The Life Appearance of Sauropod Dinosaurs

If you’re a regular TetZoo reader you’ll be familiar with my several articles on the life appearance of Mesozoic dinosaurs, an issue in which I have a special interest. Alas, several of these articles were published at TetZoo ver 2 – the ScienceBlogs years – and hosting issues at the site concerned mean that they’re currently appearing sans all of the many images I so lovingly uploaded. Which is a pain.

A really nice, life-sized model of a titanosaurian sauropod, on display in Romania and constructed by Brian Cooley. Image: Darren Naish.

Partly as a consequence, and partly because the issue is on my mind due to several concurrent projects (he says, cryptically), now is a good time to talk once more about the life appearance of Mesozoic dinosaurs, and I’m going to start with sauropods. Sauropods have been covered quite a few times on TetZoo before, as you can see from the list of links at the bottom of this article.

The head, and face especially. We’ll start with the head. Sauropod skulls were proportionally small compared to the overall size of the animals, but not comically so. It’s also worth saying that their eyes – while pretty big in absolute size (based on the size of the eye socket and the sclerotic rings preserved in fossils) – are easy to over-emphasise in illustrations. If you’ve heard that sauropods might have had trunks and thought it reasonable or plausible… well, it’s a poor idea that’s neither reasonable nor plausible, and has a lot counting against it, as covered in the ver 3 article here.

Did sauropods have trunks? NO. It’s the dumbest idea ever, and every piece of evidence counts against it. Several authors have independently explored this idea, including Robert Bakker (upper right) and Bill Munns (lower right). The two images at left are from Knoll et al.’s (2006) paper refuting the trunk idea. Images: Knoll et al. (2006), Bakker (1986), Bill Munns (original here).

Trunks are a no, but did sauropods have ‘lips’ or ‘cheeks’? A whole article could be written on this issue. For now, I’ll summarise things by saying that sauropod skull bone texture indicates that they likely did have extra-oral tissues that mostly covered their teeth (Witton 2018), meaning that they were facially similar to lizards and kin.

A suggestion that some titanosaurs had blade-like cutting sections in the posterior sections of their jaws – so-called ‘guillotine crests’ (Apesteguía 2004) – looks unlikely given that it would require the animals concerned to do their food procurement at the sides of the jaws, rather than at the front like all other sauropods and other dinosaurs. It’s more likely that a mistake has been made here and that the jaw segments thought to support those blade-like, keratinised sections were, while sharp-edged, covered in normal lip tissue. The possibility that diplodocoids and maybe other sauropods too might have had true beaks at the font of the mouth has recently been put forward, but so far only in preliminary fashion.

Apesteguía (2004) argued that some titanosaurs - this is the rhino-sized Bonitasaura - had keratinised ‘guillotine crests’ on the edges of the jaws. I think that this is likely a mistake. Image: Apesteguía (2004).

The nostrils and nose. As is now widely known, and near-universally accepted, the external or fleshy nostrils of sauropods were almost certainly not located way up on the forehead, far from the front of the snout, as was long thought based on the retracted position of the bony nostril openings. A more ‘normal’, anterior position for the nostrils was demonstrated by Witmer (2001) who pointed to evidence from blood vessel and nerve impressions and associated cranial hollows, all of which are located on the anterior part of the snout. These indicate the most likely position of the fleshy nostril and associated blood vessel clusters.

The soft dinosaur revolution comes to Sauropod Town. At left, an image by Matt Wedel which shows why we need more soft tissue on our sauropods. At right, Larry Witmer’s (2001) depiction of the different possibilities as goes nostril position in sauropods. (a) is the most likely option based on anatomical data. Image: Mathew Wedel, Witmer (2001).

There are also reasons for thinking that the giant, cavernous bony nostril openings and tall nasal bars of some macronarian sauropods supported, and were surrounded by, dome-shaped soft tissue convexities, superficially recalling the bulging nasal regions of some living monitor lizards. Part of my reason for saying this comes from the nasal anatomy of the exquisitely preserved South American titanosaur Sarmientosaurus. Here, an anteriorly projecting bony spine located along the midline and associated bar-like structures on the lateral edges of the large narial fossa – the big bony opening surrounding the bony nostril – indicate that a wide, convex mass of tissue connected the forehead with the sides and front of the snout’s upper surface (Martínez et al. 2016). Sarmientosaurus isn’t unique in this respect but is a particularly good example.

The skulls of some sauropods - this is the titanosaur Sarmientosaurus - indicate that there were bulbous nasal structures covering much of the snout region. Image: WitmerLab.

Indeed, skull openings in general were almost definitely not sunken in appearance or otherwise all that obvious, something that’s being said of archosaurian faces in general as artists and anatomists have learnt to take better attention of the conditions present in living animals (where cranial openings are just about never obvious in the live creature).

Accordingly, sauropod faces were seemingly ‘softer’ and more ‘padded’ than convention would have it, at least some of – perhaps all of – the nasal and forehead region being convex and fleshy, rather than shrink-wrapped and covered in thin skin alone (Witton 2018).

Were sauropod necks mostly semi-horizontal and with a limited range of motion, as argued by Martin (1987) and illustrated at left? Nope; it’s more likely that the necks were often held aloft and far more flexible, as argued by Taylor et al. (2009) and depicted at right. Images: Martin (1987), Taylor et al. (2009).

The neck. The most remarkable feature of sauropod anatomy is the neck. As goes how the neck was held and how flexible it was, several possibilities have been put forward, some workers arguing that it was held in a mostly horizontal attitude for much of the time and with only a limited range of lateral and vertical flexibility. I’m part of a group who argue for mostly elevated habitual neck poses (even in diplodocids) and a wide range of flexibility (Taylor et al. 2009). Arguments that sauropods must have been horizontal-necked do not take account of the flexibility permitted by cartilage, or – in living animals of all sorts – the ranges of motion that happen at zygapophyseal junctions and at the neck base and head-neck junctions.

There are good reasons for thinking that sauropod necks were habitually inclined upwards, as is typical for terrestrial tetrapods in general, and several fossils are actually preserved with the neck in this orientation. This montage by Greg Paul shows several of the fossil concerned. Image: Paul (1998).

It's been convention to show the sauropod neck as a featureless tubular structure, like a hose. This is mostly wrong, in part because the shapes of the vertebrae reveal a more unusual cross-sectional shape that varies from one sauropod group to the next. In some sauropods (like mamenchisaurs), the vertebrae are quite narrow and the neck would have looked laterally compressed in places, as it is (for at least some of its length) in giraffes. In diplodocoids – apatosaurines in particular – the neck is narrow close to the head but wide for much of its length, and subtriangular in cross-section, being widest across the neck’s underside. A neck that was quite narrow in its anteriormost quarter or so but was markedly wide for the rest of its length seems to have been the case in brachiosaurs and at least some titanosaurs.

The necks of some sauropods - a good example is the ultra-long-necked Omeisaurus, photographed at Zigong Dinosaur Museum - are somewhat laterally compressed for at least part of their length. Image: Darren Naish.

Indeed, the very base of the neck is remarkably broad in some sauropods – Camarasaurus is the classic example, where the neck base is not that different in width from the front of the chest – meaning that the neck would taper gradually along its length if you were looking at the animal from the front or back, or from above or below.

The neck bases of some sauropods - this is the Camarasaurus replica on display in London’s NHM - are shockingly broad. It would be wrong to show the neck as a narrow, hose-like object with parallel sides. Image: Darren Naish.

The vertebrae themselves are very complicated with large hollows on their sides, projecting neural spines on the apices and so on. In the most extreme version of the shrink-wrapping meme promoted by one or two palaeontologists and palaeoartists, sauropod necks have been depicted as if these structures should be visible in the live animal, Ely Kish’s apatosaurines from 1983 being the ultimate example. This was definitely not the case. It remains uncertain how much musculature and soft tissue surrounded the vertebrae, but it was almost certainly (based on the anatomy of living animals) enough to obscure the form of the vertebrae, their bulbous junctions perhaps being visible in sauropods of some or many sorts.

The brilliant, late Ely Kish, one of the best palaeoartists of all time, constructing a scale model of Apatosaurus (in preparation for a large painting). Partly on the advice of palaeontologist Dale Russell, she depicted great lateral concavities on the sides of the neck. Image: Russell (1987).

Necks as display banners. The unprecedented form of the sauropod neck makes it plausible – perhaps even likely – that the neck was used as a display structure. Phil Senter’s suggestion that the size and length of the neck was driven, in evolutionary terms, by its use as a display structure (Senter 2007) is not supported by evidence (Taylor et al. 2011). However, this doesn’t discount co-option of the neck in display, so it really is – while wholly speculative right now – worth taking seriously the possibility that display structures of various kinds could well have adorned sauropod necks. These could include wattles, dewlaps, spiky frills, spines, filaments, inflatable pouches or distensible flags or flaps. A few artists have explored these possibilities, most notably Brian Engh and Emiliano Troco.

As weird as it might seem, it is at least plausible that the sauropod neck was decorated with weird soft-tissue display structures, as depicted here on a diplodocid. Image: Emiliano Troco.

 Hands. Sauropod hands are extremely odd, and very different from the rounded, elephant-style structures, edged with big nails and/or several claws, shown so often in artistic reconstructions and museum models. Indeed, they’re sufficiently weird and interesting that I’ve written whole articles about them before but, as I said above, these are currently lacking all of their relevant illustrations and are thus all but useless.

Sauropod hands are essentially unique. They’re weird, semi-tubular structures with pillar-like metacarpals. At left, a brachiosaur hand. At right, the hand of the turiasaur Zby. Images: Anthony Maltese, Darren Naish.

The sauropod hand is essentially a semi-tubular structure formed of elongate metacarpals arranged, pillar-like, in a semi-circle. The posterior surface – corresponding to the palm – was hollow, the consequence being a semilunate area of contact with the ground. We’ve known since at least 1940 that sauropod hands had this very unusual form thanks to fossil tracks (Falkingham et al. 2014), which makes it all the weirder that people have ignored this information and merrily continued giving sauropods elephant-like hands across the decades.

Roland T. Bird’s sauropod track illustrations from the 1940s - shown here - clearly show the true, highly unusual form of the sauropod manus. Shame on those who ignored this information in the following decades. Image: Falkingham et al. (2014), CC BY 4.0.

Tracks also show that nails and claws were absent from the hands, except on the thumb where a pointed claw – which varied considerably in exact shape and size from one group to the next – projected inwards and slightly backwards. The thumb claw was lost within Titanosauria, meaning that at least some members of this group lacked nails and claws on their hands altogether. I will concede that poorly defined convexities corresponding to manual digits were present in at least some sauropods, but they still wouldn’t have looked like distinct digits.

Some tracks appear to show that tough semi-conical tubercles projected from the skin on the front and sides of the hand, perhaps giving part of the hand a spiky or tuberculated appearance (Milàn et al. 2005). Maybe these structures had a role in foraging, digging, display or combat, since it’s plausible that they made the hands gnarlier and harder than they would have been otherwise.

Vertical scores associated with sauropod hand prints indicate that at least some of them had tuberculate hand skin, as depicted here (at right) in this modified version of a Greg Paul illustration. Image: Milàn et al. (2005).

Feet. Three large, curved claws projected anterolaterally from the inner three toes. At least some sauropods possessed four such claws. The outer two toes projected as blunt, rounded convexities in some, most or all sauropods. This is obvious from at least some Brontopodus tracks (Meyer et al. 1994). They might have had nails but were more likely lacking horny structures of any sort and essentially continuous with the rest of the foot’s outer surface. In contrast to the hand, the foot did have a massive fatty pad at its back.

Until recently it was thought that the metatarsals were held at a high angle, the consequence being a short foot shaped like that of an elephant but for the claws (Paul 1987). Data from articulated skeletons and tracks, however, show that the metatarsus was not as erect as argued, in which case the toes were longer and flatter than depicted by some artists.

Tschopp et al. (2015) used data from complete Camarasaurus hands and feet to produce the skeletal and soft-tissue models you see here (in 1 and 2), and then used these to generate tracks (3). The tracks are an exact match for real fossil tracks. Image: (c) Tschopp et al. (2015).

On the subject on feet and limbs, trackways also show that sauropods differed in how they placed their hands and feet, some walking with a very narrow gait (the hands and feet being placed close to the midline), others using a wide gait (where there was some short distance between the hands and feet of the left side versus those of the right), and others being intermediate. It should be remembered that even the widest-gauge sauropods did not walk with their feet all that far apart, but that the legs were almost certainly angled inwards, as is typical in living animals. For more on this issue see Scott Hartman’s article here.

Sauropods were variable in cross-sectional shape and in whether they walked with narrow-gauge or wide-gauge gaits, as depicted here by Scott Hartman. Image: Scott Hartman’s skeletaldrawing.com

The body and tail. Sauropod bodies were variable in cross-sectional shape, length and other details. Diplodocoids, for example, were relatively narrow, deep-bodied and with a tall ridge formed from their neural spines running along the back, while titanosaurs were extremely broad across the hips and must have been just about flat across the back. The thorax may have sloped down ever so slightly in diplodocoids (in part because their forelimbs were shorter than their hindlimbs) whereas the thorax was angled upwards slightly or even markedly in some macronarians, like brachiosaurs and some titanosaurs.

Articulated skeletons show that the tail mostly projected horizontally from the pelvis (a subtle arch at the tail base is present in diplodocoids and some others, in fact), but the macronarians with the upward-sloping bodies also had a downward-sloping tail. A real curiosity is provided by the tails of some mamenchisaurs which seem to have projected upwards at an angle. This has been discussed and illustrated by Paul (2010) and more recently by Hallett & Wedel (2016), but has otherwise gone undiscussed as far as I know. It sounds so odd that surely some mistake has been made… though I really don’t think it has.

Tall neural spines, massive, wing-shaped transverse processes and other structures show that an enormous quantity of musculature would have been obvious along the proximal part of the sauropod tail. This is the tail of … ugh .. Dippy, the cast of Diplodocus carnegii until recently on show in London. Image: Darren Naish.

As is typical for non-bird dinosaurs, the musculature at the base of the tail was (so we can say from the relevant bony attachment points) evidently enormous and bulky, the proximal part of the tail likely being similar in width to the pelvis and thighs combined.

The integument. We know for definite that sauropods of all groups were scaly animals, since scaly skin impressions and actual preserved skin patches are known for diplodocids, Camarasaurus and titanosaurs at least. A few skin folds here and there were almost certainly present (say, where the limbs met the body, at the limb joints, and at mobile zones in the neck) but a fissured, wrinkled or scored elephant-like skin is a big fat no and every effort should be made to avoid it in artistic depictions.

Haestasaurus from the English Wealden (shown at left) and a few other sauropods preserve polygonal scales that vary somewhat in size. Image: Darren Naish, Czerkas (1994).

Polygonal – specifically, pentagonal, hexagonal and heptagonal – scales are known for the macronarian Haestasaurus and some diplodocids, while rounded scales possessing a papilliform texture of tiny bumps are also preserved in a diplodocid specimen (Czerkas 1994). The polygonal structures varied in size somewhat, those present in more mobile parts of the skin (say, the inner crease of the elbow) being smaller than those present in non-mobile sections. Even so, the largest scales were, at most, 60 mm across, which is not large at all on an animal more than 15 m long. This is pretty common in non-bird dinosaurs, by the way. The scales were often so small that they wouldn’t be readily visible from any distance greater than a few metres. The papilliform texture on those diplodocid scales seems to have been widespread across sauropods and means that sauropod skin would have been rough to the touch.

A diplodocid specimen from Wyoming that has scaly skin preserved also preserves tall, conical dermal spines (note: they are not horn-covered bony structures) that must have been arranged along the dorsal midline (Czerkas 1992). These are variable in height (the biggest are 18 cm tall) and seem to have formed a single row along the top of the tail’s proximal part at least. This specimen was originally implied to belong to Diplodocus but is of indeterminate identity. It might belong to Kaatedocus, since remains of this dinosaur come from the exact same quarry.

Large, laterally compressed conical and semi-conical dermal structures lined the upper surface of the tail (at least) in some diplodocids, and perhaps in other diplodocoids and sauropods too. Image: Czerkas (1994).

The conical and semi-conical dermal spines of some diplodocids were variable in height, breadth and shape, as illustrated here. Image: Czerkas (1994).

Were these structures present across all diplodocids, all diplodocoids, all neosauropods or even all sauropods, or were they exclusive to one small clade, perhaps even just to Kaatedocus itself? In the absence of further information we can’t say, but it’s appropriate to depict them on diplodocids and their close kin at least, in the absence of further information. The possibility that shorter and/or taller dermal structures of this sort were present elsewhere on other sauropods exists.

The discovery of those conical and semi-conical spines in a diplodocid led Stephen Czerkas to produce this new look for these dinosaurs. It might be correct, but it is not clear whether the structures were as extensive along the animal’s length as shown here. Image: Czerkas (1992).

Horn-covered osteoderms - that is, lumps and nodules with a bony core - were arranged across the backs and flanks of some titanosaurs, specifically the members of the clade Lithostrotia. These structures were variously rounded or oval, sometimes quite flat and sometimes slightly or strongly domed. They appear to have been numerous and prominent in some of the relevant species and would have given their backs and sides a pebbly, armoured appearance.

Colours. There’s a traditional view stating that big dinosaurs were likely grey and plain because big living mammals are, as is the Komodo dragon, I suppose. I’m going to be bold here and say that there’s no reason whatsoever to take any notice of this. Sauropods were not mammals or Komodo dragons. While an argument could be made that camouflage would have been beneficial, and that certain pigments and patterns would have been helpful or necessary for physiological reasons (heat-shedding, heat retention, UV protection and so on), bold and complex patterns and bright, even vibrant colours are all consistent with the ecophysiological demands of being a sauropod, and the good colour vision and complexity of sauropod integument, and likely reliance on visual display, mean that they very likely could have been more like gigantic lizards or birds than elephants. In any case, giraffes, perenties and others show that big, terrestrial animals don’t have to be bland and grey as has often been stated.

The idea that big terrestrial animals have to be bland never was correct. Perenties Varanus giganteus might not be that colourful, but they help emphasise the possibility that striking patterns can be present even in very large terrestrial reptiles. Image: (c) Stephen Zozaya.

In the absence of any direct evidence, it seems reasonable to me to reconstruct barred, striped, reticulated or dappled colour schemes, to depict bright colours on faces, necks or other areas considered relevant to display, and to imagine any colours reasonable for big, terrestrial animals associated with woodlands, parks, scrubby places, mangroves and all the other habitats frequented by this long-lived, diverse group of animals.

Want to know more about sauropod life appearance? It sometimes surprises people that there are scarcely any good books dedicated to specific Mesozoic dinosaur groups. For sauropods, the main must-have volume is Mark Hallett and Matt Wedel’s The Sauropod Dinosaurs. It’s really good. Dinosaurs Past and Present Vol II is also worth getting, in part because it includes Greg Paul’s (now quite dated, but still useful) article on dinosaur life appearance. Images: amazon (here and here).

If we really want to imagine sauropods as living animals, there is – of course – so much more to say. Body language, posture, gait, social life, anti-predator responses, feeding behaviours, digestive function, sleep and so much more are all things that need to be considered. But that’s where I’ll stop for now. I hope you found this interesting, and we’ll be looking at dinosaur life appearance again sometime soon.

Articles like this are possible because of the support I receive at patreon. Please consider supporting my research and writing if you don’t already, thank you so much.

For previous TetZoo articles on sauropods, see…

•  The hands of sauropods: horseshoes, spiky columns, stumps and banana shapes, October 2008

• Sauropod dinosaurs held their necks in high, raised postures, May 2009

• Getting scansoriopterygids, terrestrial-stalking azhdarchids, sauropod pneumaticity and the word palaeontography into a kid's book, July 2010

• Necks for sex? No thank you, we're sauropod dinosaurs, May 2011

• The sauropod viviparity meme, May 2011

• The Second International Workshop on the Biology of Sauropod Dinosaurs (part I), December 2011

• The Second International Workshop on the Biology of Sauropod Dinosaurs (part II), January 2012

• Junk in the trunk: why sauropod dinosaurs did not possess trunks, November 2012

• The changing life appearance of dinosaurs, September 2014

• Brian J. Ford's Aquatic Dinosaurs, 2014 Edition, February 2015

• That Brontosaurus Thing, April 2015

• 10 Long, Happy Years of Xenoposeidon, November 2017

 Refs - -

Apesteguía, S. 2004. Bonitasaura salgadoi gen. et sp. nov.: a beaked sauropod from the Late Cretaceous of Patagonia. Naturwissenschaften 91, 493-497.

Bakker, R. T. 1986. The Dinosaur Heresies. New Theories Unlocking the Mystery of Dinosaurs and their Extinction. William Morrow, New York.

Czerkas, S. A. 1992. Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology 20, 1068-1070.

Czerkas, S. A. 1994. The history and interpretation of sauropod skin impressions. Gaia 10, 173-182.

Falkingham, P., Bates, K. & Farlow, J. 2014. Historical photogrammetry: Bird’s Paluxy River dinosaur chase sequence digitally reconstructed as it was prior to excavation 70 years ago. PLoS ONE 9, 4: e93247.

Hallett, M. & Wedel, M. J. 2016. The Sauropod Dinosaurs: Life in the Age of Giants. Johns Hopkins University Press, Baltimore.

Knoll, F., Galton, P. M. & López-Antoñanzas, R. 2006. Paleoneurological evidence against a proboscis in the sauropod dinosaur Diplodocus. Geobios 39, 215-221.

Martin, J. 1987. Mobility and feeding of Cetiosaurus (saurischia, sauropoda [sic]) - why the long neck? In Currie, P. J. & Koster, E. H.(eds) Fourth Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Boxtree Books (Drumheller, Alberta), pp. 154-159.

Martínez, R. D. F., Lamanna, M. C., Novas, F. E., Ridgely, R. C., Casal, G. A., Martínez, J. E., Vita, J. R. & Witmer, L. M. 2016. A basal lithostrotian titanosaur (Dinosauria: Sauropoda) with a complete skull: implications for the evolution and paleobiology of Titanosauria. PLoS ONE 11, 4: e0151661. 

Meyer, C. A., Lockley, M. G., Robinson, J. W. & dos Santos, V. F. 1994. A comparison of well-preserved sauropod tracks from the Late Jurassic of Portugal and the western United States: evidence and implications. Gaia 10, 57-64.

Milàn, J., Christiansen, P. & Mateus, O. 2005. A three-dimensionally preserved sauropod manus impression from the Upper Jurassic of Portugal: implications for sauropod manus shape and locomotor mechanics. Kaupia 14, 47-52.

Paul, G. S. 1987. The science and art of restoring the life appearance of dinosaurs and their relatives - a rigorous how-to guide. In Czerkas, S. J. & Olson, E. C. (eds) Dinosaurs Past and Present Vol. II. Natural History Museum of Los Angeles County/University of Washington Press (Seattle and London), pp. 4-49.

Paul, G.S. 1998. Terramegathermy and Cope’s rule in the land of titans. Modern Geology 23, 179-217.

Paul, G. S. 2010. Dinosaurs: A Field Guide. A & C Black, London.

Russell, D. A. 1987. Models and paintings of North American dinosaurs. In Czerkas, S. J. & Olson, E. C. (eds) Dinosaurs Past and Present, Volume I. Natural History Museum of Los Angeles County/University of Washington Press (Seattle and Washington), pp. 114-131.

Senter, P. 2007. Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation. Journal of Zoology 271, 45-53.

Taylor, M. P., Hone, D. W. E., Wedel, M. J., & Naish, D. 2011. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology 285, 150-161.

Taylor, M. P., Wedel, M. J. & Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54, 213-220.

Tschopp, E., Wings, O., Frauenfelder, T. & Brinkmann, W. 2015. Articulated bone sets of manus and pedes of Camarasaurus (Sauropoda, Dinosauria). Palaeontologia Electronica 18.2.44A: 1-65.

Witmer, L. M. 2001. Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science 293, 850-853.

Witton, M. P. 2018. The Palaeoartist’s Handbook: Recreating Prehistoric Animals in Art. The Crowood Press, Marlborough.

Heilmann, Thompson, Beebe, Tetrapteryx and the Proavian

Many people familiar with ideas on the early evolution of birds and of bird flight will know of the Proavis or proavian, a hypothetical bird ancestor illustrated and discussed by William Pycraft (1868-1942) in 1906 but made better known by Gerhard Heilmann (1859/1861-1946) during the 1910s and 20s. Versions of this creature were later illustrated in colour by Zdeněk Burian for popular books of the 1960s, 70s and 80s. In the article that you’re about to read, we’re predominantly interested in Heilmann’s take on the Proavis.

Caption: fighting, gliding and climbing proavians, depicted in a cluttered woodland environment, by Gerhard Heilmann. This is typical of Heilmann’s many excellent scenes. He used posed plaster models of the creatures before drawing. This illustration was used in the 1916 Danish version of the book (it first appeared in one of his 1912 articles) but does not appear in the 1926 English version. As we’ll see below, the decision to exclude some images from the 1926 version might have served to make it seem more scholarly than its predecessor. Image: Heilmann (1916).

Heilmann was an artist and graphic designer by profession but his interest in bird evolution and anatomy was such that he published a series of articles on the subject for the Danish Ornithological Society between 1912 and 1916 (Nieuwland 2004, Ries 2007).

Caption: self-portrait of Gerhard Heilmann, produced in 1902. Heilmann was an argumentative man often in conflict with “various forms of authority”, and at loggerheads with his own family (Nieuwland 2004). This illustration was included in Nieuwland (2004).

Compiled, these articles described Heilmann’s take on the diversity of ancient fossil birds, embryological development in birds and reptiles, and bird anatomy, with the last in the series explaining what these data meant for the appearance and lifestyle of his Proavis, an imagined animal older and anatomically more archaic than Archaeopteryx. These articles were published together in a Danish-language book Vor Nuvaerende Viden om Fuglenes Afstamning (Heilmann 1916), the title meaning Our Current Knowledge of the Descent of Birds.

Heilmann’s The Origin of Birds. This body of work was sufficiently impressive that – while Heilmann was mostly shunned in Denmark – it was well received elsewhere, and he was encouraged by scientists in Germany in particular to translate it into English. The result was the 1926 The Origin of Birds (republished in the USA in 1927), a scholarly and well-illustrated book that was enthusiastically accepted internationally and almost immediately deemed the most authoritative work on bird origins. The 1926 volume is quite different from that of 1916 in the illustrations it includes (as we’ll see below), but also in how scientific and speculative the text is, some more fanciful sections on palaeobiology being absent from the English version.

It has been said several times that Heilmann’s work was deemed so impressive that, rather than inspire new work on the subject, it effectively bought research on bird origins to a halt. Most relevant workers (albeit not all) now considered the question of bird origins to be resolved. As noted by historian Ilja Nieuwland, things might not have gone this way had the English-speaking readers of Heilmann’s book known that he was an amateur scientist and an artist by trade (Palm 1997, Nieuwland 2004).

Caption: Heilmann’s draftmanship was superb. This illustration (fig. 140 from Heilmann 1926) shows thigh feathering on the chicks of various birds, with 8 being a gliding gecko. “The animal with which we may best compare the bird-ancestor, is the Fringed gecko” [sic] (p. 197). Image: Heilmann (1926).

Heilmann thought that bird ancestors were likely quadrupedal gliders, similar superficially to living gliding geckos (Heilmann 1926, p. 197). This statement is somewhat odd in view of his endorsement elsewhere of a more cursorial view of these animals, but it seems that he imagined them adopting very different poses when on the ground versus when climbing. Heilmann also argued that birds were not dinosaurs – despite his good understanding of the compelling anatomical similarity between theropod dinosaurs and birds – but were instead the descendants of a group closely related to – and supposedly ancestral to – dinosaurs, termed ‘pseudosuchians’*. This was because of his adherence to ‘Dollo’s Law’ – the (erroneous) idea that a lost anatomical structure cannot be regained – and his mistaken contention that dinosaurs lacked clavicles. If dinosaurs couldn’t be ancestral to birds, the true ancestors must have been another group, hence Heilmann’s use of the more archaic ‘pseudosuchians’, already mooted as possible bird ancestors by Robert Broom in his 1913 description of the South African Euparkeria.

* The term pseudosuchian is today applied to the archosaur lineage that includes crocodylians and all of their extinct relatives. Given the historical baggage that comes with the term, I personally don’t think that this is a good idea at all and would prefer it if another name were used for the lineage concerned (like Crurotarsi)… but that’s an issue for another time.

Caption: Euparkeria capensis has long been intimated as a sort of bird ancestor by those looking for such creatures outside of theropod dinosaurs. Not only is this animal only very distantly related to birds, it’s not even part of the crown-archosaur clade. Image: Taenadoman, CC BY-SA 3.0 (original here).

By combining features common to Archaeopteryx as well as to the ‘pseudosuchians’ Aetosaurus, Euparkeria, Ornithosuchus and Saltoposuchus, Heilmann (1916, 1926) described how he invented a creature that looked somewhat like a theropod but had a more archaic skull and foot, retained a fourth metacarpal, and had a smaller pelvis with far shorter pubic and ischial bones.

Caption: Heilmann’s skeletal reconstruction of Proavis, as depicted in the 1916 Danish version of the book… but not in the 1926 English version. Note the lack of long feathers on the hindlimbs. Incidentally, note also that Heilmann was partly responsible for encouraging the belief that forelimb feathers did not cover the hands during the earliest stages of bird evolution. Image: Heilmann (1916).

Heilmann, Thompson and D’Arcy Thompson grids. It’s a matter of some interest that Heilmann didn’t just guess what the imagined morphology of the proavian would be, as might be assumed given the English edition of his book. Instead, he used a technique whereby the relevant anatomical regions were mapped on to a grid and then distorted to mimic the evolutionary process. This grid-based deformation process was pioneered by D’Arcy Wentworth Thompson (1860-1948) and was explained most thoroughly in his 1917 book On Growth and Form (Thompson 1917). It was actually put forward beforehand in 1915 (Thompson 1915).

Caption: Thompson applied his Cartesian grid technique to many animal lineages, his aim being to show that mathematically predictable transformation in one or more anatomical regions could result in profound anatomical change. Here’s one of my favourite examples: how you can derive a molid sunfish (Mola was known to Thompson as Orthagoriscus) from a porcupinefish. Image: Thompson (1917).

By placing grids on top of diagrams of related animals (say, the skull of a human and a chimp), Thompson showed how the skewing of the grid in a certain direction “would result in the sort of changes that would allow the emergence of a new species” (Naish 2017, p. 116). These grids are generally termed D’Arcy Thompson transformation grids, Cartesian transformations or Cartesian grids, and their use was quite popular in the evolutionary literature of the early 20th century. Heilmann was inspired to use the technique after Thompson wrote to him about bird evolution in 1915, and the two wrote to each other on many occasions about the technique and its application to ideas on the evolution of horses, hominids and birds. They also discussed how the resulting illustrations could be used in Thompson’s On Growth and Form (Ries 2007).

Caption: Heilmann included these Cartesian transformations in the 1916 version of his book, but they weren’t included in the 1926 English version. His take on Proavis was not, therefore, simple guesswork. Image: this montage is from Witmer (1991) but the originals are from Heilmann (1916).

Heilmann’s commitment to the technique is demonstrated by the fact that the 1916 Danish edition of his book includes grids that depict the inferred evolution of the avian skull, forelimb and pelvis (I don’t own a copy of the Danish edition, but the diagrams are included in both Witmer (1991) and Ries (2007)). With Ornithosuchus and Euparkeria as ‘starting points’ and Archaeopteryx and modern birds as ‘end points’*, Heilmann used the grids to create relevant intermediates, the results allowing him to generate, piecemeal, his Proavis (Heilmann 1916). A lateral view of the hypothetical animal – showing it in a neural, non-dynamic walking pose – was included in the Danish edition (Heilmann 1916, Ries 2007).

* It should be noted that Heilmann’s Archaeopteryx was not accurate, some of its details (in the skull especially) being semi-hypothetical and informed by Heilmann’s interpretation of ‘pseudosuchians’ like Aetosaurus.

Caption: the more dynamic, climbing version of Heilmann’s Proavis skeletal reconstruction, the only version of the reconstruction included in the 1926 English version of his book. Image: Heilmann (1926).

Remarkably, none of this was included in the English edition at all, this creating the impression that Heilmann simply invented the proavian via intuition and guesswork. He didn’t: it was generated via a testable, repeatable method (albeit using flawed anatomical data). Furthermore, the fact that Heilmann’s skeletal reconstruction of Proavis included in the English edition (Heilmann 1926) is the more dynamic climbing version of the creature gives it a less scientific, more speculative, artistic air than the walking version of 1916.

Heilmann, Beebe and Tetrapteryx. When it came to feathering and other aspects of the integument, Heilmann noted his thoughts on another Proavis-type animal, this time the one invented by Charles William Beebe in 1915. Beebe (1877-1962) was an interesting person. He was a naturalist, ecologist, ornithologist, marine biologist, author and explorer, among other things. Today, he might be best known for the deep-water observations he made while in a bathysphere off the coast of Nonsuch Island off Bermuda during the 1930s.

Caption: William Beebe’s hypothetical tetrapteryx creature. A pre-Archaeopteryx glider. You’ll note that Beebe was a pretty good artist [UPDATE: this is an error. The illustrations in this publication were by Dwight Franklin, not by Beebe!]. Image: Beebe (1915).

When it came to bird origins, Beebe thought that birds originated via a ‘tetrapteryx’ phase where large feathers on the proximal hindlimb were present and functioned in slowing descent during leaping and gliding (Beebe 1915). Both the fossils of Archaeopteryx and the nestlings of living bird species demonstrated the antiquity of these ‘pelvic wings’, Beebe (1915) said. Heilmann (1926) disagreed. His own examination of bird nestlings pertaining to bird lineages across the family tree resulted in his “complete disappointment, for what I found was wholly negative; there was not in any of them the slightest trace of a “pelvic wing”” (p. 194).

As for Archaeopteryx, Heilmann (1926) argued that the hindlimbs were arranged such that they could never have had an aerodynamic function, nor was there evidence for long hindlimb feathering of the sort consistent with ‘pelvic wings’. Ergo, Heilmann illustrated Archaeopteryx without long hindlimb feathering, and he didn’t think that Proavis had them either: his skeletal reconstructions (both the 1916 walking version and 1926 climbing one) lack them (the 1926 climber has short hindlimb feathers, similar in length to those elsewhere on the body) and the gliding proavians included in his various drawings clearly lack them as well. His most famous proavian figure – the semi-erect one standing in a conifer tree (shown below) – does have long feathers on the rear edge of the thigh, though they certainly aren’t as long as they should be as per Beebe’s tetrapteryx model.

The iconic tree-dwelling proavian. The iconic tree-dwelling proavian I have in mind here is one of Heilmann’s best-known and most frequently reproduced illustrations. It has what appears to be great symbolic significance if you’ve read the English version of The Origin of Birds since it’s the very last image in the entire book (Heilmann 1926, p. 199), thus appearing as a visual metaphor for an imagined phrase: “Finally, I can reconstruct the real ancestor of birds, and it looked like this”.

Caption: Heilmann’s most iconic version of his Proavis. Note the long feathers on the back of the thigh and how they’re invisible and thus not deemed important in the gliding individual in the background. Heilmann said that the long feathers close to the base of the tail would produce the required lift in this region. Image: Heilmann (1926).

I was surprised to learn that Beebe produced what looks like his own version of this illustration, and inserted it as a plate in a book published in 1915. However, the image itself is not dated 1915 and does not definitely pre-date Heilmann’s illustration. My suspicion – and that of Paul Stewart, who kindly brought my attention to the image – is that Beebe copied Heilmann’s proavian for his own personal use (the illustration was not intended for publication*). This in itself is interesting since it could mean that Beebe personally endorsed Heilmann’s view of Proavis and regarded it as a ‘descendant’ of his tetrapteryx creature… which it wasn’t, given that Heilmann rejected Beebe’s idea, as we saw earlier. Again, my thanks to Paul Stewart for sharing this image with me and permitting its use here.

* Thanks to Jonathan Kane, I’ve learnt that it was previously published in Tim Berra’s 1977 William Beebe: An Annotated Bibliography.

Caption: William Beebe seems to have produced his own take on Heilmann’s iconic proavian scene. We’re presuming that it was produced some time after Heilmann’s illustration was, but this hasn’t yet been fully confirmed. The illustration appears here courtesy of Paul Stewart.

Today, the significance of Beebe’s tetrapteryx idea is a bit uncertain and, frankly, depends on who you ask. Archaeopteryx probably did have long feathers on the hindlimbs (Longrich 2006), and they perhaps had an aerodynamic role if this animal indulged in aerial locomotion (which it likely did). Furthermore, the discovery of prominent ‘hindlimb wings’ in other feathered dinosaurs (most notably the Chinese dromaeosaur Microraptor) has led some experts to note the predictive power of Beebe’s suggestion (Kane et al. 2016) and to favour the presence and importance of hindlimb feathers in the earliest birds (Longrich 2006, Chatterjee & Templin 2007, Zheng et al. 2013, Xu et al. 2014). However, it’s not at all certain that large, aerodynamically ‘functional’ hindlimb feathers were present consistently across the lineages concerned or that they had the role that Beebe imagined (O’Connor & Chang 2015).

Caption: recent work shows that Archaeopteryx really did have long feathers on the proximal portions of its hindlimbs (these reconstructions are by Longrich (2006)), though they weren’t as long as the feathers Beebe imagined for his tetrapteryx stage creature. Image: Longrich (2006).

As always, there’s more to say and this article is already longer than intended. I also wanted to talk about Pycraft’s initial concept of the Proavis, and also Burian’s depiction, since he didn’t simply copy Heilmann’s illustrations but actually produced yet another hypothetical creature. These things will have to wait to another time. Also worthy of further discussion is the point made earlier about Heilmann’s influence happening despite his status as a ‘mere’ artist, a theme that has parallels elsewhere in the history of vertebrate palaeontology and is relevant to my writings on meme perpetuation, the scientific acceptance of feathering in non-bird dinosaurs and much else besides. I aim to explore these topics (and others) in future articles.

Caption: before Heilmann, there was Pycraft’s proavian of 1906. We’ll have to discuss this creature and its backstory another time. Image: Pycraft (1910).

On which note, please consider supporting this blog at patreon if you don’t already do so. The more support I receive, the more time I can spend generating new content. Thanks so much.

For previous TetZoo articles relevant to the issues covered here, see…

• Flight of the Microraptor, November 2013

• The Integrated Maniraptoran, Part 1, May 2016

• The Integrated Maniraptoran, Part 2: Meet the Maniraptorans, May 2016

• The Integrated Maniraptoran, Part 3: Feathers Did Not Evolve in an Aerodynamic Context, May 2016

• Naish and Barrett's Dinosaurs: How They Lived and Evolved, November 2016

• Book Review: God's Word or Human Reason? An Inside Perspective on Creationism, March 2018

Refs - -

Beebe, C. W. 1915. A tetrapteryx phase in the ancestry of birds. Zoologica 2, 38-52.

Chatterjee, S. & Templin, R. J. 2007. Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceedings of the National Academy of Sciences 104, 1576-1580.

Heilmann, G. 1916. Vor Nuvaerende Viden om Fuglenes Afstamning. Unknown publisher, Copenhagen.

Heilmann, G. 1926. The Origin of Birds. Witherby, London.

Kane, J., Willoughby, E. & Keesey, T. M. 2016. God’s Word or Human Reason? An Inside Perspective on Creationism. Inkwater Press.

Longrich, N. 2006. Structure and function of hindlimb feathers in Archaeopteryx lithographica. Paleobiology 32, 417-431.

Naish, D. 2017. Evolution in Minutes. Quercus, London.

Nieuwland, I. J. J. 2004. Gerhard Heilmann and the artist’s eye in science, 1912-1927. www.PalArch.nl., vertebrate palaeontology 3, 2.

O’Connor, J. & Chang, H. 2015. Hindlimb feathers in paravians: primarily “wings” or ornaments? Biology Bulletin 42, 616-621.

Palm, S. 1997. The Origin of Flapping Flight in Birds. Svend Plam, Ballerop.

Pycraft, W. P. 1910. A History of Birds. Methuen & Co, London.

Ries, C. J. 2007. Creating the Proavis: bird origins in the art and science of Gerhard Heilmann 1913-1926. Archives of Natural History 34, 1-19.

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Thompson, D. W. 1917. On Growth and Form. Cambridge University Press, Cambridge.

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Xu, X., Zhou, Z., Dudley, R., Mackem, S., Chuong, C.-M., Erickson, G. M. & Varricchio, D. J. 2014. An integrative approach to understanding bird origins. Science 346 (6215), 1253293.

Zheng, X., Zhou, Z., Wang, X., Zhang, F., Zhang, X., Wang, Y., Wei, G., Wang, S. & Xu, X. 2013. Hind wings in basal birds and the evolution of leg feathers. Science 339, 1309-1312.