An epiphysean Sispyhean task today: solve this mystery that has been bothering me for >15 years. It’s about bird knees. Read on.
Stomach-Churning Rating: 1/10- bones and brief words. Nothing to worry about.
Here is an ostrich. I was interviewing undergrads the other day and looked up to see it, then thought something like: “Oh yeah, that little bit of bone really bothers me. I cannot figure it out.” What little bit of bone?
This little bit of bone. Zooming in on that ostrich’s knee:
The little bit of bone is not talked about much in the scientific literature on bird knees. But we know it’s there and it is part of the composite bone called the tibiotarsus (ancestral tibia, this bit of bone, and the proximal tarsal [ankle] bones on the other end; the astragalus and calcaneum of earlier dinosaurs).
What is it? We call it something like the proximal tibial epiphysis (PTE for short, here). An epiphysis is an end of a bone that fuses up with the shaft during growth, around the time of skeletal maturity; ultimately ending longitudinal (length-wise) growth of that bone. Mammals almost ubiquitously have them. So do lizards and tuataras. And some fossil relatives. Not much else– except birds, in this particular region (the two ends of the tibiotarsus; also in the foot region; the tarsometatarsus; which also has its share of mysteries such as the hypotarsus; I won’t go there today). You can see the PTE in mostly cartilaginous form if you take apart a chicken drumstick.
This PTE, like other well-behaving epiphyses, fuses with the tibiotarsus in mature birds, forming one bone. But the young ostrich’s knee above shows the PTE nicely; and other living birds show more or less the same thing.
It begs for explanations. I’ve talked about it in a few of my papers. But I’ve always punted on what it really means– does it have anything to do with the patella (they appear at similar times in evolution; we know that much, roughly)? Where does it come from, developmentally? (we sort of know that but more work is needed in different species and in high resolution) When did it evolve? What does it tell us? Why is it there in living birds and almost no other extinct birds/other dinosaurs? Does it have anything to do with why birds, during their evolution, seem to gradually increase the fusion of skeletal elements or ossify new ones (tendons, kneecaps, etc)? Why here and not in the femur or several other long bones of birds? How much do these PTEs vary between (or within) bird species?
This is the challenge in the post’s title. I present to you: solve this puzzle. Developmentally, biomechanically, evolutionarily, genetically, whatever– why does this PTE happen? There are hints– e.g. this paper proposes why growth rates of long bones favour the formation of “secondary centres of ossification” like this. But I’m unable to satisfy myself with any solutions I can find. Maybe you can complete The Bird Knee Challenge?
Have a go at it in the Comments below! There are plenty of papers or even a grant or something involved in sorting out this single mystery; one of the many basic mysteries about animal anatomy.
Can’t help answer the question, but for the record, pterosaurs also have epiphyses on the long bones that fuse up only later in ontogeny, so there’s some other archosaurs with them.
Ahh yes, pterosaurs! Good point. Have comparative studies of the order of fusion of these epiphyses been done between pterosaur species and/or w/birds?
Not that I know of. Chris Bennett has some stuff on basic fusion patterns in Pteranodon at least and a little on Rhamphorhynchus / Pterodactylus as I recall, but I’m not usre there’s anything specific on the order / conjunction of fusion in pterosaurs during ontogeny.
See, you called it the PTE in your papers and so I simply thought it was an epiphysis after years of thinking it was an apophysis for the patellar ligament.
But now my dreams may still come true and this is unanswered after all. Why don’t endochondral bones of the skull for epiphyses???? They sure tinker with a lot of different tissues. Lizards will mineralized their Cranial epiphyes (eg palatobasal joint) and some have an intercalary cartilage here and there in the otic joint….but no bony epiphyses. Archosaurs don’t build them at all in their heads, save whatever the predentary bone is.
Epiphyses, those and these, remain mysteries!
Let’s solve them please, during this deep freeze!
Hi, John!
My reply comes with such a great delay that you may imagine that I’ve totally lost myelin and my nerve impulses go so slowly. -;) In any case…
In 1985 or so I had a chance to study in Askania-Nova, together with my teacher F. Dzerzhinsky, the ostrich, Rhea and Dromaeus – their skeletons, limb muscles and live birds’ locomotion. We did not publish anything (lazy Sovieticos), and now I cannot easily reach my notes. But I remember some of my thoughts. I thought that in the course of the hindlimb parasagittalization in birds, there was some twisting inside the knee joint, so that the distal end of the femur pronated on the tibia from lateral to forward direction. In this case, the patella would come to the lateral side of the femur, and become non-functional. And the new patella should be formed more medially – against the new position of the femoral intercondylar groove. And the old patella could become the apophysis which you are talking about and which we, naturally, noticed in Askania skeletons too.
In my opinion, it is the major difference between the way of hindlimb parasagittalization in birds and mammals. In mammals the bones are reshaped (e.g. bending of the femoral neck), while the avian way is repositioning inside articulations (e.g. formation of false trochanter and false neck inside the hip articular bursa).
Hi Alex, thanks for the interesting message. A complexity is that the hindlimb becomes ~parasagittal early in dinosaurs but no (bony) patella appears until very late in birds. So there was no old patella that became non-functional with changes to the knee (which did happen as the limb became more crouched), although the bony patella of birds surely relates in some way to their derived crouched posture. The problem with the “tibial epiphysis”/”old patella”/”apophysis” is that there is no clear precursor in the fossil record; like the patella, it appears late in bird evolution (i.e. is a neomorph, in terms of ossification) and that’s it.
Yes, I don’t know dinosaurs well. Did they have an ossified patella in the avian lineage? If not, the old patella could have been cartilaginous until it ossified in birds to become this mysterious apophysis. As I understand, dinosaurs never have ossified epiphyses either, as they grew throughout their life. I mean, that the patella replacement, on which I hypothesize, could have taken place when the leg became parasagittal, but it became fixed in bony structures later, when birds got osiified epiphyses and adjacent structure for strength.
Hi Alex, yes I strongly suspect that there was some fibrocartilage in the knee extensor tendon, and maybe something around the “proximal epiphysis” of the tibia too, in early dinosaurs as their distal femur tends to show a canal for the extensor tendon. And yes, dinosaurs didn’t have ossified epiphyses, just cartilage (lots); like in crocs+birds. I think we sort of understand the patella in birds but the evo-devo of the apophysis/proximal tibial epiphysis remains very unclear; either there are studies on extant bird mechanobiology in this area that I am missing, or there is a need for them.