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Morphological Characters, Systematic Position and Age of Fossil Genera Pertaining to Brachiopods

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Morphological Characters, Systematic Position and Age of Fossil Genera Pertaining to Brachiopods EXPERIMENT 1 MORPHOLOGICAL CHARACTERS, SYSTEMATIC POSITION AND AGE OF FOSSIL GENERA PERTAINING TO BRACHIOPODS Outline of Experiment________________________________________ 1.1 Introduction 1.5 Precautions To Be Taken Before Studying Fossil Specimens Expected Learning Skills 1.6 Laboratory Exercises 1.2 Requirements 1.7 Results and Discussion 1.3 Brachiopods: Systematics, General Morphology and Age Range 1.8 References 1.4 Common Genera 1.9 Further Suggested Readings 1.1 INTRODUCTION You have studied the general morphology, systematic position and geological history of brachiopods in Unit 13 of Block 4 of BGYCT-137. You have learnt that the brachiopods form a major fossil group of animals. Their fossils first appeared in rocks of early Cambrian age and the distant descendants of these ancient forms are still living in the oceans and seas. Brachiopods are entirely marine animals; occur in all oceans at depths ranging from the intertidal to 5000 m and till date no brachiopod has been found in the fresh water environment. All brachiopods are sessile (non-mobile), benthic and solitary marine animals having a shell made of two unequal valves and live on the sea floor usually in clusters. Many of them remain fixed to the sea floor while others just lie on the sea-bed. In this experiment, we intend to provide you with some practical experience in studying some common fossil genera of brachiopods. It is important to note that brachiopods have around 4500 known genera, out of which merely 100 genera are surviving in today’s ocean. 9 Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 The morphological study of various genera of brachiopods or any fossil group is important to accurately identify the genera and to differentiate one form from another. It also helps us to establish the evolutionary sequence of a group with respect to its geological age range, which in turn, provides data to know the diversity, palaeobiogeography, palaeoecology and geological significance of any group or its genera. Expected Learning Skills__________________________ After performing this experiment, you should be able to: list the systematic positions of some common genera of brachiopods; identify and describe their morphological characters; prepare their line drawing with labeling of morphological features; and write their stratigraphic/geological age and palaeoecology 1.2 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 brachiopods Instructions: You are required to study Unit 13 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. 1.3 BRACHIOPODS: SYSTEMATICS, GENERAL MORPHOLOGY AND AGE RANGE 1.3.1 Systematic Palaeontology/Taxonomic Hierarchy Kingdom Animalia Subkingdom Metazoa Phylum Brachiopoda Subphyla Linguliformea Craniiformea Rhynchonelliformea 1.3.2 General Morphology The shell of a brachiopod consists of two-valves, which are held together either by teeth and socket (e.g., rhynchonelliformeans or articulates) or by muscles (e.g., linguliformeans and craniiformeans or inarticluates). The two valves of brachiopods are morphologically quite different, each valve is bilaterally symmetrical and one valve is always larger than the other. The larger valve is commonly known as pedicle or ventral valve. It contains an opening known as pedicle opening or foramen at the beak. The smaller valve is known as brachial or dorsal valve (Fig. 1.1). Each valve has a 10 Morphological Characters, Systematic Position and Experiment……………………….…………………………………………………………………………….….............…... 1 Age of Fossil Genera Pertaining to Brachiopods pointed end which is known as beak. It represents an initial point of growth of a valve. In brachiopod shells, the shell growth mostly occurs in the form of concentric lines, radial ribs or corrugations on the exterior surface of the valves. The arched part of the valve near the beak is called umbo. The beak is considered the posterior end of the valve whereas the rounded margin located opposite to the beak is the anterior end as shown in Fig. 1.1. Commissure is the line where the two valves meet (Fig. 1.1b). It may be straight or zig-zag. (a) (b) Fig. 1.1: Shells of brachiopods showing external morphology: a) dorsal view; and b) side view. Fig. 1.2: Dorsal view of a brachiopod shell showing internal morphology of the pedicle valve. (Source: simplified after Clarkson, 1979) The pedicle valve has a deep medial depressed area described as the sulcus that receives medial elevated portion termed as fold of the brachial valve. Teeth are knob-like projections that are present on the posterior end of the pedicle valve and fit into the small depressions known as sockets of the brachial valve. The contact point where teeth and sockets of the two valves rotate to open and close forms the hinge and the portion of the posterior end 11 Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 where both valves meet is known as hinge line (Fig. 1.2). A straight hinge line is described as strophic, while a curved one is named as astrophic. Brachiopod valves come in a variety of shapes like convex, concave or flat and some forms show interesting external ornamentation of costae and plicae. Costae are very fine radial ridges on the external surface of the valve originating from the beak and plicae are radial ridges present on external as well as internal surfaces of the valve. In a few forms, the valves are covered with spines. 1.3.3 Stratigraphic Range Brachiopods have a very long geological history. They first appeared near the beginning of the Cambrian period and some of the forms are still alive today on the floor of the modern oceans. 1.4 COMMON GENERA 1.4.1 Linoproductus Systematic Palaeontology Kingdom Animalia Phylum Brachiopoda Subphylum Rhynchonelliformea Class Rhynchonellata Order Productida Family Linoproductidae Genus Linoproductus Morphological Description: It is a medium to large sized genus of brachiopods. It has concavo-convex valves. Shell is of medium to large size and its outline is elongated to subcircular. The ventral profile inflated posteriorly and brachial valve gently concave. The pedicle valve convex, brachial valve starts from flat, but becomes concave. The margins of the two valves are often turned ninety degrees from the hinge and become very elongated. Surface of both valves are marked with fine striations that are slightly wavy. Hinge commonly occurs in the widest part of shell and tends to be distorted at spine bases, which are widely scattered ventrally. The cardinal process is sessile and supported by lateral ridges. Adductor scars and brachial ridges are weakly marked. The genus is also known for having some of the largest shells. Spines are occasionally present. The genus Linoproductus is distinguished by its strongly convex pedicle valve and its slightly concave brachial valve (Fig. 1.3). Stratigraphic Range: Carboniferous to Triassic. Palaeoecology: The genus Linoproductus (known originally as Productus) was a marine animal that commonly lived as sessile, low-level epifaunal suspension feeder. 12 Morphological Characters, Systematic Position and Experiment……………………….…………………………………………………………………………….….............…... 1 Age of Fossil Genera Pertaining to Brachiopods (a) (b) Fig. 1.3: Linoproductus: a) Photograph; and b) Line drawing of the external views of the valves of Linoproductus showing the shape and growth lines. 1.4.2 Spirifer Systematic Palaeontology Kingdom Animalia Subkingdom Metazoa Phylum Brachiopoda Subphylum Rhynchonelliformea Class Rhynchonellata Order Spiriferida Family Spiriferidae Genus Spirifer Morphological Description: This genus is triangular in shape and characterised by having a long and straight (strophic) hinge line. It has a biconvex shell, prominent umbo and well-developed interarea of the pedicle valve, large delthyrium and triangular brachial valve. The external surface of the shell is marked by strong radial ribs, sulcus and plica (Fig. 1.4). (a) (b) (c) Fig. 1.4: Spirifer: a) Line drawing of the side view; b) Line drawing of the dorsal view of Spirifer (Source: modified after Nield, 1987); and c) Photograph showing external view. (Courtesy: Mr. A. A. Quarshi, Srinagar) Stratigraphic Range: Silurian to Jurassic. Palaeoecology: The genus Spirifer was a marine animal, lived as sessile and epifaunal suspension feeder. 13 Stratigraphy and Palaeontology: Laboratory BGYCL………………………………………………………………………………………………….….............….…........-138 1.4.3 Athyris Systematic Palaeontology Kingdom Animalia Subkingdom Metazoa Phylum Brachiopoda Subphylum Rhynchonelliformea Class Rhynchonellata Order Athyridida Family Athyrididae Genus Athyris Morphological Description: It usually has a biconvex transverse shell. The hinge line is curved. The both valves of the shell are almost equally convex. The umbo ventrally, round foramen, small deltidial plates and teeth are prominent on the dental plates. Long and thin dental plates are present on the pedicle valve whereas the brachial valve contains cardinal plates. The concentric growth lines occur on the external surface of the shell (Fig. 1.5). (a) (b) (c) Fig. 1.5: Line drawing of Athyris: a) External; b) Side; and c) Dorsal views. Stratigraphic
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