CHAPTER XIX.

PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHARACTER; ILLUSTRATED BY THE EVOLUTION OF THE SUTURE LINES OF AMMONOIDS.

The Ammonoids Illustrate the Law of Acquirement of Differences by Gradual Modification.-The Ammonoids illustrate another of the laws of evolution in a particularly satisfactory manner.

When we examine the representatives of the same genus, or family, or order, at the beginning and at the close of its life-period, it is very common to find the two representatives differing in one or more characters, which may be described as differing in the degree or extent of their development. The number of parts has increased; some part which is small in one is large in the other; some structure which is simple in the earlier is complex in the later; or parts which are indefinite in form, or similar in the beginning are definite and particular in form and structure at the close.

It is rare, however, to be able to collect examples to show the various stages by which the one was elaborated by degrees of modification into the other. The famous case of the development of the specialized horse foot out of a five-toed ancestor is familiar to all, with the beautiful theory of the way by which the modification came about. This is a case of relative rather than of direct evolution, since the prominence of the one toe and line of connecting bones is produced by the aborting and withdrawal from use, and finally from development, of the normal number of parts which were present at the beginning of the series. The Ammonoids, as we shall see, illustrate the case of actual increase in complexity, gradually and continuously; the order of succession in the steps of progress being clearly and regularly expressed by the actual appearance of each form at the particular geological stage in

which it should appear according to the law of genetic evolution of the characters of the race.

Description of the Characters of the Ammonoids.

In order to

place before the reader a concise description of the characters of Ammonoids, the definitions of Zittel may again be followed, furnishing as they do the precise characters needed for an understanding of the problem under discussion.

Zittel's definition of the characters of the Ammonoidea is as follows:

SECOND SUBORDER: AMMONOIDEA. Shell generally enrolled or spiral, discoidal, more rarely spirally coiled, evolute, arched or straight; opening simple or furnished with lateral and ventral prolongations. Sutureline undulating, notched or with slashed or dentate lobes and saddles; siphuncle cylindrical. always marginal, without internal deposit; initial chamber spherical or ovoid, frequently an aptychus or anaptychus.

In the description of the fundamental characteristics of the sutures and their development we follow Zittel's synopsis.

A

The embryonal chamber (nucleus, ovisac) of the Ammonoids has a spherical or transversely ovoid shape (Fig. 112, a); it is smooth, separated by a contraction from the rest of the shell, and always enrolled spirally about an imaginary axis. Its anterior aspect is, in consequence, essentially different from its lateral profile, its sides having a projection in form of an umbilicus. The embryonal chamber, of which the height varies from 0.3 to 0.7 mm., is limited in front by the primary septum. The constitution of the first suture gives, according to the beautiful researches of Branco, excellent basis for classification. In the most ancient Ammonoids it forms a straight line, more or less simple, and then resembles the first suture of Nautiloids; Branco calls these forms the Asellati (Fig. 11, A)

In a second group the first sutural line proceeds forward to form an arch towards the ex

B

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terior, and forms a large simple ventral saddle, FIG. 111.-Ventral views of the Latisellati (Fig. 111, B).

The third group is distinguished by the relatively narrow ventral saddle, on each side of which is developed a lateral lobe and generally also a small lateral saddle, Angustisellati (Fig. III, C).

edges of the embryonal chamber, representing A the primary septum of an asellate Goniatite, B of a latisellate Ammonite (Arcestes), and C of an angustisellate Ammonite (Phyloceras). (After Branco.)

While the first suture of all Ammonoids is comparatively simple, more or less considerable complication is produced by the later development of the shell. Only a few of the more ancient types possess a sutural line altogether simple, like that of the Nautilids. Almost always, even in Paleozoic forms, the suture attained at least the Goniatite stadium, that

is, an undulating or notched suture formed of simple lobes and saddles (Fig. 112).

A later complication is observed in the Ceratite stadium, in which the saddles remain intact while, on the contrary, the lobes are notched by slight denticulations. The more elaborate differentiation is reached in the Ammonite stadium, in which the lobes and saddles are gashed by secondary notches in the most variable manner.

FIG. 115.-Suture of an Ammonite, Desmoceras latidorsatum. (After Zittel.)

As the Goniatites appeared, in general, before the Ceratites, and these in part before the true Ammonites, it is believed that these three genera may be considered to be the three principal stadia of development of the Ammonoids. This view is further confirmed by the fact that the suture line of all Ammonites in the course of the first whorl passes through the Goniatite stadium (Fig. 116, H to N). According to the researches of Hyatt and Branco, however, the Ceratite stadium is, in general, passed

over and the Goniatite stadium passes directly into the Ammonite stadium. The development of the sutural line by folding of the septum advances from without inwards; on the contrary, the new lobes and the new saddles are intercalated, almost always, at the lateral suture of the whorl, and rarely on the external ridge.

The second suture is distinguished from the first in almost all Ammonoids by the development of an external ventral lobe, more or less deep, simple or bifid, which gives rise to two external saddles caused by the dichotomy of the original simple saddle. It is rare that it is confined to these three elements; generally, there is added besides a lateral lobe and a lateral saddle. In the more simple forms the suture has by that time acquired its definite shape, and all the later chambers present the same design at the point of their attachment. There generally occurs, however, a multiplication of the lobes and of the saddles, and the external lobe takes part in it by one small median saddle becoming bifid.

=

M

FIG. 116-Development of the suture of an Ammonite (Trobites subbullatus). G = 1st suture, H 2d, I = 3d, L = 7th, MN = sutures of second whorl, 0= definitive suture. (After Branco.)

Such is the characteristic development of the suture in the Goniatites, the Clymenias, and a small number of the Triassic Ammonites. In the Ceratites and the true Ammonites there takes place exactly the same differentiation at the outset as in the Goniatites; but later, when the shell has reached the size of 3 mm. in diameter, begins the secondary slashing of the lobes and of the saddles of the exterior and of the interior. (See O of Fig. 116.)

At the size of 4 mm. the Ammonites are generally in possession of these characteristic suture lines, which from that time on remain constant, or at least suffer very slight change. In the determination of the several species it is necessary to compare the suture lines of only the mature forms. The external lobe does not tend to become bifid in the Goniatites and Ammonites, the most ancient geologically, as in a stadium of relatively tardy growth. In the relatively young Angustisellati the division into

FIG. 117.-Suture of Pinacoceras Metternichi. (After Zittel.)

two lobes is distinctly accomplished. In a single form, or even in series of forms, or in the most closely related species, the geologically younger representatives generally possess the more differentiated suture lines; on the contrary, however, it is not possible to deduce the geological age of an Ammonite from the structure of the suture line alone. In the Trias there are forms (Pinacoceras, Fig. 117) which present lobes so finely slashed and so complicated that one can scarcely observe similar ones in

the most recent formations; on the other hand, there are known Ammonites (Buchicera) from the Middle and Upper Cretaceous, the sutures of which represent the Ceratite stadium (Fig. 118; also compare with Fig. 114) by retrocession, if they be not quite the same genera. In all typical Ammonites there is developed, besides the external ventral lobe, which, in the forms with an external siphuncle, is called often also siphonal lobe, two main lobes on the side-the first and second lateral lobe. Besides the external lobe, there are two large external saddles; and besides the lateral lobes, the two primary lateral saddles. The external is almost always profoundly slashed into two points by the development of a secondary median saddle, while the internal lobe (dorsal lobe) opposite ordinarily remains entire. The external saddles are also able to be divided sometimes by deep secondary indentations. genera (Pinacoceras) the differentiation of the external part of the external saddle goes so far that there are intercalated between it and the external lobe a greater or less number of supernumerary saddles and lobes. All the saddles and all the lobes from the second lateral saddle to the internal contact suture of the whorl are called external; those which are within the contact sutures up to the inner saddle receive the name internal auxiliary lobes and saddles.

In some

The variability in the number and size of the lobes is, generally, in relation with the form of the shell. If the whorls are circular, one observes, ordinarily, only a few lobes, and in that case they are of nearly equal dimensions (Lytoceras); upon a wide ventral side the external lobe and the external saddle acquire considerable dimensions; the more flat the sides are and the thinner the ventral part, the larger the size of the lateral lobes and lateral saddles, and the more numerous the auxiliary lobes.

Two Divisions of the Retrosiphonatæ: Goniatites and Clymenias. -In following the course of evolution of this group, as indicated by the modifications of the suture-line, we begin with the first division of the Ammonoidea-the Retrosiphonate of Fischer. The two groups are the Goniatites and the Clymenias. The fundamental and constant difference is found in the relative position of the siphuncle. In the Goniatites the siphuncle is external and in the Clymenias always internal.

The Goniatitinæ, of Hyatt's classification, begin in the