MOLLUSCA II Bivalves Cephalopods

Bivalves Shells indicative of life habit/environment – good eco indicators, Live in all marine habitats except anoxic. Invaded fresh water by mid Paleozoic. Species tend to be long-lived – little biostrat use. Ancestor (limpet-like) developed hinged shell, lost head, sense organs, radula. First appear in Cambrian Survive P/T extinction and came to dominate (due to predation & infaunal habit?) Survive K/T extinction with little effect (except rudists & inoceramids) Survive all following extinctions

Plane of Symmetry runs between valves Plane of Symmetry runs between valves. Standard orientation is hinge at top. Body consists of mantle, adductor muscles, foot, gills, digestive system. No diductor muscles – use ligament as spring

Shell interior contains muscle scars, pallial line (attachment line of mantle), pallial sinus (notch marking siphon) Also see teeth and sockets at hinge Externally beak – usually points forward prosogyral (can be opisthogyral) ligamental groove or resilifer – for ligament

Gills used for respiration and filter feeding. Gill Types Protobranch – primitive Filibranch – W-folded gill sheets Eulamellibranch – folded gills with cross partitions Septibranch – divide mantle cavity

Teeth and Sockets None Taxodont (Arca) Schizodont Dysodont Isodont Heterodont Pachydont Desmodont

Taxonomy based on gills, hinge structure Class Bivalvia Subclass Protobranchia Order Nuculoidea – (Palaeotaxodonta) nut clams Order Solemyoidea - (Cryptodonta) awning clams Subclass Pteriomorpha – (Filibranchia) Order Arcoida – taxodont ark shells & bittersweet clams Order Mytiloida – dysodont, unequal muscles Order Pterioidia – scallops, oysters, inoceramids, unequal muscles or 1 muscle, reduced foot Subclass Heterodonta – (Eulamellibranchia) Order Veneroida – “clams”, tridacna, razor, tellines, cockles Order Myoida – boring clams and ship-worms Order Hippuritoida – rudists Subclass Paleoheterodonta Order Modiomorpha – Paleozoic, heterodont Order Unionoidia – fresh water heterodonts Order Trigoniodia – Paleoz-Mesoz schizodont Subclass Anomalodesmata Order Pholadomyoida – burrowing/boring desmodont

epifaunal suspension byssus attached boring infaunal susp. infaunal nonsiphonate suspension siphonate carnivore siphonate susp infaunal deposit feeders infaunal mucus tube feeder siphonate deposit

For burrowing bivalves shell shape related to burrowing deep = smooth thin shells shallow – thick ornamented shells discoidal, bladed, cylindrical shells = faster divaricate surface pattern = rapid deep pallial sinus = long siphon = deep burrow Swimming bivalves = large umbonal angle, monomyarian Byssally attached = byssal sinus Cemented = asymmetrical shells Lots of convergence

Evolution Early Cambrian – Taxodont nut clams – probably descended from early rostroconch Early Ordovician – great radiation, most subclasses and orders found, not very diverse. Occupied nearshore habitats and muddy substrates Post P/Tr increased due to predation and ability to burrow and siphons Free-swimming scallops appeared in Triassic Free-living forms in Mesozoic: Gryphaea, Exogyra, Inoceramus, Rudists

CEPHALOPODS Most intelligent of the molluscs (possibly of all invertebrates) Dominant predators throughout most of Paleozoic. Extremely useful for biostratigraphy in Dev-Cretaceous

Physiology based on modern Nautilus Body – 38 tentacles surrounding mouth, horny beak in center Pair of well developed eyes. Hood covers body when retracted Mantle cavity contains gills, water can be expelled through hyponome for jet action Shell consists of body chamber and phragmocone. Chambers (camerae) closed by septa (suture is line of join of septa with outer shell, only visible on steinkern) Siphuncle extends through septa and connects body with camerae – controls fluid/bouyancy

Taxonomy Class Cephalopoda Subclass Nautiloidea – living Nautilus, orthocone, crytocone, involute coiled forms simple sutures Subclass Endoceratoidea – Paleozoic orthocones, huge siphuncle on ventral, endocone deposits O-Sil Subclass Actinoceratoidea – Paleozoic orthocones, septa w necks around siphuncle, siphuncular deposits O-Carb Subclass Bactritoidea – Dev -Tr orthocones w globular protoconch, ancestors to Ammonites Subclass Ammonoidea – Dev-K, Goniatite, Ceratite, Ammonite sutures Subclass Coleoidea – lower Carb-R, squids, octopi, cuttlefish, belemnites

Endoceratoidea Nautiloidea Bactritoidea Actinoceratoidea Ammonoidea Coleoidea

Coiling shapes

Suture Complexity Buttressing Ion exchange Muscle attachment

Bouyancy solutions B. Endoceratoidea C. Actinoceratoidea D. Liquid retention E. Cameral deposits G. Shed chambers

Variations on the basic Ammonite Some bizzare forms thought to be “racial senescence”. Good examples of iterative evolution. “Haeckel’s Law”