The trust in tree hierarchies like DNSSec and a public key infrastructure flows from a root to the leaves. Take a look at a hierarchy like VeriSign's, which has three PKI trees. The trust in each tree begins at the Class 1, 2, and 3 self-signed root certificate authorities. Those CAs are the trust anchors for each tree. All other public CAs have a similar structure where a self-signed root sits atop the tree and trust flows downward to the leaves. That flow of trust is the trust chain, which you can follow back to a trusted root. If you look Amazon.com's digital certificate, it was issued by the VeriSign Class 3 Secure Sever CA, which was in turn signed by the VeriSign Class 3 Public Primary Certification Authority, which is the trust anchor.
The hierarchy in DNS is no different. There is a single root, the root zone, at the top of DNS that refers all queries to the top-level domain (TLD) servers. I recognize that DNS is a single tree where the plethora of public CAs are multiple trees, but that recognition simply demonstrates that a single trust anchor is unnecessary. The TLDs could be their own trust anchors, and the trust anchor signing keys could be distributed the same way that C A certificates are distributed today, which is through software updates. Or a mechanism to update trust anchor signing keys could be distributed through DNS, making sure that keys don't expire before the new ones are distributed.
A singed root zone is a more elegant and potential efficient solution because there are fewer keys to update and can be managed through a single entity, but it's not necessary.