Quarterly Journal of the Geological Society of London/Volume 34/On Argillornis longipennis, Ow., a large Bird of flight, from the Eocene Clay of Sheppey

10. On Argillornis^[1] longipennis, Ow., a large Bird of flight, from the Eocene Clay of Sheppey. By Prof. Owen, C.B., F.R.S., F.G.S., &c. (Read December 19th, 1877.)

[Plate VI.]

The fossils on which the above genus and species of extinct bird are propounded were discovered in the London Clay of Sheppey Island, and form part of the collection of W. H. Shrubsole, Esq., of Sheerness-on-Sea, by whom they have been kindly submitted to me, with permission to take casts of them for the Geological Department, British Museum.

They consist of parts of fractured humeri, the right and left, of the same species or individual, and include the articular head of the bone, with portions of the upper and lower parts of the shaft. They are in the usual petrified condition of the Sheppey fossils, more or less impregnated with pyrites, and, from the fractured and abraded condition of the more prominent parts of the bones, seem to have been subject to the forces of transport and rolling.

The texture of the shaft, the thinness of the compact outer bony wall, and the large size of the obviously pneumatic cavity, recall the characters of the wing-bones of the large Pterodactyles of the Cretaceous period. This leads me to refer to the 'Supplement' No. iii. to the Monograph on the Cretaceous Pterosauria (Palæontographical Society's volume, 4to, for 1860), in which descriptions and figures are given of the proximal portion of the right humerus of Pterodactylus Sedgwickii, Ow., a species about the size of a large vulture (Vultur monachus, L.), of which bird comparative views of the answerable bone and part are added in the same plate (pl. iii.).

A comparison of figs. 1-3 (Plate VI.) of the present paper with figs. 1 and 2 of the plate cited will show the difference of the Eocene fossil from the Cretaceous one; a similar comparison with figs. 6-8 of plate iii. Monogr. cit. will show the avian characters of the Sheppey specimen.

In giving the results of comparisons with the humerus in different kinds of birds, I avail myself of the same terms indicative of aspect and position as are defined in that Monograph. Proximal signifies the upper, distal the lower end of the bone as it hangs in Man. In the natural position of the humerus, as at rest, in Birds, the distal end is usually higher than the proximal one. When the palm of the hand is turned forward in the pendent arm of Man, the corresponding surface of the humerus is "palmar," the opposite surface is "anconal;" the outer side of the humerus is "radial," as being that which the radius holds; the opposite or inner side is "ulnar." The answerable surfaces or aspects bear the same names in the humerus of the bird. "Proximad," "palmad," &c. are adverbial inflexions, meaning "toward the proximal end" and "toward the palmar side" of the bone.

In the proximal end of the humerus, figured in Plate VI., the following parts are noted:—

a. Articular head.

b. Radial tuberosity (answering to the "greater tuberosity," or tuberculum majus, of anthropotomy).

c. Ulnar tuberosity ("lesser tuberosity," or tuberculum minus of anthropotomy).

d. Scapular groove, lodging the lower border of the glenoid surface of the scapula (part of the "neck of the humerus" of anthropotomy).

e. Ligamentous pit or surface, receiving the anterior coracoid ligament.

f. Subtuberous fossa, with "foramen pneumaticum."

g. Radial border of ditto.

h. Tricipital ridge.

i. Pectoral crest.

k. Ancono-deltoid ridge.

l. Distal radial border of humerus.

m. Prebrachial depression.

n. Ectepicondylar process.

The subject of figs. 1–3 includes, of the left humerus, the articular head (a), the base of the radial tuberosity (b), that of the ulnar tuberosity (c), with the intervening scapular groove (d), the beginning of the tricipital crest (h), and that of the pectoral crest (i). In what remains of the subtuberous fossa part of a pneumatic foramen (f) is preserved; a greater proportion of the pneumatic fossa is shown in the right humerus (fig. 3, f').

The head (fig. 3, a), or proximal articular surface, is, as usual, elongate and moderately convex in the radio-ulnar direction, more convex across the shorter ancono-palmar diameter. It is characterized by its large relative size as compared, for example, with that in the albatross (Diomedea exulans), ib. figs. 4–6; and differs from the humerus in that and other birds in the degree and extent of the con- cavity longitudinally, or in the direction of the long axis (fig. 3, a') at the radial third part of the surface. The humerus of the Marabou (Ciconia marabou, Temm.) offers a feeble approach to this character.

The radial or outer tuberosity (b) has lost its outer layer; the cancellous structure is exposed by abrasion. The rising or smooth ridge (fig. 2, h), continued from the anconal part of the tuberosity upon the shaft below, is better marked than in Vultur monachus, still more so than in the marabou, pelican, and albatross, in which it is barely definable. The ulnar or inner tuberosity (c) is bent anconad, leaving a (scapular) groove or canal (d) between it and the head, wider and shallower than in the vulture, marabou, crane, and pelican, but less wide and shallow than in the albatross (fig. 4, d). Of the shape and extent, however, of this tuberosity the worn and fractured state of the fossil prevents a judgment.

The palmar part of the head (fig. 1, a), though abraded, projects further over that surface of the shaft than in any other bird with which I have compared the fossil. In the extent of the flat expanded surface so overhung the fossil is remarkable. The albatross, again, most nearly resembles it in this character, but without the overhanging production of the head (compare figs. 1 & 5). The smooth outer crust of this surface is impressed in the fossils with minute points, and shows very faint longitudinal striæ.

There is a trace of the foramen pneumaticum (fig. 2, f) at the bottom of the fossa beneath the ulnar tuberosity, the walls of the fossa being broken away.

The thinness of the compact outer wall of the shaft (with the concomitant wide air-cavity), and the size and conformation of this part of the wing-bone, bespeak a bird of flight, and a species as large, at least, as any of the existing birds which enjoy the characteristic locomotion of the class.

The portion of the right humerus includes, with the head (fig. 3) and indications of proximal structures above described, five inches of the shaft. From a preserved breadth of 2 inches it diminishes to that of 10 lines at the fractured end. The smooth ridge noted in the left humerus (fig. 2, h) is continued distad for 21/2 inches before subsiding.

With the head of the left humerus were brought three portions of a long limb-bone with the pneumatic structure of the humerus in longipennate birds of flight (Pl. VI. figs. 9 & 10), and corresponding in size with the portion of the shaft of the right humerus, but less crushed and distorted.

The first of these portions I infer to be from the proximal half of the shaft; it is three-sided, with the angles broadly rounded off, but least so at that angle which shows the base of an outstanding ridge. An inch and a half of this base or origin is preserved (figs. 7 & 8, i), continued from the proximal fractured end of the bone. I infer it to be part of the origin of the "pectoral crest." Anconad of this origin, and about 3 lines distant therefrom, is a linear ridge (fig. 7, k) running longitudinally parallel therewith, and bending slightly more anconad before terminating at the distal fractured end of the portion. This ridge denotes the insertion of the ancono-deltoideus, or "posterior deltoid muscle."

Now, as to the trihedral shape of this proximal portion of the humerus, which may include one fourth or one third of the shaft, such shape is not shown by the humerus in the majority of birds: I find it in certain Raptores, Longipennates, and Totipalmates. In all that do show it one of the sides is palmad, the other two sides are anconad (as in figs. 7, 13). In the eagles and vultures (Vultur monachus, monogr. cit. pl. iii. fig. 7) the angle dividing the radial from the ulnar sides of the anconal surface is sharper than in Argillornis.

Pelecanus and Diomedea come nearest to the fossil in the obtuseness or degree of rounding-off of that angle, but fall short of that degree shown in the fossil. In all the characters above compared, the portion of shaft (fig. 7), like the head of the bone (Pl. VI. fig. 1), is from a left humerus.

The next character which I find available in steering toward the port of determination is the linear ridge above mentioned, viz. the "ancono-deltoid ridge." It varies, in different birds, in its relative position to the pectoral ridge. In Raptores it is relatively closer to the base of that ridge than in Argillornis. In Pelecanus its relative position agrees better with that of the ancono-deltoid ridge in the fossil, but it inclines distally toward the radial border of the bone instead of from that border. In Diomedea both its course and relative position (fig. 13, k) at the part of the base of the pectoral ridge answering to that preserved in the fossil best agree therewith; but there is in Diomedea a second linear intermuscular ridge (ib. k') anconad of the first, of which there is no trace in the fossil.

But the sum of my comparisons of the present portion of humerus inclines me to see the nearest affinity to be to the longipennate natatorial or aquatic birds, and among them to the largest existing kind, viz. the albatross (Diomedea exulans), but with a difference of size indicated by the subjoined admeasurements:—

Argillornis.
lines.

Diomedea.
lines.

Transverse breadth of humerus at distal termination of the origin of the pectoral ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A guiding modification of the distal portion of the humeral shaft in birds is the difference of form of the radial and ulnar sides of the bone. The radial side becomes, in most birds, the narrowest, as the shaft descends and expands toward the distal articular end; and in the albatross it is carried to the extreme of becoming a mere ridge for near 3 inches from that end (figs. 14, 15, l). The portion of the distal end of the shaft of the present fossil humerus, wanting the articular termination of the bone (figs. 10-12), presents this character, and, as in Diomedea, shows the narrow longitudinal groove on the palmar side of the ridge (fig. 11). This groove is the radial border of the triangular flat and shallow depression (figs. 11, 15, m) for the origin of the "brachialis anticus" muscle, which depression shows the usual linear roughness or sculpturing due to such relation of muscular attachment. The same sculpturing marks the triangular" prebrachial depression" (fig. 11, m) in Argillornis; but the triangle is longer and narrower in the fossil, and the free surface of the palmar part of the shaft, ulnad of the depression, rises more abruptly from it in Argillornis, and presents a more uniform transverse convexity than in Diomedea.

Unfortunately the characteristic ectepicondylar process (fig. 15, n) of Diomedea has been broken away, if it ever existed in Argillornis, with the rest of the distal end of the humerus in the present fossil (fig. 11); but it may be remarked that the presence of such a projecting part of the humerus, like the pectoral and subtuberous ridges in Diomedea (figs. 4 & 5, c' & i) would render the rolled fossil more liable to such fracture and loss.

In relation to other species and genera of birds based on fossil remains from the London Clay, the cranial evidence of Dasornis londinensis^[2] indicates a bird too big to be upborne by wings of a size to which the present fossil bone would belong; and, besides, the characters of that fossil skull were rather those of a large flightless or ground-bird. The skull of Odontopteryx, on the other hand, seems too small in proportion to the humerus of a bird exceeding in size that in Diomedea.

It is much more probable that the avifauna of the Eocene period should supply a palæontologist with many more species than he has already determined, than that so singular a form should have existed as a bird with a head of the size of that of the Solan goose and wings of vaster expanse than those of the albatross. The sternum of the still smaller Eocene bird, Lithornis vulturinus^[3], at once removes that genus and species to a distance from Argillornis.

The humerus of Pelagornis miocænus, Lartet^[4], of similar size to that of Argillornis, differs, as far as its mutilated state permits comparison, in the form of the articular head and the narrower scapular groove.

In Pelagornis the head is relatively small and less prominent proximally than in other birds; the transverse ligamentous groove (e) is well marked, as in Totipalmates; the bicipital surface is very narrow, and develops below a tuberosity more prominent than in the Boobies (Sula), but not projecting beyond the ulnar border of the shaft; the ulnar tuberosity is large and projects anconad.

Prof. Seeley^[5] refers an ornitholite in the Woodwardian Museum to the Lithornis emuinus of Bowerbank, and accepts its supposed emuine or cursorial affinity, with the remark:—"Taking the ostrich as a type, this bird diverges from the typical Struthionidæ on the other side of the emu, yet appears to conform to the Casuarine allies;" and he proposes to refer the fossil to a genus Megalornis.

If the Woodwardian fossil should prove to have formed part of the longipennate volant bird, the subject of the present paper, I would refer to the 'Proceedings of the Zoological Society of London,' February 1843, for the following entry:—"A communication from Prof. Owen was read, proposing to substitute the name Dinornis for that of Megalornis, applied to the Great Bird of New Zealand in his paper read at the previous Meeting. The change is rendered desirable to prevent confusion in nomenclature, Mr. George Gray having previously used the term Megalornis for a genus of Birds in his 'List of Genera' &c." To this notice the Editor adds a footnote:—"This change in the name has been made in the paper referred to whilst passing through the press."

 

EXPLANATION OF PLATE VI.

Fig. 1.	Argillornis longipennis. Proximal end of left humerus, palmar surface.
2.	The same, aneonal surface.
3.	Proximal end of right humerus, proximal surface.
4.	Diomedea exulans. Proximal end of left humerus, anconal surface.
5.	The same, palmar surface.
6.	The same, proximal surface.
7.	Argillornis longipennis. Proximal portion of the shaft of left humerus, anconal surface.
8.	The same, radial border.
9.	The same, transverse section distad of pectoral ridge, showing size of pneumatic cavity and of its compact wall.
10.	Distal portion of the shaft of left humerus, transverse section above l, fig. 11.
11.	The same, palmar surface.
12.	The same, radial border.
13.	Diomedea exulans. Proximal portion of shaft of left humerus, anconal surface.
14.	The same, transverse section distad of pectoral ridge.
15.	Distal end of left humerus, palmar surface.
16.	Argillornis longipennis. Ulnar side of distal end of fig. 7, showing a groove, o, leading to the pneumatic cavity.
17.	Diomedea exulans. Corresponding side of fig. 13, showing the groove leading to the "foramen arteriæ nutritiæ" in Diomedea. This additional evidence of the humeral nature of the portion of bone of Argillornis (fig. 7) was detected and pointed out to me by the accomplished artist Mr. C. L. Griesbach, F.G.S.

 

Discussion.

Prof. Seeley said that it was impossible to form a judgment upon the matters brought forward by Prof. Owen without an opportunity of closely examining the specimens. At the same time he had no doubt that the remains were those of a bird; and he had himself, several years ago, obtained in the same locality a bone (a cast of which he exhibited) which he regarded as part of the tibia of a very large bird, and referred to a genus which he called Megalornis. At the time that his paper on this specimen was written, he was not aware that the name had been already appropriated, although he had consulted competent ornithologists on the subject. With regard to the specimens described by Prof. Owen, there might perhaps be some doubt whether all of them belonged to the fore limb, since two of them, he thought, closely resembled parts of a tibia such as the tibia of Megalornis. If this resemblance were not an accidental coincidence, the remains exhibited might furnish indications of two genera.

The Author, in replying, remarked that the bone which Messrs. Bowerbank and Seeley had held to be "tibial," and of an Emuine or wingless bird, was "pneumatic," therefore part of a bird of flight. But if tibial, the proportions of the humerus of such a bird would far exceed those of the fossils exhibited and referred to that bone. Prof. Owen then pointed out in the supposed tibial fragment intermuscular ridges which, with the more obvious characters he had described, would receive from palæontologists the same interpretation as his own.

1. ἄργιλλος, clay; ὄρνις, bird.}}
2. Trans. Zool. Soc. vol. vi. p. 144.
3. 'British Fossil Mammals and Birds,' 8vo, 1846, p. 549, fig, 232. One of the fossils described in the present paper had a label attached, with the name Lithornis emuinus; if the specific name referred to Dromaius, the evidence of the size of wing and concomitant power of flight renders the reference of such fossil ('Comptes Rendus de l'Acad. des Sciences,' 6 Avril, 1857) to the Australian struthious genus inappropriate.
4. Alph. M.-Edwards, 'Oiseaux Fossiles de la France,' 4to, 1868, pl. xlv.
5. Seeley, "Note on some new Genera of Fossil Birds in the Woodwardian Museum" (Annals and Magazine of Natural History, 1866, 3rd ser., vol. xviii. p. 110).