Morphological Characters, Systematic Position and Age of Fossil Genera Pertaining to Bivalves
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EXPERIMENT 2 MORPHOLOGICAL CHARACTERS, SYSTEMATIC POSITION AND AGE OF FOSSIL GENERA PERTAINING TO BIVALVES Outline of Experiment________________________________________ 2.1 Introduction 2.4 Common Genera Expected Learning Skills 2.5 Laboratory Exercises 2.2 Requirements 2.6 Results and Discussion 2.3 Bivalves: Systematics, General 2.7 Reference Morphology and Age Range 2.8 Further/Suggested Readings 2.1 INTRODUCTION In the previous experiment, you have made line drawing and described morphological characters along with palaeoecology and stratigraphic range of various genera of brachiopods. Every genus of brachiopods displays a set of distinctive morphological characters, on which basis they can be differentiated from each other. You are aware that brachiopods are marine animals; hence, their occurrence in a sedimentary bed is an indicative of marine environment of deposition for that bed. In this experiment, you will work with some common genera of bivalves. Bivalvia is the second largest class of molluscs. They consist of a twin-valved shell (i.e., shell made up of two almost similar valves) that gives them a superficial resemblance to brachiopods, but varies in shell morphology and symmetry. In many bivalves, the valves are virtually mirror images of each other, but a very few forms like oysters and rudist bivalves have one valve larger than the other. Bivalves have also been described as ‘pelecypods’ and ‘lamellibranchs’. They represent a wide range of adaptation that is from marine to freshwater and from infaunal burrowers, epifaunal to active swimmers. As a consequence, a genus of marine environment possesses different morphological characters as compared with those of non-marine genera. Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 Expected Learning Skills__________________________ After performing this experiment, you should be able to: list the systematic positions of some common genera of bivalves; identify and describe their morphology characters; prepare their line drawing with labeling of morphological features; and write their stratigraphic/geological and palaeoecological range. 2.2 REQUIREMENTS To perform this experiment successfully, following are the requirements: Pen, pencil, eraser, sharpener, scale, stencil and hand lens. Practical note-book. Fossil specimens of bivalves. Instructions: You are required to study Unit 14 of BGYCT-137 course (Stratigraphy and Palaeontology) before performing this experiment. Bring this practical manual along with Block 4 of BGYCT-137 course while attending the Practical Counselling session. 2.3 BIVALVES: SYSTEMATICS, GENERAL MORPHOLOGY AND AGE RANGE 2.3.1 Systematic Palaeontology/Taxonomic Hierarchy Kingdom Animalia Subkingdom Metazoa Phylum Mollusca Class Bivalvia Subclass Palaeotaxodonta Cryptodonta Pteriomorphia Palaeoheterodonta Heterodonta Anomalodesmata 2.3.2 General Morphology The soft parts of bivalves are covered by hard exo-skeleton, which is made up of two valves. In most cases, the valves are equal in size, asymmetrical in outline and essentially mirror images of each other. The two valves are named as the right and left valves (Fig. 2.1), because they are located to the right and left sides of the animal. Each valve has a nose-like pointed apex, which is known as beak. Beak usually curves toward the anterior (i.e., front) end of valves. Umbo (plural umbones) is the region of maximum curvature of each valve close to the beak and situated on the dorsal margin of the shell (Fig. 2.1). In most of the bivalves, the two valves are joined together along their 24 Morphological Characters, Systematic Position and Experiment 2 ……………………….…………………………………………………………………………….….............…...Age of Fossil Genera Pertaining to Bivalves dorsal margin by means of elastic ligaments and by an interlocking mechanism of teeth and sockets. Fig. 2.1: Dorsal view of the shell showing the external morphology of a bivalve. Orientation of the bivalve shell If you place the beaks of the shell uppermost in your front, the valve appears on the right side is right valve and that on the left is left valve. The margin closest to the beak is anterior of the shell and opposite margin is posterior. The margin of the shell near the hinge is dorsal and the opposite margin is ventral (Fig. 2.1). If you see the internal surface of the valves, you can find the following features: muscle scars, pallial line, pallial sinus, teeth and sockets. Muscle scars are one of the important features of the bivalve morphology. Indeed, these are the impressions or attachment sites left by the soft (fleshy) adductor muscles and commonly referred to as adductor impressions or adductor scars. Many bivalves have two adductor scars; one on the posterior margin, known as posterior adductor scar and the other on the anterior margin described as anterior adductor scar (Fig. 2.2). Fig. 2.2: Left valve of a bivalve showing internal (interior) bivalve morphology. 25 Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 The portion of the dorsal margin of valves, where teeth and sockets occur is known as hinge line or cardinal margin. The area lying between the hinge line and the umbo of each valve is called hinge area. In some cases, the hinge area is divided into a lunule and an escutcheon (Fig. 2.1). Lunule is a heart-shaped area that occurs in front of the beak and escutcheon is an elongated depression present posterior to the beak. The teeth present below the beak are termed as cardinal teeth and those that occur in front or towards the posterior side of the cardinal teeth are the lateral teeth (Fig. 2.2). In bivalves, the term dentition is used to describe the arrangement of teeth, sockets and other closely related features. There are three types of dentitions present in the bivalves: taxodont, schizodont and heterodont (Fig. 2.3). a) Taxodont: it is a primitive type of dentition, where teeth are numerous, more or less equal in size and arranged in a subparallel pattern. b) Schizodont: it includes two or three thick teeth with prominent grooves. c) Heterodont: it comprises both cardinal and lateral teeth, which are not uniform, and less in number. (a) (b) (c) Fig. 2.3: Internal view of valves of bivalves showing different types of dentition: a) Taxodont; b) Schizodont; and c) Heterodont dentitions. The bivalve shells lacking dentitions are termed as edentulous shells. Turning to the outside surface of the valves, you can see that in some cases it is smooth while in others it is ornamented with fine growth lines. The growth lines are concentric about the umbones and show the earlier positions of the edge of a valve. The outer surfaces may also be ornamented with other surface features such as radial ribs, concentric ridges and spines. 2.3.3 Geological History Like brachiopods, bivalves also have a very long geologic history. The earliest bivalves are known from the early Cambrian, but they became diverse during the Ordovician. The non-marine bivalves first arose in the Devonian and became abundant in the Carboniferous. Some of the groups of bivalves are surviving till date. 2.4 COMMON GENERA 2.4.1 Pecten Systematic Palaeontology Kingdom Animalia Phylum Mollusca 26 Morphological Characters, Systematic Position and Experiment 2 ……………………….…………………………………………………………………………….….............…...Age of Fossil Genera Pertaining to Bivalves Class Bivalvia Order Pectinida Family Pectinidae Genus Pecten Morphological Description: The shell of Pecten shows trigonal to subcircular outline. The shell is gently convex and equilateral to nearly equivalve. Its right valve is convex and the left valve is flat. It has a straight hinge line with wing- like expansions. Well developed radial ribs and costae are present on the external surface of the shell. It has well developed ears, a central pit for ligament and large adductor impressions (Fig. 2.4a). (a) (b) Fig. 2.4: Photographs showing external views: a) Pecten; and b) Inoceramus. (Courtesy: Prof. R. Nagendra, Anna University, Chennai) Stratigraphic Range: Carboniferous to Recent. Palaeoecology: The genus, Pecten is a free-swimming suspension feeder and occurs in a variety of environments ranging from marine, marginal marine to freshwater. 2.4.2 Inoceramus Systematic Palaeontology Kingdom Animalia Phylum Mollusca Class Bivalvia Order Pterioida Family Inoceramidae Genus Inoceramus Morphological Description: The shells of Inoceramus are ovate, circular, inequivalved to inequilateral in shape. Hinge line is usually long, which may be straight or oblique. It possesses taxodont dentition, ears and prominent umbones. It is a monomyarian bivalve. It has transverse ligament area and feebly present adductor scars. The external surface of shells bears prominent, broad, rounded and curved concentric ribs. The margins of the shell are usually rounded (Fig. 2.4b). Stratigraphic Range: Jurassic to Cretaceous. Palaeoecology: The genus was a mobile epifaunal suspension feeder. 27 Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 2.4.3 Ostrea Systematic Palaeontology Kingdom Animalia Phylum Mollusca Class Bivalvia Order Ostreoida Family Ostreidae Genus Ostrea Morphological Description The shells of Ostrea have various shapes, ranging from triangular, irregular, inequilateral to inequivalve (Fig. 2.5). Ostrea is a monomyarian bivalve, which means the genus has only a single large adductor muscle scar. Its left