologists place these in a wastebasket "paragastropods" outside true Gastropoda.)

Orthogastropoda

The remaining gastropods are the Orthogastropoda (the "right" or "correct gastropods"). In nearly all of these the shell is coiled. Eyes of orthogastropods are on eyestalks. The radula is on a flexible membrane. Many of the organs (nephridium, osphradium, gill) are unpaired. The orthogastropods include the Vetigastropoda (abalones, turbans, top shells, slitshells), which are almost all epifaunal grazers, and the more diverse Apogastropoda. (That said, some molecular phylogenies find rather different combinations).

The apogastropods have two major divisions: Heterobranchia and Caenogastropoda.

Heterobranchia

• Some basal heterobranchs are shelled freshwater and marine forms
• The marine Opisthobranchia (Carboniferous to Holocene), in which the body is often detorted (and the gills point backwards again: hence "opistho-" (backwards) "branch" (gill)). Opisthobranchs might be paraphyletic with respect to the Pulmonata. Divisions include but are not limited to:
  • Nudibranchia (sea slugs), possibly paraphyletic. Shell-less. Benthic or nektonic. Some even converge on the body shape of fish
    • A possible case of convergence is the Carboniferous Typhloesus of the Bear Gulch Limestone Lagerstäe. This is an untorted unsegmented bilaterian with a belly full of conodont elements. Initially thought in 1973 to be the elusive "conodont animal", and then as the conodont animal predator, new study in 2022 reveals it to have a radula on an extensible protrusible organ. Is it an independent evolution of a swimming pelagic pseudo-fish gastropod?
  • Pteropoda (sea butterflies), possibly diphyletic. Some are shell-less, others have very thin, often internalized shells. Pteropods are planktonic, and make up a sizable fraction of the biomass in some parts of the sea.
• Pulmonata (land snails and slugs), known from the Carboniferous to Holocene (but pre-Middle Jurassic examples may actually be terrestrial littorinimorphs caenogastropods, and in fact the oldest pulmonates may be only Cretaceous onward.)
  • Gills are absent, and instead the mantle has become highly vascularized to absorb oxygen from the air: thus, their name "the lunged ones"
  • "Slugs" (shell-less land snails) are polyphyletic, arising multiple times from different land snail groups
  • Hermaphroditic. In at least some taxa a chitinous or calcareous "love dart" is produced. These are not spermatophores; instead, the physiological response to the mucous coating of the dart in the wound produces anatomical changes in the recipient to make them better able to accept sperm.
  • At least some studies find pulmonates as a whole to be polyphyletic!

Caenogastropoda

Caenogastropods (Ordovician to Holocene) are primarily marine snails, but some are freshwater or land-dwelling forms. Basal branches have separate sexes, and include diverse taxa such as apple shells, perwinkles, cowries, moon shells, turrids, and the like. Many of these are carnivorous rather than epifaunal grazers.

One diverse clade within this is Neogastropoda (Cretaceous onward), siphonate taxa. The gill is monopectinate in these forms. Whelks, conchs, cone shells, olive shells, and murexes are all part of the neogastropod radiation. Most are predators, with some (such as cone shells) being extraordinarily venomous.

Boring Gastropods

Not a statement about the ability of gastropods to maintain interest, but rather about a particular life habit. A wide variety of gastropod taxa make a living by drilling into, killing, and eating hard-shelled mollusks (bivalves and other gastropods). The radula is typically used to carve into the prey; in some neogastropods an accessory boring organ in the foot has evolved to supply a mixture of various substances to dissolve and weaken the shell. Once pierced, the snail sends its proboscis into the victim to feed. (In the case of some venomous snails, it first injects the bivalve victim with venom. The muscles relax, and the ligament opens the shell, allowing the snail to rasp away at the clam.)

Scaphopoda (tusk shells)

• A single tall conical valve, open at both ends
• Tubular body
• Gills very reduced: respiration over mantle surface
• Mostly sediment-sitting detritivores and foram-eaters, grasping them with captacula (tiny tentacles)
• The latter also serve as the only major sense organs: no eyes, no osphradia
• Ordovician to Holocene record, but very simple shell design shows little variation throughout Phanerozoic. Some consider the Ordovician taxa to be convergences onto scaphopods, and that the oldest definitive scaphopod is Devonian or Mississippian
• Different analyses have allied them with cephalopods, bivalves, and rostroconchs

Stenothecoida

(Do NOT confuse them with the equally-poorly-known Stenothecidae, a helcionellid group.) Stenothecoids are a problematic group of early-through-mid-Cambrian mollusks with a dorsal and ventral valve: essentially molluscan brachiopods. Unlike brachiopods, however, the valves are not bilaterally symmetrical, but show a slight spiral.

They appear to have been sediment sitters. When their valves were found isolated they were considered limpet-like gastropods. They have been allied with bivalves by some authors, but on no real strong basis.

Or... maybe they are really stem-brachiopods! New specimens show possible attachments for a pedicle from between the valves (in the manner of linguliforms). So maybe in the future they will be moved out of this lecture.

Rostroconchia (rostroconchs)

• Cambrian-Permian, but peak is Ordovician
• Univalved, resemble a non-hinged bivalve
• Begin as limpet-like protoconchs, grow a pair of valves downwards (forms the adult dissoconch)
• Appear to have had a protrusible foot out an anterior permanent gape in the shell, and an exhalent siphon
• Some inferred similarity to scaphopods; additionally, make relatively reasonable "transitional forms" to Bivalvia (by means of development of a hinge along the dorsal surface, dividing the dissoconch into right and left valves

A phylogeny of the bivalves:

Bivalves aka Lamellibranchiata aka Acephala aka Pelecyopoda: second only to gastropods among mollusks in terms of species diversity. They range from the Cambrian onward, but with greater diversity in the post-Paleozoic. Their basic anatomy:

• Bivalved, with right & left valve
• Ligament to open, adductor muscles (ancestrally one anterior, one posterior) to close
• Dentition of hinge to assist closure and opening, used in classification schemes
• Foot has become large and protrusible
• Radula lost
• Except for basal forms, are suspension feeders
• Enlarged ctenidia
  • Used for feeding as well as breathing: lophophore-analogs
  • With reduced activity level, gills can function double-duty
  • Gill morphology, broken down into:
    • Protobranch: small, leaf-like (primitive condition, found in polyplachophorans, cephalopods, basal gastropods); present in non-suspension feeders
    • Lamellibranchs: by far the most common, further subdivided into:
      • Filibranch: lamellar sheets of individual filaments in a W-shape
      • Eulamellibranch: as above, but with cross-partitions
    • Septibranch: (only in rock-borers in Anomalodesmata) small gills transverse to inner chamber, subdividing the mantle cavity horizontally

The phylogeny of Bivalvia is become better understood. Basal members of Bivalvia (Early Cambrian forms like Pojetaia and Fordilla) are exceedingly small (~4-5 mm long), and likely epifaunal animals similar to protobranchs. Crown bivalves--Eubivalvia consists of the Protobranchia and Autobranchia, both extant.

Protobranchia

Cambrian to Holocene forms, with protobranch gills and taxodont dentition. These taxa are not suspension feeders:

• Nuculida and Nuculanoidea are algae-sluprpers, using small tentacles and palps to push algae into their mouths. This is likely the ancestral mode of life for Bivalvia
• Solemyoida use chemosymbionts, and are typically very small

Autobranchia

Vastly more diverse, autobranchs are suspension-feeders (or their descendants), and have lamellibranch gills and any of a wide-variety of dentitions. They include three vast clades: Pteriomorpha, Palaeoheterodonta, and Heterodonta (the latter two united as Heteroconchia

Pteriomorpha

Ranging from the Ordovician onward. These forms are almost exclusively suspension feeders, and many are epifaunal, attached to the substrate by a byssus. This group includes well known taxa such as Arcoida (ark shells and bittersweet clams); Mytiloida (mussels); Limoida (file shells); Pteroida (pen shells), and three groups of special note for paleontologists:

• Ostreoidea (true oysters and foam oysters)
  • Monomyarin (only a single adductor muscle)
  • Shells lacking the nacreous layer (NOTE: pearls don't come from ostreoids, but from the genus Pinctada ("pearl oysters") in Pteroida!)
  • The animal lives with the left valve downward. In true oysters (Ostreidae) it is typically cemented to a hard substrate; in foam oysters (Gryphaeidae) it is typically resting on a muddy surface. The left valve is often very large and massive. The right valve is flat, and functions as an operculum.
  • Their durable skeleton allows for great preservation.
• Petcinoidea (scallops and spiny oysters)
  • As with oysters, monomyarin
  • In contrast with ostreoids, they typically rest on their right valve
  • In some they are soft-sediment sitters or attach to a hard substrate, and in many they use byssal threads to stabilize their position
  • In Pectinidae proper (true scallops), however, the animal is free living and motile, capable of swimming by clapping their shells and squirting the jet of water past their auricles (winglets)
  • Pectinoids are among the few bivalves with eyes (which form along the mantle)
• Inoceramidae
  • Largest of bivalves (rivaled only by modern Tridacna).
  • Once considered close relatives of solemyoids, discovery of mineralized fossils of their enormous complex eulamellibranch gills showed they were autobranchs, and the best present placement is with Pterioda.
  • Permian to the end of the Cretaceous; their heyday was the Late Cretaceous
  • They were capable of producing golfball-to-softball-sized pearls
  • Initially thought to be Tridacna-like inhabitants of shallow clear water, feeding with mantle-hosted zooxanthellae
  • Discovery of their gills and paying attention to the sedimentary environments they were preserved in showed that they lived in the bathyal and neritic environments, often in methane seeps: their gills almost certainly hosted chemosymbionts

Palaeoheterodonta

Silurian to Holocene, typically equivalve with hinge teeth in a single (rather than double) row. Two major groups:

• Devonian-Holocene Unionida, the only freshwater clams, with larvae (glochidia) with parasitize the gills of fish
• Silurian-Holocene Trigonioida, with highly complex dentition. Like coelacanths, they had thought to have died out in the Cretaceous until a discovery in 1802 off of Tasmania

Heterodonta

Ordovician onward, containing a huge diversity of clams. Marine, eulamellibranch (other than septibranch anomalodesmatans). The nacreous layer is reduced or absent. Most are siphonate: consequently, this clade includes the deepest infaunal forms.

A major extinct clade is Hippuritoida (Late Jurassic-end of Cretaceous), the rudists. These are epifaunal sessile forms, ecologically equivalent to Tridacna. Some are recumbent; others were elevators (where one valve formed a tall cone, and the other a flat operculum). Almost certainly they fed by means of zooxanthellae. During the mid-to-Late Cretaceous they formed vast reefs, displacing scleractinian corals in the hot tropical regions of the world. Like their distant inoceramid (and even more distant belemnoid and ammonoid) cousins, they were extinguished at the K/Pg boundary.

Although some heterodonts have rudist-like habits (such as the enormous cockle Tridacna, aka the "giant clam" of the Indo-Pacific), most are infaunal suspension feeders. In Myoida the siphon can become exceedingly long, such that the clams MUST live infaunally or risk having their flesh exposed, unable to protect it.

Among the myoids evolved the Teredinidae (shipworms), in which the shell becomes reduced and used simply as a boring organ, to drill into driftwood, ships hulls, or docks. In boring bivalves, the wood acts to protect the body, so that only the ends of the siphons are exposed

To Syllabus.

Last modified: 20 October 2022